impact of air travel on global warming

Aviation is responsible for 3.5 percent of climate change, study finds

• September 3, 2020

New research that provides the most comprehensive calculations of aviation’s impact on the climate finds that global air travel and transport is responsible for 3.5 percent of all drivers of climate change from human activities.

The study, published in the journal Atmospheric Environmen t , evaluated all of the aviation industry’s contributing factors to climate change, including emissions of carbon dioxide (CO 2 ) and nitrogen oxide (NOx), and the effect of contrails and contrail cirrus – short-lived clouds created in jet engine exhaust plumes at aircraft cruise altitudes that reflect sunlight during the day and trap heat trying to escape at night. 

The findings show that two-thirds of the impact from aviation is attributed to contrails, NOx, water vapor, sulfate aerosol gases, soot, and other aerosols. The remainder is  due to the cumulative heat-trapping effects of long-lived CO 2 emissions – 32.6 billion tonnes between 1940 and 2018, or roughly the total global CO 2 emissions for the year 2010. 

The five-year research effort was led by the UK’s Manchester Metropolitan University in collaboration with numerous academic and research institutions across the globe.

Co-author David Fahey, director of NOAA’s Earth System Research Laboratories in Boulder, Colorado, said the research strengthens the scientific foundation for understanding the role of aviation in climate change. 

“Our assessment will aid decision makers and the industry in pursuing any future mitigation actions, while protecting this important sector from any inaccurate assertions concerning its role in the climate system,” Fahey said. 

The study presents the complete first set of calculations for aviation based on a new metric introduced in 2013 by the Intergovernmental Panel on Climate Change called ‘effective radiative forcing,’ or ERF. ERF represents the increase or decrease in the balance between the energy coming from the sun and the energy emitted from the Earth since pre-industrial times.

Using the new ERF metric, the team found that while contrail cirrus has the largest climate warming impact, it is less than half that previously estimated. The effects of CO 2 emissions generated by aviation last for many centuries, and represent the second largest contribution. Approximately half the total cumulative emissions of CO 2 were generated in the past 20 years.

“Given the dependence of aviation on burning fossil fuel, its significant CO 2 and non-CO 2 effects, and the projected fleet growth, it is vital to understand the scale of aviation’s impact on present-day climate change,” said lead author David Lee, professor of Atmospheric Science at Manchester Metropolitan University and Director of its Centre for Aviation, Transport, and the Environment research group. 

Lee said that estimating aviation’s non-CO 2 effects on atmospheric chemistry and clouds was a complex challenge. “We had to account for contributions caused by a range of atmospheric physical processes, including how air moves, chemical transformations, microphysics, radiation, and transport.”

The new study will allow aviation’s impact on climate change to be compared with other sectors such as maritime shipping,  ground transportation and energy generation as there is now a consistent set of estimates.

This story was adapted from a press release issued by Manchester Metropolitan University .  

For more information, contact Theo Stein, NOAA Communications, at [email protected] . 

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‘Worse Than Anyone Expected’: Air Travel Emissions Vastly Outpace Predictions

The findings put pressure on airline regulators to take stronger action to fight climate change as they prepare for a summit next week.

By Hiroko Tabuchi

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Greenhouse gas emissions from commercial air travel are growing at a faster clip than predicted in previous, already dire, projections , according to new research — putting pressure on airline regulators to take stronger action as they prepare for a summit next week.

The United Nations aviation body forecasts that airplane emissions of carbon dioxide , a major greenhouse gas, will reach just over 900 million metric tons in 2018, and then triple by 2050.

But the new research, from the International Council on Clean Transportation , found that emissions from global air travel may be increasing more than 1.5 times as fast as the U.N. estimate. The researchers analyzed nearly 40 million flights around the world last year.

“Airlines, for all intents and purposes, are becoming more fuel efficient. But we’re seeing demand outstrip any of that,” said Brandon Graver , who led the new study. “The climate challenge for aviation is worse than anyone expected.”

Airlines in recent years have invested in lighter, more fuel-efficient aircraft, and have explored powering their planes with biofuel.

Over all, air travel accounts for about 2.5 percent of global carbon dioxide emissions — a far smaller share than emissions from passenger cars or power plants. Still, one study found that the rapid growth in plane emissions could mean that by 2050, aviation could take up a quarter of the world’s “carbon budget ,” or the amount of carbon dioxide emissions permitted to keep global temperature rise to within 1.5 degrees Celsius above preindustrial levels.

The decision by Greta Thunberg, a young climate activist, to sail across the Atlantic rather than travel by air ahead of her speech at the United Nations next week, has refocused attention on aviation’s role in causing climate change and its consequences, including sea-level rise and more intense heat waves, hurricanes, flooding and drought.

Climate protesters have said they plan to gather in Montreal next week, where airline regulators are set to hold their own summit.

William Raillant-Clark , a spokesman for the U.N. aviation body, stood by its emissions projection , which he said was “the most up-to-date” and provided “a clear picture on the future environmental trends.” He added that the group “endorses and welcomes wholeheartedly” calls for the aviation industry to address climate change with greater urgency.

Underlying the growth in aviation emissions is the rapid expansion of air travel worldwide, propelled by a proliferation of low-cost airlines and a booming tourism industry catering to a growing middle class.

A separate study released this week by the industry group Airports Council International found that the world’s fastest-growing airports were in emerging economies; 12 of the top 30 were in either China or India.

Still, the new data from the clean transportation council found that flights from airports in the United States were responsible for almost one quarter of global passenger flight-related carbon dioxide emissions. China was the next biggest source of passenger aviation emissions, followed by the United Kingdom, Japan and Germany ; the lowest-income countries that contain half the world’s population accounted for only 10 percent of all emissions.

The study underscored the heavy carbon-dioxide footprint of domestic flights, often left out of negotiations over global emissions-reduction targets. Domestic travel accounted for a large majority of departures in countries including the United States, China, Indonesia, Brazil and Australia.

Governments have pledged to take major steps to improve fuel economy in their routes and fleets. Under a plan adopted by the U.N. body, the International Civil Aviation Organization , three years ago, airlines will start to voluntarily offset most of the growth in their carbon dioxide emissions beginning in 2020. Carbon offsets compensate for emissions by canceling out greenhouse gas emissions elsewhere in the world. (For example, the offset may involve paying for renewable energy or other programs designed to reduce emissions.)

Some governments have suggested going further. In Germany, the Green Party has suggested banning domestic air travel altogether to force Germans to travel by train, which pollutes less.

“At a time when students are going on climate strikes around the world, this will really put pressure on the aviation industry to be much more ambitious,” said Annie Petsonk, international counsel for the Environmental Defense Fund. “They’re beginning to understand that for most people who fly, aviation is the biggest part of their personal carbon footprint.”

For more news on climate and the environment, follow @NYTClimate on Twitter .

An earlier version of this article misstated the nature of a global aviation summit meeting in Montreal next week. While industry representatives will be present as observers, the meeting is for airline regulators and diplomatic delegations, not executives.

How we handle corrections

Hiroko Tabuchi is a climate reporter. She joined The Times in 2008, and was part of the team awarded the 2013 Pulitzer Prize for Explanatory Reporting. She previously wrote about Japanese economics, business and technology from Tokyo. More about Hiroko Tabuchi

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It turns out planes are even worse for the climate than we thought.

By Michael Le Page

27 June 2019

Contrails produce a global warming effect

Roman Becker/EyeEm/Getty

The contrails left by aeroplanes last only hours. But they are now so widespread that their warming effect is greater than that of all the carbon dioxide emitted by aeroplanes that has accumulated in the atmosphere since the first flight of the Wright brothers.

Worse still, this non-CO2 warming effect is set to triple by 2050, according to a study by Ulrike Burkhardt and Lisa Bock at the Institute of Atmospheric Physics in Germany.

Altogether, flying is responsible for around 5 per cent of global warming, the team says, so this figure will soar even higher – and no meaningful actions are being taken to prevent this.

“Lots of people talk about the need to stop air traffic increasing all the time, but this is not taken that seriously,” says Burkhardt.

And the discussions that are taking place focus almost entirely on the associated CO2 emissions . “That’s a problem if the non-CO2 effects are larger than the CO2 ones,” she says.

“The non-CO2 warming is the elephant in the room,” says Bill Hemmings of Transport & Environment, a Belgium-based campaign group.

All aircraft that burn fuels leave behind a trail of exhaust fumes and soot. At high altitudes, water vapour often condenses on the soot particles and freezes to form a cirrus cloud that can persist for seconds to hours, depending on temperature and humidity.

Clouds can have both a cooling and warming effect. They reflect some of the sun’s rays back into space, but also block some of the heat radiated by Earth’s surface. On average, both thin natural cirrus clouds and contrails have a net warming effect .

Burkhardt and her colleagues used a computer model of the atmosphere to estimate how much warming contrails caused in 2006 – the latest year for which a detailed air traffic inventory is available – and how much they will cause by 2050, when air traffic is expected to be four times higher.

Read more: Earth could warm by 14°C as growing emissions destroy crucial clouds

The model accounts for not only of the change in air traffic volume, but also the location and altitude of flights, along with the changing climate.

The team concludes that the warming effect of contrails will rise from 50 milliwatts per square metre of Earth’s surface in 2006 to 160 mW/m 2 by 2050.

In comparison, the warming due to CO2 from aviation will rise from 24 to 84 mW/m 2 by this time.

In a scenario in which the airline industry increases fuel efficiency and reduces the number of soot particles  emitted by improving fuels and engines, the warming from contrails by 2050 is limited to 140 mW/m 2 and the warming from CO2 to 60 mW/m 2 .

Hard to cut

But reducing contrail warming won’t be easy. “It’s much harder than CO2,” says Burkhardt, and we aren’t doing anything effective about that either.

“There’s absolutely no doubt that aviation CO2 needs to be addressed properly, and there is absolutely no doubt that it is not being addressed at all effectively,” says Hemmings.

An international scheme called Corsia is supposed to limit aviation emissions. But its plan is instead to offset emissions, an approach known to be ineffective . What’s more, the airline industry is trying to use Corsia to block additional measures such as taxes on aviation fuel.

Read more: EU set to resist air industry attempts to limit climate change action

And then there is the non-CO2 warming. “The attitude has been that there are uncertainties, so let’s sit on our hands and do nothing,” says Hemming.

There are indeed large uncertainties whenever clouds are involved , says Burkhardt, but these go both ways. The study could be underestimating contrail warming by as much as 70 per cent.

The one bit of good news is that as contrails become more common, they reduce natural cirrus cloud formation by using up all the water available. This cuts the overall warming effect attributable to contrails by a fifth.

Atmospheric Chemistry and Physics DOI: 10.5194/acp-19-8163-2019

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Climate Matters • December 6, 2023

Climate Change is Disrupting Air Travel

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KEY CONCEPTS

Burning fossil fuels for aviation contributes to global warming — and the resulting warming is disrupting air travel.

Flooding caused by rising seas and storm surges threatens access and operations at coastal airports.

More extreme weather events, warmer air temperatures, and shifts in the jet stream can also disrupt air travel and increase in-flight safety risks.

Last year, 917 million passengers took over 15 million U.S.-based flights — an average of more than 42,000 flights per day.

Fossil fuels burned for global air travel and airport operations contribute to the carbon pollution that is warming the planet. In 2022, global air travel emitted more than 780 Mt CO2 , accounting for about 2% of global energy-related CO2 emissions that year. Emissions from aviation have steadily increased, quadrupling from 1966 to 2018.

Air travel not only contributes to heat-trapping pollution — the resulting warming now poses new and growing challenges for air travel.

Weather currently causes more than 75% of air traffic delays in the U.S. As climate change worsens coastal flooding and extreme weather events, more flights could be grounded from weather-related delays. A warming atmosphere can also increase in-flight turbulence.

Five ways climate change is disrupting air travel

1. Coastal airports are at risk from rising seas and storm surges.

Rising seas due to human-caused warming are worsening coastal floods during both regular high tides and coastal storms; and storm surge is affecting larger areas in  many U.S. cities  because of rising seas.

Runways at some major airports in U.S. cities and abroad are at risk of closures, delays, and damage due to coastal flooding, particularly after major storms . Access for passengers and employees can also be impacted if access roads are inundated.

2. Heat can affect plane capacity and restrict takeoff.

Warm air is less dense than cool air. Hotter temperatures at ground level therefore make it more difficult for airplanes to gain enough lift to take flight. 

Hotter temperatures can cause weight restrictions for flight take-off — meaning fewer passengers and reduced capacity for luggage, cargo, and fuel. In some cases, planes may require longer runway distances to generate enough lift. 

3. Climate change is increasing the risk of lightning strikes in flight.

The potential for severe storms is increasing in some parts of the U.S., particularly in the eastern half of the country. One study projects that annual lightning strikes in the U.S. could increase by 12% for every 1°C (1.8°F) of global warming. 

Lightning strikes can damage electrical systems and equipment on large commercial aircraft. The average passenger plane is struck by lightning one or two times per year . Following lightning strikes, planes are subject to inspection and repairs, which can take them out of service and cause delays.

4. A shifting jet stream could mean a longer round trip journey.

In recent decades, scientists have observed changes in the jet stream — narrow bands of strong wind high in the atmosphere that move west to east along the boundaries between hot and cold air. The influence of global warming on these observed changes is not yet fully understood.

Research suggests that changing wind patterns could impact travel times in the Northern Hemisphere — potentially making west-bound flights longer, while speeding up east-bound flights. These changes could affect route planning, scheduling, and fuel consumption.

5. Increased wind shear in the jet stream is causing more hazardous turbulence.

One observed change to the jet stream includes stronger wind shear at flight cruising altitudes, which can increase turbulence during flights. 

A certain type of turbulence known as clear-air turbulence can’t be seen by pilots or detected by radar. Clear-air turbulence is more likely to occur during winter months.

A recent study found a 41% increase in severe clear-air turbulence over the U.S. between 1979 and 2020 — and it is projected to increase further due to climate change.

The future of flying

Reducing heat-trapping pollution from air travel now is critical to limit future warming. But some consequences from rising temperatures are unavoidable, even with rapid cuts to carbon pollution. The aviation industry is faced with a need to adapt air travel for a warming world to protect passengers and employees, as well as keep costs under control.

Adaptation measures, including seawalls or other coastal defenses , can help protect existing airports from rising seas and storm surges, but they can be costly and complicated. The Shoreline Protection Program at San Francisco Airport is one example of how U.S. airports might build climate resilience.

In especially hot locations or during summer, airports may choose to schedule flights during cooler parts of the day to mitigate heat effects. Where possible, airports may expand runways to accommodate longer takeoff distances.

LOCAL STORY ANGLES

Find out if your local airport is vulnerable to coastal flooding..

Read Climate Central’s  assessment of 23 major U.S. cities that are vulnerable to coastal flooding  during this decade. Or use Climate Central’s Coastal Risk Screening Tool  to map localized vulnerability to sea level rise and storm surges around your city’s airport.

See which airports are experiencing delays.

Extreme weather events, heat, or flooding can ground planes and disrupt domestic and international air travel with cascading impacts. The Federal Aviation Administration provides status updates on delays at airports across the U.S. (but specific flight information and delay causes aren’t reported here.)

CONTACT EXPERTS

Paul D. Williams, PhD Professor of Atmospheric Science. University of Reading Relevant expertise:  atmospheric turbulence, jet streams, climate change, and aviation Contact:  [email protected] * Available for interviews on or after December 11, 2023

FIND EXPERTS

Submit a request to SciLine from the American Association for the Advancement of Science or to the Climate Data Concierge from Columbia University. These free services rapidly connect journalists to relevant scientific experts. 

Browse maps of climate experts and services at regional NOAA, USDA, and Department of the Interior offices.  

Explore databases such as 500 Women Scientists , BIPOC Climate and Energy Justice PhDs , and Diverse Sources to find and amplify diverse expert voices. 

Reach out to your State Climate Office or the nearest Land-Grant University to connect with scientists, educators, and extension staff in your local area. 

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• Open access
• Published: 22 June 2021

Evaluating the climate impact of aviation emission scenarios towards the Paris agreement including COVID-19 effects

• Volker Grewe   ORCID: orcid.org/0000-0002-8012-6783 1 , 2 , 3 ,
• Arvind Gangoli Rao   ORCID: orcid.org/0000-0002-9558-8171 2 , 3 ,
• Tomas Grönstedt 3 , 4 ,
• Carlos Xisto   ORCID: orcid.org/0000-0002-7106-391X 3 , 4 ,
• Florian Linke   ORCID: orcid.org/0000-0003-1403-3471 3 , 5 ,
• Joris Melkert 2 , 3 ,
• Jan Middel 3 , 6 ,
• Barbara Ohlenforst   ORCID: orcid.org/0000-0002-5793-6059 3 , 6 ,
• Simon Blakey   ORCID: orcid.org/0000-0001-6478-7170 3 , 7 , 8 ,
• Simon Christie   ORCID: orcid.org/0000-0003-2631-5425 3 , 9 ,
• Sigrun Matthes   ORCID: orcid.org/0000-0002-5114-2418 1 , 3 &
• Katrin Dahlmann 1 , 3  

Nature Communications volume  12 , Article number:  3841 ( 2021 ) Cite this article

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• Climate change
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• Environmental impact
• Projection and prediction

Aviation is an important contributor to the global economy, satisfying society’s mobility needs. It contributes to climate change through CO 2 and non-CO 2 effects, including contrail-cirrus and ozone formation. There is currently significant interest in policies, regulations and research aiming to reduce aviation’s climate impact. Here we model the effect of these measures on global warming and perform a bottom-up analysis of potential technical improvements, challenging the assumptions of the targets for the sector with a number of scenarios up to 2100. We show that although the emissions targets for aviation are in line with the overall goals of the Paris Agreement, there is a high likelihood that the climate impact of aviation will not meet these goals. Our assessment includes feasible technological advancements and the availability of sustainable aviation fuels. This conclusion is robust for several COVID-19 recovery scenarios, including changes in travel behaviour.

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Introduction

Fuel efficiency of jet aircraft has been increasing right from the dawn of jet aviation in the late ’50 s and early ’60 s. This improvement cannot be attributed to one single source but has been achieved by a combination of factors such as improvements of the airframe aerodynamics, weight reductions due to better engineering, materials and manufacturing techniques, larger engines with a lower specific thrust, higher overall pressure ratios and component efficiencies, lighter structures and lighter on-board systems. Kharina and Rutherford 1 report an average reduction in fuel consumption per passenger-km at the global fleet level of 1.3% per year over the years 1960–2014. Without any further specific measures this reduction is expected to continue at a similar rate until 2037 2 in a business as usual scenario.

Air transport as a sector has been growing rapidly in most regions of the world. The total number of passengers transported annually passed 4 billion in 2017. The number of flights in all regions of the world has increased (Supplementary Fig. 1 ) and aircraft have on average greater seating capacity and are operated with a higher load factor (Supplementary Fig. 2 ). It is expected that air transport will continue to grow in the coming decades. Airbus 3 predicts in its Global Market Forecast continued annual growth of 4.4% in revenue passenger kilometre (RPK) for the next two decades. Boeing 4 expects in its Commercial Market Outlook an annual growth of 4.6%. The effects of the COVID-19 pandemic are expected to only have a temporary effect on this growth.

Without any measure the climate impact of aviation will continue to grow. Several measures, both political and technical, are in place or will be introduced in the near future. Via a number of scenarios, we analyse their effect on global warming and assess the effectiveness of these measures. Since many of these measures are set top-down we also want to assess the technical feasibility. Therefore, we have performed a bottom-up expert assessment on the feasibility of technical advances and their effect on climate change. We confront the two approaches with each other.

The profitability for the airlines is small. Their average net profit per passenger is <10 USD (Supplementary Fig. 3 ). Competition amongst airlines is fierce and therefore sensitive to airline costs differences. Fuel costs play an important role, which is of particular concern for the uptake of sustainable alternative fuels (SAF) that currently have a significantly higher cost than conventional fossil fuels. The COVID-19 pandemic has led to a large decrease in the number of flights and passenger load factors in 2020. In May 2020, the International Civil Aviation Organisation (ICAO) estimated a decrease of global total available seat kilometres of 94% in April 2020 compared to the 2019 baseline. However, they expect a recovery leading to an annual decrease in available seat kilometres of 45% to 63% for 2020 5 , but assume growth will resume beyond 2020.

Approximately 5% of the current anthropogenic climate change is attributed to global aviation 6 , 7 and this number is expected to increase since aviation passenger transport is projected to grow by ~4% per year whilst other sectors continue to decarbonise. Aviation emits carbon dioxide (CO 2 ), water vapour (H 2 O), nitrogen oxides (NO x ), sulphate aerosols, compounds from incomplete combustion (unburnt hydrocarbons, UHC) and particulates (soot). The emitted species are transported in the atmosphere and alter a wide range of atmospheric processes including the formation of contrail-cirrus and ozone and the depletion of methane 7 , 8 , 9 .

The formation of persistent contrails-cirrus depends on aircraft and fuel parameters as well as atmospheric conditions, as the propensity of contrail formation is higher in the cold and saturated atmosphere 10 , 11 , 12 . Contrail-cirrus influence the incoming solar radiation and the outgoing infrared radiation emitted by the Earth and its atmosphere. The net change, the radiative forcing (RF), is on average positive and hence contrail-cirrus act to warm the climate 13 . The emitted nitrogen oxides (NO x ) react with hydroxyl radicals (HO x ), which eventually form ozone and contribute to the depletion of methane in the atmosphere. Therefore, emissions of nitrogen oxides increase the ozone concentration and decrease the methane concentration (which itself leads to a reduction in ozone production and is called primary mode ozone, PMO). Ozone and methane are greenhouse gases and changes in their concentrations cause changes in the RF, which are in total positive, i.e. leading to warming 7 , 14 , 15 . The net direct impact of aerosol emissions on RF (soot: warming and sulphate: cooling) is small 7 and are not further regarded in this study, whereas the impact of soot emissions on contrail-cirrus properties are important 16 and considered in our calculations (see ‘Methods’). An open question, which is currently under investigation is whether aerosol emissions significantly alter or even induce natural clouds, both low-level and cirrus clouds 17 .

The Advisory Council for Aviation Research and Innovation in Europe (ACARE) has set targets for the reduction of emissions in its Flightpath 2050 document 18 . Among these targets is a reduction of 75% of CO 2 and 90% of NO x emission per passenger-km by 2050. The datum for these reductions is a typical new aircraft in the year 2000. These targets are set for the research, with intended outcomes to be realised at a technological readiness level (TRL , The European definition of TRLs range from 1 to 9, i.e. from ‘basic principles observed’ to ‘actual system proven in operational environment’) of 6.

The ICAO of the United Nations has agreed on a global market-based measure scheme to abate the growth of CO 2 emissions from international aviation. This scheme is the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA). According to this scheme, the post-2020 growth in the sector must be offset such that the net carbon emissions do no longer grow. They must either be reduced via more efficient aircraft and/or the use of SAF or must be compensated via offsets. CORSIA starts as a voluntary pilot scheme in 2021 and becomes mandatory, with some exceptions, in 2027 for all member states 19 . Aviation is a growing sector that has committed to reduce net CO 2 emissions and thus contributes to the international goals of limiting climate warming ‘to well below 2.0° C above preindustrial levels and pursuing efforts to limit the temperature increase to 1.5 °C above preindustrial levels’, as stated in the Paris Agreement 20 . The Paris Agreement does not set emission targets for specific sectors. Furthermore, international aviation and shipping are not included in the national contributions that countries have to make to comply with the agreement. However, we assume that the international aviation community will contribute to the goal of the Paris Agreement. We will investigate the effect of measures and policies on global warming and also assess their feasibility. Thereby we will not distinguish between domestic and international aviation but treat the sector as a whole. There are two bridges to cross between the emission goals set by ACARE and ICAO and the climate targets set by the Paris Agreement: First, how do the emission goals translate into near-surface temperature changes, i.e. climate change. Second, how large are the non-CO 2 effects?

Here we close these gaps and show that the emissions goals set by Flightpath 2050 very likely will stabilise aviation’s climate impact, though the sector’s contribution to global warming remains considerable. Contrarily, we find that ICAO’s offsetting scheme, CORSIA, will surpass the climate target set to support the 1.5 °C goal between 2025 and 2064 with a 90% likelihood. In both cases non-CO 2 effects will have a considerable contribution to aviation’s climate impact, however, they are currently not included in ICAO’s goal of climate neutral growth and only partly addressed in Flightpath 2050. We assess the feasibility of achieving the Flightpath 2050 goals by technological improvements and the availability of sustainable alternative fuels as an ECATS (Environmentally Compatible Air Transportation System) expert group and reveal the risk of a large discrepancy, leading to an increasing aviation induced global warming effect rather than stabilisation.

Results and discussion

Top-down scenarios for future aviation.

Figure  1a presents the global growth of revenue passenger kilometres, showing an exponential increase between 5.2 and 6% per year (dotted lines). From this basis, we developed eight top-down scenarios, which consider a further increase in aviation, though with a decreasing rate of growth (down to 1.2%/year). These rates are based on simulations of the aviation sector, relying on the Randers scenario 21 , which is independent from aircraft manufactures. This scenario was employed within the WeCare project 22 and considers worldwide saturation effects of economic growth. This Randers scenario leads to a growth rate of 1.2%/year in 2050 which we extrapolate to 0.8%/year in 2100 (see also Supplementary Material). Our industry-independent scenario shows lower estimates of the transportation volume for the coming two decades compared to the Airbus and Boeing forecasts (see above), though still slightly higher than other estimates for 2050 23 , 24 . Advances in airline operating efficiency, including changing the type of aircraft, the number of seats and load factor lead to a reduced increase of flown kilometres (Fig.  1b ; green line) compared to the transport volume measured in RPKs (violet line). More fuel-efficient technologies even lead to a smaller increase in fuel use compared to flown distances (blue and orange lines). Taking the targets of Flightpath 2050 into account, a more aggressive reduction in emissions can be achieved up to 2050. In the scenario FP2050, we consider a development of these technologies until 2050 followed by an introduction into the market. In the scenario FP2050-cont we apply a continuous introduction of these innovative technologies into the market (Fig.  1b , early and continuous/late introduction light/dark brown, respectively). Using these assumptions, the modelled results show that after 2050 the increase in RPK is balanced by technology enhancements leading to almost constant fuel consumption until 2100.

They include the future use of current technology, i.e. without technology improvements (CurTec), with a business-as-usual future technological improvement (BAU), the offsetting scheme of the international civil aviation organisation (CORSIA), and 2 Flightpath 2050 scenarios which differ in the speed of technology improvements (FP2050 and FP2050-cont). a Revenue passenger kilometres as provided by ICAO; dotted lines provide exponential growth rates. b Future changes relative to their respective values in the year 2000 for revenue passenger kilometres (violet), flown distances (green), the fuel consumption of the scenarios BAU and CORSIA (blue), and the FP2050 scenarios (brown). c Future CO 2 emissions for the scenarios CurTec (red), BAU (blue), CORSIA (light blue), FP2050 with late technology advancements (dark brown) and continuous technology advancements (brown). Note that for CORSIA the effective CO 2 emission is considered, including reductions due to the use of sustainable alternative fuels (SAF) and capping net emissions. d as bottom-left, but for NO x emissions; note that the NO x emissions for BAU and CORSIA are identical. The order in the legend is the same as the lines appear in the graph.

We take into account five different scenarios (Table  1 ): (1) Current Technology (CurTec), which describes the emission pathways with current (2012) technology, (2) Business-as-usual (BAU), which, in addition, takes into account some of the future improvements in technology, (3) CORSIA, which is identical to BAU, but yearly CO 2 emissions are reduced by offsetting CO 2 emissions beyond 2020 values, (4) and (5) Flightpath 2050 (FP2050 and FP2050-cont), which utilise the targets of FP2050 (Fig.  1c, d ). Note that for the CORSIA scenario, we assume an optimistic future availability and a price premium of SAF based on an analysis of feedstocks and the evolution of SAF production. As a result, approximately half (53%) of the CO 2 reduction that is required to achieve CORSIA’s CO 2 -neutral growth stems from the use of SAF and the other part results from carbon caps. This leads to a larger reduction in climate impact compared to a scenario where the total amount of CO 2 is capped. The explanation is that SAF do not only reduce the climate impact via CO 2 but also the reduction in contrail-cirrus climate impacts since a change in their chemical composition changes the contrail-cirrus properties (see ‘Methods’).

Aviation climate impact

We use these five scenarios to calculate their climate impact with the non-linear climate-chemistry response model AirClim 25 , 26 in terms of near-surface temperature change by taking into account effects from CO 2 as well as NO x and H 2 O emissions and contrail-cirrus (Fig.  2 ). The three scenarios CurTec (red line), BAU (dark blue line), and CORSIA (light blue line) show an increase in temperature until the end of the simulation (2100), though the rate of increase slows down. For CurTec, since the technology is frozen in this scenario, the rate of increase arises from the assumed development of the transport volume (Fig.  1 ). The increased efficiency in scenario BAU in comparison to the scenario CurTec clearly shows a substantial temperature reduction of roughly 25% in 2100. The temperature reduction is even larger for CORSIA (35–40%), due to a reduction in the effective CO 2 emissions from the CORSIA scheme and changes in contrail-cirrus properties from the extensive use of SAF. Terrenoire et al. 27 calculated a temperature increase in 2050 for a CORSIA scenario of 32 mK, which is consistent with our calculated value of 30.4 mK. The two implementations of the Flightpath 2050 scenarios (FP2050 and FP2050-cont) show a clear stabilisation of their climate impact, though with an overshoot around 2050. Allowing 5% of the anthropogenic temperature increase to be contributed by the aviation sector, as motivated by the current estimate of aviation to global warming, both scenarios show compliance with a 2 °C target and the scenario FP2050-cont even with the 1.5 °C target. The inertia of the climate system delays the impact of both FP2050 scenarios, which overshoot these targets around the year 2050. However, after 2050, the FP2050 measures are sufficient to cause significant temperature decreases beyond these targets from this point on.

The horizontal lines indicate 5% of a 2 °C and 1.5 °C climate target. The scenarios describe a future use of current technology, i.e. without technology improvements (CurTec, red), a business-as-usual future technological improvement (BAU, blue), the offsetting scheme of the international civil aviation organisation (CORSIA, light blue), and 2 Flightpath 2050 scenarios which differ in the speed of technology improvements (FP2050 and FP2050-cont, brown and orange, respectively).

Temperature change is a complex response to the individual measures through the various climate agents. The reductions of the CO 2 emissions (Fig.  1 ) for all scenarios compared to the CurTec scenario lead to a significant reduction of aviation’s absolute contribution to climate change (Fig.  2 ). However, the relative contribution to climate change, i.e. the share of CO 2 to the aviation’s climate impact, increases from 25% in 2005 to between 33% and 56% in 2100. The reason is that the reduction in NO x emissions reduces the temperature increase via ozone faster than the reductions in CO 2 emissions. The short lifetime of both NO x and ozone in the atmosphere compared to CO 2 enables this faster response. On the other hand, the contrail-cirrus climate impact is largely driven by the distances flown. Here two factors play a role, the increase in the efficiency of the transportation system and the use of sustainable alternative fuels. These two effects lead to a reduction in the contrail-cirrus climate impact by roughly 20% in the scenarios BAU and CORSIA compared to CurTec (Fig.  3 ). The relative contribution of contrail-cirrus to the climate impact (Table  2 ) shows a reduction from 33% in 2005 to around 20% and 24% in 2100 for BAU and CORSIA scenarios, respectively, and is only slightly reduced for the FP2050 scenarios (27% and 30%). Recently, the non-CO 2 effects of aviation were revised concerning NO x emissions 15 and contrail-cirrus 13 . While Grewe et al. 15 stressed methodological improvements, like how to correctly attribute ozone concentrations to aviation NO x emissions, Bock and Burkhardt 13 focussed on improved contrail-cirrus microphysics. Our results include most aspects of these new developments and hence show, e.g. a larger ozone-RF as well as NO x -RF compared to earlier studies, such as Lee et al. 28 (Fig.  3 , left bars). The current results are in accordance with those new findings.

The individual bars are grouped into four categories. (1) The two bars on the left describe the radiative forcing of aviation in the year 2005 (RF 2005). Results from Lee et al. (2009) are expanded by contrail-cirrus estimates based on Bock and Burkhardt (2019), denoted by L09 + BB19, respectively; (2) temperature change in the year 2005 (dT 2005); (3) temperature change in the year 2050 for the 5 scenarios (dT 2050); (4) as (3), but for the year 2100 (dT 2100). For 2100, i.e. the right-hand columns, the scenarios are presented in the same order as for 2050. The order in the legend is the same as the colours appear in the individual boxes. The scenarios describe a future use of current technology, i.e. without technology improvements (CurTec), a business-as-usual future technological improvement (BAU), the offsetting scheme of the international civil aviation organisation (CORSIA), and 2 Flightpath 2050 scenarios which differ in the speed of technology improvements (FP2050 and FP2050-cont).

Hence to summarise, the increase in transport volume leads to an increase in the overall climate impact from aviation, which also increases the relative importance of CO 2 (25% in 2005 Base to 39% in 2100 CurTec, Table  2 ), even if aviation net CO 2 emissions are regulated and capped to 2020 values. The increase in fuel efficiency of aviation technologies at a current rate decreases the overall climate impact, especially for CO 2 and NO x . By this, it mainly reduces the relative contribution of NO x (41–16%). The introduction of the CORSIA scheme further reduces the climate impact of CO 2 emissions and that increases the relative importance of contrail-cirrus and NO x . The technological measures from FP2050 have a similar reduction efficiency for CO 2 as CORSIA, however, the strong measures for NO x largely reduce the overall climate impact so that the remaining climate impact from aviation is due to CO 2 (50–60%) and contrail-cirrus (around 30%).

Contrasting aviation climate impact with 1.5 °C and 2 °C climate targets

The climate impact of aviation emissions has a considerable uncertainty range, especially, for the non-CO 2 effects 22 , 28 , which influences not only the absolute change of near-surface temperatures but also the importance of the individual climate agents. In this work, we take into account uncertainties in the atmospheric lifetime of aviation-related species, uncertainties in the RF of individual species and the climate sensitivity parameter, the last of which relates the RF to temperature changes. These parameters are varied in a Monte–Carlo analysis with 10,000 simulations to obtain a range of possible atmospheric responses. The results of the Monte–Carlo analysis provide a basis for estimating a range when the temperature thresholds, 5% of 1.5 °C and 5% of 2 °C, are surpassed. For example, Fig.  4a shows the first 20 simulations of the CORSIA scenario. Figure  4b shows the probability density function (blue) and cumulative probability density function (green) of these times of surpassing the 5% of 1.5 °C for the CORSIA scenario. The mid 90% range (between the 5% and 95% percentile) indicates that this threshold is surpassed between 2025 and 2064 in the CORSIA scenario. The 5% of 2 °C is surpassed roughly 10 years later (Fig.  4c ). Both, the CurTec and BAU scenario, show that both thresholds are surpassed very likely well before 2050 (Fig.  4c ).

a Potential pathways (first 20 realisations of the Monte–Carlo simulation) for the CORSIA Scenario (grey). 5% of the 1.5 °C ( = 75 mK) is indicated as a black line. Crossings of the brown line with the grey line indicate the year when the threshold is surpassed. b Probability density function (PDF, blue) and cumulative probability density function (CPDF, green) for the year in which the climate target of 5% of 1.5 °C is surpassed (and stays above). The horizontal bar indicates the 95%, 50% and 5% percentiles. c 95%, 50% and 5% percentiles of the year in which the climate target is surpassed for 5% of 2 °C (thin top lines with crosses) and 5% of 1.5 °C (thick bottom lines). For both FP2050 scenarios, the 5% of 2 °C target is not surpassed in >95% of the cases, hence there is no thin line, and for FP2050 scenario with continuous improvements (FP2050-cont) the 50% percentile is beyond 2100. The scenarios describe a future use of current technology, i.e. without technology improvements (CurTec), a business-as-usual future technological improvement (BAU), the offsetting scheme of the international civil aviation organisation (CORSIA), and 2 Flightpath 2050 scenarios which differ in the speed of technology improvements (FP2050 and FP2050-cont) and bottom-up estimates based on a group of experts from ECATS (Environmentally Compatible Air Transportation System).

ECATS technology scenarios and their climate impact

The emission reductions formulated in the Flightpath 2050 are aspirational goals, which the aviation community is aiming to achieve. Here we now contrast this with technologies which are currently discussed in the research, such as boundary layer ingestion, distributed propulsion, laminar flow control, lightweight structures, advanced geared turbofan engines, etc., and assess their potential to reduce fuel use and NO x emissions (Table  3 and Supplementary Material for more details). The majority of technology enhancements for a 2050 aircraft should, at least as an idea, be available today since the time from the development of basic research ideas (TRL 1) to having this aircraft operational in service (TRL 9) takes decades. We take into account developments for different aircraft segments, such as single-aisle and twin-aisle aircraft for entry into service between 2035 and 2050. General aviation, regional aircraft and business jet have been left out from this study, as their current contribution to total aviation CO 2 emission is around 5–6%, only. We take into account a large range of technologies and engine airframe integrations (see Supplementary Figs. 7, 12–16 ) and find a 18–22% improvement in fuel efficiency, which is similar to the analysis presented by Cumpsty et al. 2 , which indicates an 18% reduction. For the far future (2050), we consider one variant for a single-aisle aircraft, while three variants are considered for a future long-range twin-aisle aircraft. These include (1) a conventional tube-and-wing wide-body aircraft (TW), (2) the so-called Flying-V (FV) or multi-fuel blended wing body (MF-BWB). Both have similar aerodynamic characteristics and were developed by TU Delft 29 , 30 , 31 , 32 and (3) NASA’s N3-X (N3) blended wing body 33 , 34 . We find that the fuel consumption of a 2035 aircraft might be reduced between 18% and 22% compared to new 2015 aircraft and between 34% and 44% in 2050. Note that though far future technologies, i.e. in 2075 or later, are in principle of interest they do not significantly impact our results, since their diffusion into the fleet delays their impact and more importantly, the impact on global temperatures will mainly occur beyond 2100 due to the inertia of the atmosphere-ocean system in the order of decades. These findings result in 9 ECATS emission scenarios with 3 variants (TW, FV, N3) including a pessimistic base and an optimistic implementation, which differ by ±10%. The scenarios are developed consistently with the top-down scenarios following the same transport volume development and SAF usage as in the scenario CORSIA. Figure  5a presents the fuel use and NO x emissions relative to the year 2000, resulting in a roughly fivefold increase in fuel consumption by 2100 and a fourfold increase in NO x emission. The new technologies introduced from 2035 onwards lead to a reduction in fuel use and NO x emission around 2050, which is then offset by the further increase in transport volume, resulting in a slight increase in fuel use and NO x emission until 2100. This analysis shows that an emission pathway better than BAU might be feasible, but that the goals set by Flightpath 2050 are unlikely to be achieved. The fuel use and NO x emission from the FP2050 scenario (Fig.  1 ) are drastically lower than the range of our ECATS scenarios (Fig.  5 ).

a as in Fig.  1 , changes in fuel use (red) and NO x emissions (blue) taking into account a bottom-up analysis of aviation technologies; three far future technology pathways are taken into account, with a ± 10% uncertainty range, each leading to a scenario range; b Transport volume for the scenario taking into account a reduction of flight due to COVID-19 with three assumptions: a short recovery of 3 years (red); a longer recovery of 15 years (brown) and in addition to a long recovery a behavioural change after COVID-19 (yellow). c Resulting temperature changes as in Fig.  2 for the range of ECATS scenarios (green) and the BAU scenario for comparison (blue). d Resulting temperature changes from the 3 COVID-19 scenarios (red, brown, and yellow) in comparison to the BAU scenario (blue). The scenario BAU describes a business-as-usual future technological improvement and ECATS bottom-up estimates based on a group of experts from ECATS (Environmentally Compatible Air Transportation System).

The climate impact of the ECATS aviation scenario (Fig.  5c ) shows clearly a reduction compared to the BAU scenario. However, the stabilisation of the temperature, as it was found for the Flightpath 2050 scenarios, is not achieved. The ECATS scenarios fall in between the BAU and FP2050 scenarios. The absolute change in temperature and the contribution from individual climate drivers (Table  2 ) contribute to climate warming in 2100 from CO 2 of 33–37% and the effects from non-CO 2 emissions roughly equally shared between contrail-cirrus and NO x emissions.

Sensitivities to growth, global targets, sustainable fuels and technologies

The future evolution of the aviation system and the resulting impact on climate relies on too many variables to be predicted with one outcome. To tackle this problem, we present a range of scenarios. Those are based on either an analysis of climate impacts based on set emission targets, the five scenarios mentioned in Table  1 , which we call top-down scenarios, or an analysis of the climate impact of technological changes that can be expected in future aircraft, which we call the bottom-up scenarios (see ‘Method’). Both approaches define possible future pathways. Even though this approach includes a large range of uncertainties, we feel that such analysis should be an important part of the debate around the impact of aviation and the potential for change within the sector. A major uncertainty is the future demand for air travel. Here we present a scenario, which lies between the estimates from Boeing and Airbus (see above) other estimates from academia 23 , 24 which levels off in the future. In this sense, we present a more conservative estimate of the future climate impact of aviation as compared to industry forecasts. A variation of the future growth rates by ±50% on top of the general declining growth rate leads to a change of fuel usage in the scenario BAU of roughly ±20% in 2100 and a shift in the median surpass year of 3 years (Table  4 ). Demand-suppressing effects from the use of more expensive SAF might end up at about 10–15% reduction of demand by 2050 for an elasticity of −1 35 and a SAF price, at best, two times that of conventional kerosene 36 . Hence, our ‘−50% growth rate’ sensitivity simulation can be taken as an indicator for the impacts of such demand-suppressing effects, implying that the median year at which the temperature rise of 5% of 1.5 °C is surpassed will be delayed by a few years only. Most other scenarios lead to a similar shift in the median surpass year. A change in future efficiency improvements has in principle similar effects. The overall setting of the climate target and a shift from 5% to either 3.5% or 6.5% leads to a shift of the median surpass year in the order of one to two decades (Table  4 ). Sustainable aviation fuels are an important means in reducing the climate impact of aviation. However, according to CORSIA, whether a cap in net CO 2 is achieved by offsetting or the use of SAF has only a limited impact on the temperature evolution. And hence a reduction of the SAF availability by 50% leads to negligible changes in the distribution of the surpass years.

COVID-19 effects on aviation climate impact

The recent COVID-19 pandemic might question the discussed future aviation pathways we analysed so far. To better understand the possible implications of this pandemic on the climate impact of aviation, we altered the BAU scenario in a parametric way to assess three different pathways for the international recovery from the lock-down of nation states and the associated dramatic reduction in air travel, based on reported transport volumes and scenario projections 5 . We take into account a fast recovery of 3 years, a slow recovery of 15 years (C19-03, C19-15) and a change in habits due to experiences during the lock-down, for example, a shift towards web conferences instead of face-to-face meetings. Figure  5b shows a drop in RPK due to COVID-19 and the three recovery pathways. The respective, resultant temperature change (Fig.  5d ), however, is only significant if a sustained reduction in RPKs follows the crisis (yellow curve). Otherwise, the changes in 2020 due to COVID-19, as dramatic as they are for individuals and the global economy, only have a minor effect on the overall climate impact of aviation as long as a recovery follows. From the experience of other crises (e.g. SARS, 9-11, etc. see Fig.  1 ) we might expect a fast recovery. However, the consideration of which COVID-19 scenario is more likely is outside the scope of this study.

Top-down-scenario building

In the top-down scenario building, we combine top-level assumptions on the evolution of aviation (transport volume, technologies, SAF availability) with a detailed description of the air transport system for specific years. Details are given in the Supplementary Material as textual description and EXCEL sheet. Five scenarios are assessed, which all have some common characteristics (Table  1 ). They have identical evolution in transport volume, defined by the revenue passenger kilometres, which resemble ICAO data for the past (1971–2017) and are extrapolated to future with the assumption of a slow decrease in traffic growth rates in future. The observed increase rate in transport volume of roughly 6% per year in the decade 2008–2017 are reduced by to roughly 1% per year in 2050 following the results from the WeCare analysis and the Randers scenario. We employed the Randers scenario named 2052 that includes the temporal development of socio-economic factors, such as population and Gross-Domestic Product, for different world regions and is complemented by reasonable narratives and scientific evaluations. Within the WeCare project, it was combined with an air passenger demand model that calculates the demand between settlements. The resulting air traffic scenario shows lower estimates of the transportation volume for the coming two decades compared to the Airbus and Boeing forecasts. The resulting air traffic scenario is not based on an extrapolation of historical trends and manufacturer expectations but considers realistic assumptions for the socio-economic growth and an associated expected saturation around 2040. Details on the forecasting methodology developed and applied in WeCare can be found in Terekhov 37 and Ghosh 38 . Future fuel efficiency improvements are based on the ICAO’s environmental report 39 , with 1%/year in 2018 decreasing to 0.25% in 2100. These two assumptions lead to a fuel consumption of 823 Tg in 2050, which agrees well with the mean of the ICAO scenarios 39 . The geographical distribution follows the emission inventories developed within the WeCare project 22 . Two time horizons are taken, one for the recent past ( = 2012) and one representative for the future (2050), describing the geographical and vertical distribution of the emissions. All scenarios are identical between 1940 and 2018, and deviate afterwards, according to scenario assumptions, derived from the basic storylines. Thereby, we obtain 5 scenarios CurTec, BAU, CORSIA, FP2050, FP2050-cont (see main text and Table  1 ). The carbon-neutral growth from 2020 onwards in the CORSIA scenario is achieved by using a combination of sustainable aviation fuels (SAF) and emission offsets. Based on the EU-Renewable Energy Directive (RED-II), we assume an effective 65% net CO 2 reduction in SAF production and use compared to conventional kerosene in the year 2020. We assume a mix of different feedstocks, such as agricultural residues, algae, dedicated energy crops and also e-fuels (power-to-liquid), which enables an improvement of the overall CO 2 reduction potential to 80% in 2100. An analysis of the current growth rates and forecasts of the availability of SAF are used to optimistically estimate future availability of SAF and to allow a conservative estimate of the climate impact of the CORSIA scenario. Note that we have not explicitly considered any closed loop demand-supressing effects of increased costs 35 , such as SAF costs, since EUROCONTROL has indicated that these effects might be marginal 40 and there is a high degree of uncertainty in the prediction of these costs. Instead, we have addressed this sensitivity by changing the growth rates (see below) by ±50% as open loop scenarios, which would cover a number of changes in transport volumes including those arising from demand suppressing costs increases. These assumptions lead to a scenario where 1/3 of the fuel used in 2100 is assumed to be SAF. We consider two different pathways of achieving the Flightpath 2050 objectives, late and continuous (FP2050 and FP2050-cont). Both scenarios have the same transport volume as BAU and consider technological improvements by 2050, which are formulated as ‘CO 2 emissions per passenger kilometre have been reduced by 75%, NO x emissions by 90% and perceived noise by 65%, all relative to the year 2000.’ 41 .

In addition to these five main scenarios, we introduce three possible development pathways related to the COVID-19 pandemic by varying the timing and degree of recovery (see main text).

Bottom-up-scenario building

In the Bottom-up scenario building, we present possible different development pathways and analyse how those scenarios influence the contribution of future aviation to climate change. Evolutionary technology scenarios are developed by expert judgement (TU-Delft, Chalmers, DLR, TU-Hamburg) with comprehensive knowledge on the possible availability of advanced technologies in future aircraft programmes along with in-house tools and models for engine performance, aircraft design and aircraft performance (explained in detail within the Supplementary Material). We assess a broad spectrum of possible aircraft configurations, technologies, systems and procedures currently under research and development and evaluate their viability and provide best estimates on fuel consumption and NO x emission reduction potentials (Table  3 ). Comparing with the work by Schäfer et al. 42 , the improvement rates are quite similar when matching our 2035 single-aisle aircraft with the evolutionary year 2035 configuration presented by Schäfer et al. The reference used in our paper is more recent and is comparable to Schäfer’s ‘intermediate’ aircraft. They predict an 18% fuel burn reduction of the evolutionary aircraft over the intermediate aircraft, which is similar to that obtained in our analysis. In a similar approach, Hileman et al. 43 investigated at the US domestic market considering single-aisle aircraft, only. According to them, a double bubble fuselage design 44 with lower cruise speed would have 42% lower fuel consumption when compared to B737-800, which is an older generation of aircraft than the A320neo. However, it is less likely that the next generation of single-aisle aircraft will deviate from a tube and wing geometry.

In this work, the fuel efficiency and emission analysis are done for both single-aisle and twin-aisle aircraft market segments, as those two segments will account for about 95% of globally available seat kilometres. Single-aisle aircraft serving short and short-to-medium distance routes are responsible for 47% of the worldwide aviation fuel consumption. Single-and twin-aisle aircraft serving the medium and long-range routes are responsible for another 47% of the fuel consumption. Hence, differently to the top-down FP2050 scenarios, we analyse possible future technology developments and derive the expected fuel efficiencies and NO x emission evolutions in a bottom-up approach and combine that with the same overall scenario definition as for the top-down scenarios, e.g. with respect to transport volume.

We compute emission inventories based on global fleet forecast data developed in the WeCare project 22 for the years 2015–2070, in 5-year steps, for single-aisle and twin-aisle market segments. As a simplification, we assume that for each segment there is one representative aircraft type which can be used to model the entire market segment appropriately, while multiple aircraft generations are considered. The aircraft Airbus A320neo and A350 are selected as best of class for the current generation and serve as reference aircraft types for the single-aisle and twin-aisle markets, respectively. Entry into service year of the current generation is assumed to be around 2015. The next generations of single-aisle aircraft are assumed to be conventional tube-and-wing configurations entering into service in 2035 and 2050 with the fuel consumption and NO x emission improvement factors as shown in Table  3 relative to the reference aircraft. For the twin-aisle market, we estimate the next generation aircraft entering into service in 2035 being a tube-and-wing configuration. In 2050, three different options, viz. a conventional tube-and-wing widebody aircraft, an aerodynamically improved aircraft, the so-called Flying-V or multi-fuel blended wing body (MF-BWB) with an advanced turbofan engine, both developed by TU Delft, and NASA’s N3-X blended wing body with a turbo-electric propulsion system, are considered and used as possible twin-aisle aircraft configurations. For each of the years considered, the actual fleet composition is calculated considering a fleet diffusion of the new aircraft generations, i.e. introducing and partly replacing old aircraft. The market penetration of an aircraft generation is modelled as an S-curve applying the Bass diffusion model that has been calibrated to reach >95% market penetration within roughly 15 years, which is a typical diffusion time for new aircraft 45 , 46 , starting from their respective entry into service (EIS) [2015, 2035, 2050].

For the calculation of the reference emission inventories (those based on the reference aircraft types), we apply the GRIDLAB methodology developed in DLR 47 . In a next step, those inventories are multiplied with the improvement factors (CO 2 and H 2 O inventories scaled according to fuel improvement, NO x inventory scaled according to NO x improvement) to determine the emissions for the respective aircraft generations. Finally, for all years, the corresponding emission inventory is obtained by combining the inventories of the individual aircraft types and generations according to their market share.

Climate modelling

We use the non-linear climate-chemistry response model AirClim 25 , 26 to analyse the climate impact of the various scenarios. AirClim is a surrogate model, which relies on a multitude of pre-calculated responses to emissions with a global climate-chemistry model and has been verified against reference models to correctly simulate scenarios, such as flying lower or higher 26 . AirClim considers changes in concentration of CO 2 , water vapour, ozone, methane and the formation of contrail-cirrus, and takes their lifetimes, effects on the Earth radiation budget and eventually the changes in the near-surface temperature into account. The spatial resolution of the relation between emission location and response depends on the kind of effect and related atmospheric lifetimes. For CO 2 , with a very long atmospheric perturbation, the emission location is unimportant and hence CO 2 concentration changes are simulated in a box model. The relation between emission location and chemical concentration changes largely depends on the altitude and geographical location of the emission. The lifetime of aviation NO x and aviation ozone is in the order of several weeks and months, respectively 48 . Accordingly, chemical responses are dependent on emission altitude and latitude, whereas for short-term contrail-cirrus effects, the longitude is also taken into account. As a background atmosphere, we take the RCP2.6 scenario into account, assuming a world which tries to achieve the Paris Agreement. The effect of sustainable aviation fuel on contrail-cirrus properties is taken into account by utilising the results from Moore et al. 49 and Burkhardt et al. 16 : A linear scaling between SAF use and reduction of soot number particle emissions is assumed, taking into account the results from measurements, which indicate that a 50–50 blend reduces the number of emitted soot particulates by 50% 49 and the change in contrail-cirrus properties and lifetime changes the contrail-cirrus RF following the results of Burkhardt et al. 16 by parameterising their results in their Fig.  1f :

where \(\triangle {{RF}}^{{contr}}\) is the relative change in contrail-cirrus radiative forcing (dimensionless value between 0 and 1) and Δ pn the relative change in particle number emissions (dimensionless value between 0 and 1). Note that the formula is only valid for Δpn \(\ge 0.1\) .

The effect of SAF use on contrail-cirrus properties and lifetime changes are qualitatively in agreement with Caiazzo et al. 50 . The increase in RF when using SAF in comparison to a kerosene baseline as calculated by Caiazzo et al. 50 stems from the increase in the calculated potential contrail-cirrus coverage, which is caused in their calculations by the change in the Schmidt-Appleman criterion.

Monte–Carlo analysis

Uncertainties in climate impact estimates are quantified by using a Monte–Carlo Simulation. As indicated in Lee et al. 7 , 28 the climate impact of aviation emissions upon the atmosphere is associated with large uncertainties. The approach has been tested in Dahlmann et al. 26 and successfully applied to obtain a robust climate impact for the mitigation option Flying slower and lower 51 . Here we categorise the uncertainties into three groups following Dahlmann et al. 26 : (1) uncertainty in atmospheric residence time ( ± 20%), (2) strength of RF ( ± 5% for CO 2 , ± 10% for CH 4 , and ± 50% for H 2 O, O 3 (incl. PMO), and contrail-cirrus), (3) relation between RF and near-surface temperature change (climate sensitivity parameter; ±5% for CO 2 , ± 10% for CH 4 and contrail-cirrus, ±30% for H 2 O and O 3 (incl. PMO)). Hence, we consider 11 uncertainty parameters, which are drawn individually for each simulation. A total of 10,000 simulations are performed to assess the uncertainty ranges, which are displayed in Fig.  4 for the top-down scenarios. A total of 3400 simulations combined with nine different ECATS scenarios resulting in 30,600 simulations are utilised for the Monte–Carlo analysis employed in the ECATS scenarios.

Data availability

The scenario data and result data are available on Zenodo 10.5281/zenodo.4627860.

Code availability

The code for deriving the scenarios is given in an excel spreadsheet and available on Zenodo 10.5281/zenodo.4627860. The software code AirClim is confidential proprietary information of DLR. Therefore, the code cannot be made available to the public or the readers without any restrictions. Licensing of the code to third parties is conditioned upon the prior conclusion of a licensing agreement with DLR as licensor. The codes used for analysing the data and plotting the analysed data are available from the corresponding author upon reasonable request.

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Acknowledgements

The authors like to thank Dr. Christoph Kiemle for providing an internal review. The non-profit ECATS-Association IASBL (Environmentally Compatible Air Transportation System, http://www.ecats-network.eu/ ) promotes and supports its Members’ joint activities and interests in the field of aviation and environmental impact. Its higher-level aim is to help making aviation sustainable. This study was launched and performed by ECATS members.

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V.G. developed the paper idea, prepared the emission data excel sheet, and performed the AirClim simulations. A.G.R., T.G., C.X., F.L. and Jo.M. analysed the top-level objectives, gave advice on how to use them in the top-down emission calculation and developed the bottom-up scenario for technical improvements. Ja.M. and B.O. analysed the legislative objectives and advised on how to use them in the top-down emission calculation. S.B., S.C. and A.G.R. analysed the effects of SAF, their potential for future use, gave advice on how to use them in the top-down emission calculation and developed the SAF part of the bottom-up scenario. K.D. and S.M. supported the AirClim simulations and interpretation.

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Grewe, V., Gangoli Rao, A., Grönstedt, T. et al. Evaluating the climate impact of aviation emission scenarios towards the Paris agreement including COVID-19 effects. Nat Commun 12 , 3841 (2021). https://doi.org/10.1038/s41467-021-24091-y

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Flying a plane through windy, stormy conditions can cause a dangerous, turbulent ride. But a clear blue sky does not guarantee smooth cruising.

Williams: “Clear air turbulence is completely invisible, as its name suggests, so it’s nothing to do with flying through a storm. … It comes out of the blue, often without warning.”

And Paul Williams , a professor of atmospheric science at the University of Reading in the UK, says it’s getting more common.

He explains that clear air turbulence is caused by instability in the jet stream, which is increasing as the atmosphere warms.

In a recent study , his team found that over the past 40 years, severe clear air turbulence over the North Atlantic increased by more than 50%.

Scientists expect that this type of turbulence will grow even more common as global warming continues.

Williams says people who fly do not need to be scared, but they should take precautions to stay safe.

Williams: “It’s really, really, really rare for someone seat-belted to be injured in a turbulence encounter. It almost never happens.”

So whether the seat belt sign is on or off, Williams recommends that travelers buckle up as flying gets bumpier in a warming world.

Reporting credit: Sarah Kennedy / ChavoBart Digital Media

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Air travel is a huge contributor to climate change. A new global movement wants you to be ashamed to fly.

Greta Thunberg gave up flights to fight climate change. Should you?

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It was 2007, during a trip to visit her sister in Norway’s pristine Lofoten Islands , when Maja Rosén had an unsettling thought.

As she took in the breathtaking archipelago north of the Arctic Circle that is dotted with mountains, carved with fjords, and circled by sea eagles, she remembered she was looking at one of the fastest-warming regions of the planet.

And she realized that how she got there was part of the problem.

She’d carpooled with friends to Oslo from her home in Gothenburg, Sweden. The final leg was a short boat ride to the islands. And in between was a 500-mile flight from Oslo to Bodø.

For the distance, short flights produce a larger amount of greenhouse gas emissions per passenger compared to longer routes. That fact wasn’t something that struck her on her previous jaunts, like her flights to the United Kingdom to visit friends.

But upon basking in the fragile and sublime wonders of Lofoten, Rosén began to consider how her own actions might be threatening the region. The contradiction between her admiration for the scenery and her pollution from getting there, she decided, was too much to bear.

“It felt so wrong that my flight there was contributing to destroying that place,” Rosén, now 38, said. Soon after, she drastically curbed her flying, but in 2008, she concluded it wasn’t enough. “That’s when I decided not to fly again, and I have not regretted that decision,” she said.

Rosén has only become more alarmed and more determined to reduce emissions from air travel since then. Last year, she gave up her spot in medical school to focus on convincing other people to join her.

She founded a group called We Stay on the Ground in 2018 to recruit people to pledge to give up flying for one year. But the pledge only kicks in once 100,000 people in a given country have committed to doing the same. The threshold is a way to show participants that they’re not alone.

“For most people, it’s to know that others have made this decision. That’s really the most powerful way to make people change their minds,” Rosén said. So far, more than 8,000 people around the world have made the pledge.

Her effort may now be getting a boost from another Swede, 16-year-old climate change activist Greta Thunberg . She gained recognition when she went on strike from school last year to protest her government’s inaction on climate change, sparking a series of worldwide demonstrations, most recently the September 20 strike that drew an estimated 4 million people around the world.

But even after becoming a global celebrity, Thunberg has led by example, traveling to events around Europe mainly by train . She’s currently sailing from the US to Portugal to attend the UN climate meeting in Madrid in December.

Some Swedish airports have now reported a decline in travelers , which some activists attribute to the “ Greta effect ,” a newfound awareness of humanity’s impacts on the planet and a desire to make a difference.

The Swedes have even coined a word for the shame that travelers are beginning to feel about flying: flygskam , pronounced “fleeg-skahm.”

Rosén is trying to use flygskam to her advantage. She resolved last year to swallow her squeamishness about making her friends reckon with their own travel “because I sort of got fed up with being more scared of being socially inconvenient than climate collapse,” she said.

It’s not just Sweden; environmental activists, scientists who study the climate, and ordinary people in other countries like Switzerland , the United Kingdom , Germany , and the United States are curbing their air travel, if not giving it up outright.

However, the growing global alarm about the environmental impacts of aviation comes as air travel continues to rise. A record 31.6 million passengers are expected travel on US airlines this week for the Thanksgiving holiday, CNN reported . Our global economy is tightly interwoven with aviation as it carries goods and facilitates commerce. Leisure flights are also increasing, and growing demand for services like two-day and overnight shipping has led some companies like Amazon to invest more in cargo aircraft.

All this demand is expected to soar higher, particularly as prices for flights decline and wealth grows in emerging economies.

For regular flyers, air travel is often the dominant contributor to their greenhouse gas footprints. With the window rapidly closing to limit global warming to a bearable level — scientists warn that the planet has as little as 12 years to halve global emissions to restrict warming to 1.5 degrees this century — it is more critical than ever to find a way to shrink aviation’s carbon footprint. Every bit of carbon dioxide we emit now will linger in the atmosphere and warm the planet for decades, but completely decarbonizing aircraft will likely require technologies that are decades away. Reducing the number of flights is one of the few surefire ways to curb emissions in the meantime.

But unlike many other activities that contribute to climate change, air travel serves a valuable social function. It gives remote towns a lifeline to critical fuels, food, and medicines. It helps families stay connected across continents. It opens the door to life-changing experiences.

So reducing air travel demands a difficult moral reckoning, even if we make the decision solely for ourselves. But activists like Rosén say these actions have consequences for the whole world, so we cannot afford to make them without forethought.

Flying’s growing effect on the environment

If you’re a regular flyer, odds are that your biggest single source of greenhouse gas emissions each year is air travel. It likely dwarfs the footprint of all the lights in your home, your commute to work, your hobbies, and maybe even your diet.

“Euro for euro, hour for hour, flying is the quickest and cheapest way to warm the planet,” said Andrew Murphy, aviation manager at Transport & Environment, a think tank in Brussels.

That’s alarming because humanity can only emit so much more carbon dioxide to limit warming this century to 1.5 degrees Celsius, the more ambitious goal under the 2015 Paris climate agreement. An international team of researchers last year reported that meeting this target would require halving global emissions by as soon as 2030 , reaching net-zero emissions by 2050, and even getting to negative emissions thereafter.

Right now, the world is flying in the opposite direction. Global emissions reached a record high last year, and so did atmospheric concentrations of carbon dioxide .

Air travel is a big reason why. A one-way flight across the Atlantic from New York City to London emits one ton of carbon dioxide per passenger. There are upward of 2,500 flights over the North Atlantic every day.

And that’s just one air corridor. Around the world, aviation emits about 860 million metric tons of carbon dioxide every year, or about 2 percent of total global greenhouse gas emissions. Those numbers are poised to soar. The International Civil Aviation Organization projects that emissions from air travel will grow between 300 and 700 percent by 2050 compared to 2005 levels.

Those emissions in turn stand to have a devastating impact. The planet has already warmed by 1 degree Celsius since the dawn of the Industrial Revolution, which has caused rising seas and more frequent and intense heat waves. Every metric ton of carbon dioxide emitted leads to 3 square meters of Arctic sea ice loss . Aircraft also emit several other pollutants at altitude, like particulates, sulfur compounds, and nitrogen compounds, which have an additional warming effect. In some parts of the Arctic under busy air routes, these pollutants combined contribute one-fifth of the warming .

So the environmental costs of air travel are huge and growing, and the worst impacts will befall future generations. At the same time, there are very few options to limit those emissions except to not fly.

But that’s if you fly to begin with. In the United States, fewer than half of travelers in 2017 took a trip by air, according to an industry survey . Globally, less than one-fifth of the population has ever buckled in for a flight. That means a minority of frequent flyers contribute a disproportionate share of emissions. So reducing air travel is one of the most effective things individuals can do to shrink their carbon footprints.

Why flying is such a challenge for the environment

The fundamental problem behind decarbonizing air travel is the physics. To fly, you need an energy source that crams a lot of power into a small space, and right now, there is nothing as energy-dense as jet fuel , which has a specific energy of 11,890 watt-hours per kilogram.

Batteries aren’t even in the same airport. The best lithium-ion batteries top out at 265 watt-hours per kilogram , which is nowhere near enough get an airliner across the Pacific. The technology is improving, but one estimate shows that electrification of airliners will only start to make a dent in air travel emissions by midcentury.

At the same time, there is very little room left for making air travel more efficient. The current generation of jet engines is already closing in on its maximum efficiency. Fuel is also often the largest single expense for airlines, so they already face intense pressure to go farther with less.

One strategy to deal with aircraft emissions is to purchase credits or offsets. Many websites will calculate the emissions of your flight and sell you means to offset them, whether through planting trees that take up a given quantity of carbon dioxide or financing renewable energy projects to displace fossil fuels. But these offsetting programs are only as good as the accounting behind them, and for some, their effectiveness so far in limiting greenhouse gas emissions is questionable .

“The research shows that three-quarters of the offsets don’t deliver the reductions they claim to deliver,” said Anja Kollmuss, a policy analyst in Zurich who studies emissions trading.

Another option is to use a carbon-neutral fuel. Airlines are experimenting with biofuels derived from plants. Since plants recycle carbon that’s already in the atmosphere rather than introducing new carbon into the air, in theory, fuels derived from these crops have no net effect on the climate. In practice, it can be tricky to manage the energy balance of growing biofuels such that you aren’t expending more energy than you get out of them. Fuel crops also require land, and it’s not clear where all the land needed to sustain a wholesale shift of the global aviation industry will come from. Right now, biofuels are also expensive.

Yet another possibility is electrofuels . That’s where you use electricity to power a mechanism that stitches carbon dioxide from the air into longer molecules that can serve as fuels. However, it requires gobs of zero-emissions energy, and the technology is still in a gestational phase.

While there may be technology solutions for cutting the emissions for aviation in the future, there are few options available today beyond simply flying less. “We see this as individuals taking this into their own hands after governments have failed to act,” Murphy said.

Shorter flights have a disproportionately large carbon footprint

It takes a lot of energy to get a fully loaded airliner 6 miles into the air. On short flights, upward of 25 percent of the fuel used is consumed during takeoff.

Once at cruising altitude, though, the aircraft becomes much more fuel-efficient. That means longer, direct journeys have a smaller carbon footprint than shorter connecting hops. But only to a point.

For extremely long hauls, the extra fuel needed for the journey adds enough weight that the flight’s fuel efficiency is reduced, thereby increasing its carbon footprint per mile.

Depending on the aircraft and the route, there is an optimal distance for an air route that minimizes carbon dioxide emissions per passenger per mile — it follows a bathtub curve. One estimate from the Worldwatch Institute pegged the most fuel-efficient flight length at 2,600 miles, a bit longer than the distance between New York and Los Angeles.

But short-haul flights are increasing as countries like China, India, and Brazil open new routes to accommodate a voracious demand for domestic air travel.

Flying first-class also carries a larger carbon footprint, upward of three times larger than passengers in coach — partly because first-class seats are heavier and take up more floor space than cheaper sections of the aircraft.

A worldwide movement is growing. Sweden is its current epicenter.

Sweden is a somewhat odd place to emerge as the leader in flying shame and staying on the ground: It’s not the country with the most air travel or the highest per capita emissions . But in recent years, Swedish celebrities started pushing the idea into the mainstream. In 2015, Swedish Olympic biathlon gold medalist Björn Ferry committed to stop flying. Then in the fall of 2017, 10 Swedish celebrities published an article about deciding to no longer fly.

In 2018, the Swedish government began debating a tax on flying, and more national celebrities began to weigh in against air travel; the renowned Swedish writer Jens Liljestrand published a well-read article with the memorable title “I’m fed up with showing my child a dying world.”

The potential impacts of climate change also became startlingly vivid to many Swedes last year as an oppressive heat wave baked the country and dried out its forests. That heat helped fuel wildfires, with several igniting north of the Arctic Circle .

“It’s the first time Swedish people felt the consequences of climate change themselves,” Rosén said. “[L]ast summer was so dry and things were just looking yellow, and we were lacking water.”

Then in August 2018, Thunberg began her strike outside the Swedish parliament building, an action that soon launched her message worldwide.

Birgitta Frejhagen, 76, was so inspired by Thunberg that she founded a group called “Gretas Gamlingar” (Greta’s oldies). Her goal is to encourage older people to get involved in climate activism. She is currently aiming to recruit 10,000 Swedish seniors to participate in the World Action Day for the Climate on September 27 to coincide with a global youth climate strike .

Frejhagen noted that despite the alarm about the climate, flying is hard for Swedes like her to avoid. Many have family spread out over the large, sparsely populated country. Frejhagen broke her hip earlier this year, so long train or bus journeys are a painful ordeal.

“There is a shame of flying, but sometimes you have to fly,” she said.

Rosén said there isn’t anything unique in the Swedish soul that has made so many across the country so concerned about flying. “This could have happened anywhere,” she said. “We’ve had some good coincidences that have worked together to create this discussion.”

Nonetheless, the movement to reduce flying has created a subculture in Sweden, complete with its own hashtags on social media. Beyond flygskam, there’s flygfritt (flight free), and vi stannar på marken (we stay on the ground).

Rosén said that judging by all the organizing she’s seen in other countries, she thinks Sweden won’t long hold the lead in forgoing flying. “I wouldn’t be surprised if the Germans would follow us soon,” she said.

Scientists are having a hard time overlooking their own air travel emissions

Kim Cobb, a climate scientist at the Georgia Institute of Technology, has curbed her air travel by 75 percent.

“I really started thinking about my carbon footprint after Trump was elected,” she said. “Doing my climate science and donating to the right candidates was never going to be enough, even if you took that to scale.”

She created a spreadsheet to track her personal carbon footprint and found that flying formed the dominant share of her emissions. “By the end of 2017, 85 percent of my carbon footprint was related to flying,” she said.

Much of Cobb’s research — examining geochemical signals in coral to reconstruct historical climate variability — required her to travel to field sites in the equatorial Pacific.

While she doesn’t anticipate giving up those visits entirely, Cobb has taken on more research projects closer to home, including an experiment tracking sea level rise in Georgia. She has drastically reduced her attendance at academic conferences and this year plans to give a keynote address remotely for an event in Sydney.

I have begun replying to invitations “Due to the climate emergency, I am cutting down on air travel ...” Have been pleasantly surprised how many take up my offer of pre-recorded talk & Skype Q&A’s @GreenUCL @UCLPALS @UCLBehaveChange https://t.co/Hlxc4R6Lj3 — Susan Michie (@SusanMichie) June 29, 2019

Cobb is just one of a growing number of academics , particularly those who study the earth, who have made efforts in recent years to cut their air travel.

While she doesn’t anticipate making a dent in the 2.6 million pounds per second of greenhouse gases that all of humanity emits, Cobb said her goal is to send a signal to airlines and policymakers that there is a demand for cleaner aviation.

But she noted that her family is spread out across the country and that her husband’s family lives in Italy. She wants her children to stay close to her relatives, and that’s harder to do without visiting them. “The personal calculus is much, much harder,” she said.

She also acknowledged that it might be harder for other researchers to follow in her footsteps, particularly those just starting out. As a world-renowned climate scientist with tenure at her university, Cobb said she has the clout to turn down conference invitations or request video conferences. Younger scientists still building their careers may need in-person meetings and events to make a name for themselves. So she sees it as her responsibility to be careful with her air travel. “People like me have to be even more choosy,” she said.

Activists and diplomats who work on international climate issues are also struggling to reconcile their travel habits with their worries about warming. There is even a crowdfunding campaign for activists in Europe to sail to the United Nations climate conference in Chile later this year.

But perhaps the most difficult aspect of limiting air travel is the issue of justice. A minority of individuals, companies, and countries have contributed to the bulk of greenhouse gas emissions from flights and profited handsomely from it. Is it now fair to ask a new generation of travelers to fly less too?

Airlines and climate-concerned travelers

At least one airline is beginning to acknowledge the concern around flying. KLM CEO Pieter Elbers wrote in a letter in June that “we invite all air travellers to make responsible decisions about flying.” The letter showed no sign of the airline itself changing its ways, but the fact that KLM was even hinting at shaming its own passengers shows that climate concerns are difficult to ignore.

Cultural changes could become a big part of reshaping demand for air travel. Shifting tastes away from impressing friends with distant, Instagram-perfect destinations and more staycations could eventually yield some reductions in greenhouse gases from aircraft.

Transport & Environment’s Murphy also noted that for a long time, aviation fuels in many countries weren’t taxed, nor were their greenhouse gas emissions, so the aviation sector hasn’t faced the same pressure to decarbonize as the automotive industry. In fact, many countries directly and indirectly subsidize air travel, whether through tax breaks for aircraft manufacturers or government ownership of airlines. While this is slowly changing — France is set to introduce a new tax on airlines, for example — much more drastic policy action is needed to curb emissions from air travel.

However, targeting the consumers of goods and services rather than just their producers is a much more fraught political debate. It’s a more direct way of changing behavior and it shifts some of the costs directly to buyers, making the costs of curbing emissions much more visible, and contentious. Cutting consumption also brings up concerns about justice. Many activists argue that the heaviest burden of fighting climate change should be borne by large institutions rather than individuals. So while some airlines would prefer to embarrass their customers, climate campaigners say it’s the airlines themselves that should feel most ashamed.

Should you, dear traveler, feel ashamed to fly?

“Travel is fatal to prejudice, bigotry and narrow-mindedness, and many of our people need it sorely on these accounts,” wrote Mark Twain in The Innocents Abroad . “Broad, wholesome, charitable views of men and things can not be acquired by vegetating in one little corner of the earth all one’s lifetime.”

Air travel has yielded immense benefits to humanity. Movement is the story of human civilization, and as mobility has increased, so too has prosperity . Airplanes, the fastest way to cross continents and oceans, have facilitated this. And while some countries have recently retreated from the world stage amid nationalist fervor , the ease of air travel has created a strong countercurrent of travelers looking to learn from other cultures.

Compared to other personal concessions for the sake of the environment, reducing air travel has a disproportionately high social cost. Give up meat and you eat from a different menu. Give up flying and you may never see some members of your family again.

So it’s hard to make a categorical judgment about who should fly and under what circumstances.

But if you’re weighing a plane ticket for yourself, Paul Thompson, a professor of philosophy who studies environmental ethics at Michigan State University, said there are several factors to consider.

No need to tell me about your feelings of guilt. I see no reason for you to feel guilty. You already excel at ethical thinking in many other areas of your life and relationships. Judge for yourself what the times require of you, personally and politically. Act or don't act. — flyingless (@flyingless) July 17, 2019

First, think about where you can have the most meaningful impact on climate change as an individual — and it might not be changing how you are personally getting around. If advocacy is your thing, you could push for more research and development in cleaner aviation, building high-speed rail systems, or pricing the greenhouse gas emissions of dirty fuels. “That’s the first thing that I think I would be focused on, as opposed to things that would necessarily discourage air travel,” Thompson said. Voting for leaders who make fighting climate change a priority would also help.

If you end up on a booking site, think about why you’re flying and if your flight could be replaced with a video call.

Next, consider what method of travel has the smallest impact on the world, within your budget and time constraints. If you are hoping to come up with a numerical threshold, be aware that the math can get tricky. Online carbon footprint calculators can help.

And if you do choose to fly and feel shame about it, well, it can be a good thing. “I think it’s actually appropriate to have some sense of either grieving or at least concern about the loss you experience that way,” Thompson said. Thinking carefully about the trade-offs you’re making can push you toward many actions that are more beneficial for the climate, whether that’s flying less, offsetting emissions, or advocating for more aggressive climate policies.

Nonetheless, shame is not a great feeling, and it’s hard to convince people they need more of it. But Rosén says forgoing flying is a point of pride, and she’s optimistic that the movement to stay grounded will continue to take off.

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Reducing air travel by small amounts each year could level off the climate impact

Postdoctoral Researcher in Weather and Climate Modelling, University of Oxford

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Milan Klöwer receives funding from the UK's Natural Environmental Research Council, the Copernicus Programme of the European Commission and the European Research Council.

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Just before the pandemic, aircraft engines were burning one billion litres of fuel a day. But then the number of daily civil aviation flights fell from 110,000 to less than 50,000 during 2020, on average. With the easing of travel restrictions, air traffic is increasing back towards its pre-pandemic peak.

Most world leaders and delegates will have flown to Glasgow to attend COP26 – the 26th annual UN climate change summit – in person. But as they haggle over emissions targets to limit global warming to 1.5°C, and not 3°C or more , aviation is unlikely to be included in them, given the lack of low-carbon alternatives to long-haul flights.

But it should be. In new research , my colleagues and I calculated that if the aviation sector continues to grow on its present trajectory, its jet fuel consumption will have added 0.1˚C to global warming by 2050 – half of it to date, the other half in the next three decades.

Aviation is responsible for 4% of the 1.2°C rise in the global mean temperature we have already experienced since the industrial revolution. Without action to reduce flights, the sector will account for 17% of the remaining 0.3°C left in the 1.5°C temperature target, and 6% of the 0.8°C left to stay within 2°C. Airlines effectively add more to global warming than most countries.

Warming footprints

At the current rate, the world will have warmed by 2°C within three decades . To quantify how different activities contribute to warming, scientists measure carbon emissions. This is because how much the Earth warms is proportional to cumulative carbon emissions in the atmosphere. This is a very good approximation in many cases, but it is inaccurate for emissions caused by aeroplanes travelling at altitudes of up to 12 kilometres.

As well as CO₂, aircraft engines emit nitrogen oxides, water vapour, sulphur and soot, causing contrail cirrus clouds and other complicated chemical reactions in the atmosphere. The sum of these so-called non-CO₂ effects adds more warming on top of the CO₂ emissions. So the total warming footprint of aviation is between two and three times higher than a conventional carbon footprint.

While a large share of a flight’s CO₂ emissions remain in the atmosphere for many thousands of years, the non-CO₂ effects diminish over time, vanishing within about ten years . So any growth in aviation, measured in global jet fuel consumption, has an amplified impact as both CO₂ and non-CO₂ effects add up.

But a decline in aviation can partly reverse some warming, as the non-CO₂ effects disappear over time until only the CO₂ effects remain. Think of the non-CO₂ effects like a bathtub – it fills up when the taps are turned further and further, despite a slow outflow down the plughole. But the same bathtub will eventually empty if the taps are gradually turned down.

The non-CO₂ effects of flights on the atmosphere will slowly disappear if fewer and fewer flights are taken, so that aviation’s contribution to warming eventually levels off. In that situation, the increase from continued CO₂ emissions would balance the fall in non-CO₂ effects, and although aviation would still contribute to climate change, the total warming from both would remain constant over time. How much would aviation need to shrink to level off its influence on global warming?

Our calculations show that flying does not need to stop immediately to prevent aviation’s contribution to global warming expanding. Flying has already caused 0.04°C of warming to date. But with a yearly decrease of 2.5% in jet fuel consumption, currently only achievable with cuts in air traffic, this warming will level off at a constant level over the coming decades.

When do we really need to fly?

COVID-19 had a huge impact on the aviation sector. Air traffic is still approximately 10-20% below pre-pandemic levels, but is rebounding quickly . Politicians should shift subsidies from flying to more sustainable modes of transport, such as train journeys. And there is much more that can be done.

Lockdowns and the shift to remote working made many people rethink the necessity of flying. People resolving to fly less can contribute considerably to reducing the number of unnecessary flights. Combining in-person and virtual attendance in hybrid meetings wherever possible is a great way to support that shift.

Reducing the space that business classes take on aeroplanes is another way to cut the number of flights, as it allows more passengers to travel on one flight.

Not allowing airport expansions could also have a big impact. The UK’s Climate Change Committee, an expert body which advises the UK government, has recommended not expanding airports to align the sector with climate targets. Yet the expansion of Heathrow airport is currently planned to go ahead .

Sustainable aviation fuels, and hydrogen or electric planes, are being developed, but none of these technologies are currently available at the necessary scale. At the moment, there is little chance of the aviation industry meeting any climate targets if it aims for a return to its pre-pandemic rate of growth.

This story is part of The Conversation’s coverage on COP26, the Glasgow climate conference, by experts from around the world. Amid a rising tide of climate news and stories, The Conversation is here to clear the air and make sure you get information you can trust. More.

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Climate Scientists Take Their Closest Look Yet at the Warming Impact of Aviation Emissions

A new study reaffirms that contrail clouds produce more global warming than carbon dioxide from flights, a finding that could help reduce emissions from air travel..

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An international team of prominent scientists has published what they say is the most comprehensive study to date calculating the complex climate impact of aviation emissions, reaffirming that contrail clouds produce more warming than carbon dioxide. 

This kind of comprehensive analysis has only been performed a handful of times, starting with a report from the Intergovernmental Panel on Climate Change (IPCC) in 1999. The new study, looking at data from 2000 to 2018, notes that while the understanding of the impacts of aviation emissions “remains incomplete,” a series of new calculations considered “factors not previously applied in a common framework.”

“We wanted to produce a very high-quality benchmark assessment,” said David Lee , a climate scientist at Manchester Metropolitan University, who led the study and was also part of the IPCC assessment.

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The study , which had been in the works since 2015, looked at both carbon dioxide and several types of “non-CO2” emissions in aviation. Carbon dioxide emissions are fairly well understood at this point, Lee said, but the impacts of non-CO2 emissions, which the study found account for about two-thirds of the net warming effect, are considerably harder to calculate.

The primary non-CO2 impact results from the emission of nitrogen oxides, water vapor and soot that can create heat-trapping contrail clouds. They form as emissions of hot gases and soot from aircraft engines activate water particles that freeze, producing the contrails, those straight, wispy white markings of a plane’s path through the sky.   

Other non-CO2 emissions involve what the study calls “aviation aerosols”—small particles composed of black and organic carbon known as soot, sulfur and nitrogen compounds. 

The team measured the “radiative forcing” of each item—a measure of how much potential it has to exert a change on the global climate.

Over the last decade the science around these emissions has improved drastically, Lee said, but he added that the better science doesn’t always mean the uncertainties will narrow. “We find out other stuff that we didn’t know before,” he said.

The climate impact of contrails has been a topic of contention in recent years. Contrails form in certain atmospheric conditions when the water vapors from airplane engines condense and freeze in the air—or when soot particles allow water in the atmosphere to condense around them—creating artificial clouds. The trails can become cirrus, or curling, clouds and hang around for hours, trapping atmospheric heat which contributes to global warming. 

Of all the emissions measured, the researchers found these contrails to be the most impactful, but there’s a caveat.

“Contrail cirrus forcing is not as powerful as we used to think it was,” Lee said. The contrails appeared to be less than half as effective as previous estimates found. Carbon dioxide emissions, meanwhile, were the second most impactful, with roughly 60 percent as much effect on the climate as the vapor trails.

Measuring the effects of nitrogen oxides was tricky because they both eliminate atmospheric methane, which has a cooling effect, but create ozone which can act as a greenhouse gas. Between these two mechanisms, the researchers estimate that nitrogen oxides have a net warming effect that is about 30 percent as powerful as the vapor trails. The other emissions didn’t appear to have strong effects.

Even though contrails showed less effect than previous estimates, their impact is still considerable, given that warming from the contrails is greater than from carbon dioxide—and that the carbon dioxide emissions alone from the airline industry equal 2.4 percent of global CO2 emissions, according to a 2019 study by the International Council on Clean Transportation. 

The magnitude of these emissions is also growing quickly. The International Civil Aviation Organization estimated pre-Covid-19 pandemic that demand for air travel would grow 4.3 percent annually for the next two decades.

“The airlines did not dispute that there was an impact of CO2 on the atmosphere,” said Annie Petsonk, the international counsel at the Environmental Defense Fund, who was not involved in the study. But until now, she said, they have claimed the science isn’t in on non-CO2 airline emissions. 

This paper, in filling that knowledge gap, deprives airlines of excuses to avoid dealing with non-CO2 emissions, said Petsonk. While the European Union has voluntarily adopted stricter standards for aviation emissions, the United Nations body that governs international aviation standards recently adjusted an international agreement after airlines asked for reprieve, citing lost revenue because of Covid-19. 

“The airlines are in the midst of a Covid crisis, which has hit them with a gut punch and they’re trying to get back on their feet. If they fail to put the climate crisis central to their rebuilding, then their efforts… will fail,” Petsonk said.

She said that getting aviation “on a trajectory for net zero climate” needs to be at the core of the industry’s recovery efforts from Covid-19 setbacks.

Leto Sapunar

Leto Sapunar is a science journalist and graduate student at NYU’s Science Health and Environmental Reporting Program. From Oregon, he studied physics at Oregon State University and taught astronomy in California prior to moving to New York. He interned with Retraction Watch where he covered science accountability, and has written for Scientific American, Medscape Medical News, and Alpinist. In his free time he enjoys rock climbing, science fiction and tea.

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Yearning to fly

We all know the benefits of travel — but a substantial increase in flying seems unsustainable at a time when aviation’s share of emissions is set to rise and most of the world has pledged to keep global warming at 1.5 C.

From “PPE” to “flattening the curve,” the COVID-19 era has added a host of new phrases to our popular lexicon.

One of the most perplexing has to be “revenge travel,” which speaks to the zealous need many of us have to experience sunnier climes after a year or two in a pandemic holding pattern.

No one is quite sure who minted the expression; some people in the tourism business find it a bit distasteful . But it was on full display this past summer, as the mortal risk of COVID-19 subsided and travel returned in force. Airports around the world — including, infamously , Toronto’s Pearson — buckled under the strain.

While the episode produced endless accounts of flight delays and estranged luggage , it showed that, pandemic or no pandemic, we have an unrelenting desire to travel. Just look at the backlog of passport applications, which became a minor scandal for the Canadian government.

One surprising effect of the lockdowns in the early days of the pandemic was a marked drop in carbon emissions, including from aviation. This, combined with a new reliance on video-conferencing, led to “a realization among many of us that there are alternatives to travelling long distances by air to do business,” said Daniel Bresette, executive director at the Environmental and Energy Study Institute (EESI) in Washington, D.C.

But that hasn’t dimmed the desire to fly for fun.

“There is a segment of the population that really feels that COVID set them back in their aspirations for visiting places or catching up on holidays and seeing family,” said Adeniyi Asiyanbi, an assistant professor of geography at the University of British Columbia Okanagan who studies forest-based climate action. “And the last thing they want to think about is precisely emissions.”

Many observers say the current growth trajectory is unrealistic — and that the aviation industry isn’t being frank about it.

The subtext of revenge travel is that getting on a plane to see the world is our right. And the tourism industry, battered by a couple of years of severe turbulence, is happy to accommodate.

To give a sense of just how much we fly, there were nearly 39 million flights worldwide in 2019 ; that was up from 25.9 million in 2009.

The Environmental and Energy Study Institute estimates that global air travel traffic is at about two-thirds of pre-pandemic levels, but projects that North America will return to pre-pandemic levels this year or next, with international markets likely to catch up a couple of years later. Thereafter, EESI expects air passenger traffic to grow by about three to four per cent per year.

We all know the benefits of travel — it allows us to marvel at Earth’s riveting beauty and gain a finer understanding of other cultures. But a substantial increase in flying seems unsustainable at a time when aviation’s share of emissions is set to rise and most of the world has pledged to keep global warming at 1.5 C below pre-industrial temperatures.

This summer alone, we’ve seen drought and forest fires in Europe and devastating flooding in Pakistan, which climate scientists see as linked to climate change — and that’s at (about) 1.2 C of warming.

Many observers, including some who have worked in aviation, like former pilot Todd Smith, say the current growth trajectory is unrealistic — and that the industry isn’t being frank about it.

Said Smith, “There’s a hesitancy to be honest and objective about this.”

Taking to the skies

Seeing the scenes of anguish at airports in recent months, one could forget that air travel used to carry a certain romance.

Commercial flight dates back to about 1919, but it didn’t really become popular until the 1940s.

“After the Second World War, you’re really into the age of mass tourism,” said Emily Thomas, author of The Meaning of Travel: Philosophers Abroad .

The notion of jet-setting grew during the so-called golden age of flying from the ‘50s to the ‘70s, when Western economies boomed and boarding a plane held the promise of fine dining and impeccable service. While the images of stewardesses for airlines like Pan Am now feel jarringly sexist and retrograde, the flying experience had an undeniable glamour.

Corporate cost-cutting in the intervening years has eliminated some of the perks and made flying more mundane, but that hasn’t hurt demand. In fact, while worldwide air travel has been on an upward trajectory since 1945, the trend greatly accelerated after 2010 — before being brought down to Earth, so to speak, by the COVID-19 pandemic.

What gets lost in the fervour to fly is just how few people have the wherewithal to do it. In 2017, Dennis Muilenburg, then-CEO of Boeing, told CNBC that less than 20 per cent of the world had ever been on a plane . Muilenburg framed it as a growth opportunity.

Most of that growth in the coming years is likely to come from Asia — thanks largely to a growing middle class in China, Vietnam and the Philippines.

While more people can afford to fly, the question is whether the planet can afford to accommodate more flying — at least with existing jet propulsion technology. Aviation currently accounts for about two per cent of global carbon emissions; as a point of comparison, electricity generation is about 40 per cent and road transport about 20 per cent.

Aviation’s share may not seem that big, but if it were a country, it would be producing more emissions than Germany.

EESI notes that while other sectors may exceed the environmental impact of flying, “passenger air travel was producing the highest and fastest growth of individual emissions before the pandemic.”

A stubborn emissions problem

Thanks to technological advances in renewable energy and battery storage, there has been significant progress in decarbonizing sectors like electricity and ground transportation. But keeping an airplane aloft with something other than a conventional fossil fuel is a different proposition.

Jet fuel is a mix of hydrocarbons, although it is largely based on kerosene. What comes out of an airplane’s exhaust is about 70 per cent carbon dioxide (CO2), but there’s other pollution as well, including soot, sulphates, nitrous oxides and contrails (water vapour). Contrails stay in the atmosphere for a limited time, but under certain conditions, trap infrared rays and produce a warming effect up to three times that of CO2.

The industry has long teased the possibility of cleaner travel, but a couple of developments increased the urgency to act. One was the release of the 2018 report from the UN’s Intergovernmental Panel on Climate Change (IPCC), which said that we had 12 years to enact significant climate action or face “catastrophic” effects.

The other was Swedish climate activist Greta Thunberg, who declared “our house is on fire” at the World Economic Forum in January 2019 and popularized the phrase flygskam (“flight shame”).

Rising public concern about emissions prompted the leading aerospace manufacturers — Airbus, Boeing, Dassault, General Electric, Rolls-Royce, Safran and United Technologies — to issue a rare shared statement at the Paris Airshow in June 2019. In it, they laid out the main planks of their collective climate action plan: reducing emissions by continuing to look for efficiencies in operations; expanding the use of sustainable aviation fuels (SAF); and designing “new aircraft and jet propulsion technology.”

EESI says “the growth of demand for passenger and freight traffic is a central barrier to controlling commercial aviation emissions.”

The statement also noted that “for the last 40 years, aircraft and engine technology has reduced CO2 emissions by a yearly average of over one per cent per passenger mile.”

Air Transat, for example, has put great emphasis on building a fleet “that is as modern and efficient as possible,” said Chrystal Healy, the company’s vice-president of corporate responsibility and ESG (environmental, social and governance). “That’s been our strategic priority over the last couple of years and will continue to be our focus.” This includes the company’s recent deployment of A321neo aircraft, which Healy says produces 15 per cent fewer emissions.

But the figure that matters is absolute emissions — the fact that individual flights are less carbon-polluting is all well and good, but any progress on energy efficiency is being cancelled out by growing demand. As EESI notes , “emissions from aviation have accelerated in recent years as increasing commercial air traffic continued to raise the industry’s contribution to global emissions.”

The International Council on Clean Transportation says that in 2013, commercial aviation produced 707 million tons of global carbon emissions. By 2019, it was 920 million tons, having jumped about 30 per cent in six years.

“The growth of demand for passenger and freight traffic is a central barrier to controlling commercial aviation emissions,” says EESI.

In the travel industry, there has been excited chatter about offsets, which allow people to fund carbon-mitigation projects to countervail their aviation emissions. There are two main categories: carbon removal offsets, which are activities that take emissions out of the atmosphere, such as tree-planting or technologies like carbon capture and storage (CCS) and direct-air capture; and emission reduction offsets, which reduce the amount of carbon that escapes into the atmosphere, through energy efficiency initiatives and land conservation.

UBC’s Adeniyi Asiyanbi, who researches offsets, says their efficacy is “modest at best,” and that they come with “a whole range of problems.”

The big ones are the difficulties with ensuring that emissions reductions are additional (i.e. wouldn’t have happened without the offset initiative) and permanent (e.g. that a forest won’t later be cut down or destroyed by wildfires). Then there’s the more immediate human impact — that is, some carbon offset projects marginalize local communities, which can mean excluding them from their lands. Asiyanbi said research shows many carbon offset initiatives are actually counterproductive, distracting from more effective climate action and hurting people, too.

The industry has its own initiative: the Carbon Offsetting and Reduction Scheme for International Aviation ( CORSIA ), a global initiative in which airlines and other aircraft operators offset any growth in CO2 emissions above 2020 levels. But Asiyanbi points out that it is currently a voluntary scheme. “It doesn’t impose anything on the aviation industry. There are no sanctions. It’s not there to enforce.”

Sola Zheng, a researcher at the San Francisco-based International Council on Clean Transportation, said the emphasis on offsets has led people “to think that offsets are going to be one of the main solutions, when they’re not really solving any problems.”

She pointed out that there are almost no offsets within the airline industry itself. “It’s mostly … other sectors trying to reduce emissions on behalf of the aviation sector.”

Technological challenges

The push to decarbonize has led to a growing emphasis on sustainable aviation fuels (SAF), which Healy says are a “huge part of the industry’s road map to get to carbon neutrality by 2050.”

Air Canada has operated some flights this year from San Francisco to Vancouver, Calgary, Toronto and Montreal using SAF.

SAF is more of a category than a specific formula. For example, most SAF available today is made of fats, oils, and greases (such as cooking oil from McDonald’s) that have been refined and turned into fuel. This is known as first-generation SAF. Second-generation SAF is made up of biomass (which includes algae, animal waste and forest residue) and solid waste.

The International Air Transport Association has said that use of SAF “has been shown to provide significant reductions in overall CO2 lifecycle emissions compared to fossil fuels, up to 80 per cent in some cases.”

Air Canada wouldn’t agree to an interview for this story, but in response to a series of email questions, a spokesperson noted the airline has “long participated in research and development of SAF, with our first flights using a biofuel blend occurring in 2012.” Air Canada is also a founding member of the Canadian Council for Sustainable Aviation Fuels and a signatory to the World Economic Forum’s Clean Skies for Tomorrow 2030 Ambition Statement, whose mission is to “accelerate the supply and use of sustainable aviation fuel to reach 10 per cent of global jet aviation supply by 2030.”

Healy said part of the problem is that there is too little SAF available right now, and none of it is produced in Canada.

While SAF is more expensive than traditional jet fuel, its use is likely to increase — for example, the recent U.S. climate bill includes incentives for airlines to buy more sustainable fuel . There is also a proposal in the European Union for a “blending mandate,” in which jet fuel would contain two per cent SAF in 2025 and gradually ramp up to 63 per cent in 2050.

Another fuel source with promise is hydrogen, but both Air Transat and Air Canada see that as a longer-term development.

Given the current level and planned growth of travel, the progress on alternate fuels such as SAF and hydrogen is “negligible,” said Finlay Asher, a mechanical engineer based in Bristol, England, who spent seven years working on aircraft engine design for Rolls-Royce.

Asher says biofuel currently makes up less than one per cent of jet fuel, and ramping up production of crop-based biofuels would require land-use changes (such as deforestation), which could actually lead to the release of more carbon. He says there are also issues in producing more biofuels from waste sources or “e-fuels” from renewable electricity because of scarce global resources and intense competition from other sectors.

Hydrogen gas, on the other hand, poses a different dilemma. In order to be viable, it needs to be compressed and turned into liquid hydrogen, which means cooling it to cryogenic temperatures of -253 C. This requires complex tanks, Asher says, with insulation, thick walls and a complicated control system. Because the tanks are so large, they increase the weight and drag of the aircraft. Incorporating them would require complete aircraft redesign, and the whole process of design, development and certification “is likely to take decades.”

There is hope for electrified air travel. Earlier this month, Air Canada announced the purchase of 30 electric-hybrid aircraft from Sweden’s Heart Aerospace, which Air Canada CEO Michael Rousseau hailed as “a step forward to our goal of net zero emissions by 2050.”

But as this piece from MIT explains , batteries will need to become significantly more energy-dense if they hope to move people and cargo for great distances. For the foreseeable future, electrified aviation will only be able to accommodate small groups for short distances.

As EESI’s Bresette put it, “It’s going to be a while before me and 200 of my closest friends board an all-electric plane in D.C. and land in Los Angeles.”

The planes Air Canada recently announced will only have capacity for 30 passengers, and won’t be ready for service before 2028.

Asher says that accommodating alternative fuels or new methods of jet propulsion ultimately requires “a radical step-change” in aircraft design.

“Right now, airplanes are still more or less the way they were 50, 60 years ago,” Asher says, referring to the “tube-and-wing structure,” which includes a cylindrical fuselage with wings and engines mounted off the wings. To accommodate hydrogen cooling systems or electric batteries would require investment in developing brand new aircraft, which he says can cost about $20 billion US.

A number of alternative concepts have been floating around the aviation sector in recent years, including the “blended wing” design , which features a wider and more aerodynamic fuselage and utilizes the aircraft’s entire body to produce lift (rather than just the wings). Asher says there are big questions with the concept — including where to put passengers. Such aircraft would also require reconfiguring airport infrastructure, which isn’t currently being planned.

Asher says he joined the aviation sector with the belief that he could help make it more sustainable, but found that there wasn’t much appetite for change.

“What became clear is that the technology was not being advanced as quickly as it needed to be. It was not getting adequate resources,” he said.

The reason, he says, is that “fossil fuel is just super cheap.”

The future of flying

While aviation is having an impact on the environment, the inverse is also true.

As Asher points out, higher air temperature can make it difficult for planes to take off . Climate change is also leading to more turbulence and bigger storms, which can complicate air traffic control. Meanwhile, sea-level rise is imperiling low-lying runways around the world.

So what is the future of aviation?

The industry is quite bullish, projecting continuing growth while aiming for net-zero emissions by 2050. The industry is regulated by the International Civil Aviation Organization (ICAO), a UN agency. Climate Action Tracker, which assesses the emissions targets of both countries and business sectors, says ICAO’s climate goals are “critically insufficient” and “nowhere near Paris Agreement compatible.”

Like a number of airlines, Air Transat touts the idea of “travelling responsibly,” even dedicating a page of its website to it . Indeed, the phrase “responsible tourism” has taken hold in recent years, with a number of sites and blogs dispensing advice that includes reducing plastic waste, respecting wildlife and observing local traditions. The UN has a guide of its own, called “Tips for a Responsible Traveller,” which includes “plan[ning] your transport to cut carbon emissions.”

But none of these sites suggest you stop flying.

Flying less has become a cri du coeur in some quarters. For example, a group called Flyingless touts itself on Twitter as “a petition calling on universities and professional associations to greatly reduce flying.” Greta Thunberg was so committed to reducing her dependence on air travel that she took a zero-carbon racing boat from England to New York to attend the UN climate conference in 2019.

The movement has spent a lot of time trying to convince people to take the train instead of flying. Some governments have rallied around the idea — for example, earlier this year, the French government banned short-haul flights in locales where a train or bus ride under 2½ hours exists. The notion of flying less has gained some awareness, but the growth projections for aviation suggest feelings of flygskam are not widely shared.

Healy said Air Transat really believes in “the positive power of travel.”

“People want to travel, and it’s not for us to tell people what they should or should not do,” she said.

While airlines envision decades of steady business, some observers have imagined a different approach to the seemingly intractable problem of emissions.

One of them is California–based sci-fi author Kim Stanley Robinson, who earned rapturous praise for his 2020 novel The Ministry for the Future , notably from the New York Times and former U.S. president Barack Obama. Drawing on climate science as well as social psychology and economic theory, Robinson’s bracing book has been touted as necessary reading because it raises difficult questions about the challenges ahead. (Robinson even spoke at the COP26 climate summit in Glasgow, Scotland, last November.)

The book is essentially a 500-page thought experiment — namely, how would we respond to a climate disaster on a scale of the Holocaust? Robinson’s novel begins with 20 million people dying in a heat wave in India. The tragedy spurs desperate action, and not all of it is civil. While the United Nations creates a department called the Ministry for the Future, some non-governmental groups take a more extreme approach. Among them is a group of environmental radicals who, in an effort to quickly drive down aviation emissions, use drones to shoot passenger planes out of the sky, effectively scaring the industry into retirement.

Asher doesn’t envision anything quite so chilling. But he says getting through the next decade — which he deems “crucial” in the fight against climate change — calls for a combination of bold regulation and changes to individual behaviour.

He and former pilot Todd Smith felt so strongly about the issue that they established Safe Landing , an organization made up of concerned current and former aviation employees who want airlines to bring down emissions not only to save the planet, but the industry itself.

One way is to reduce the amount of flying done by the rich. The U.K.-based consultancy Yard recently assembled a list of the celebrities with the most emissions from private jets, suggesting the worst offenders were Taylor Swift, boxer Floyd Mayweather and rapper/business mogul Jay-Z.

“We’ve got one per cent [of people] creating 50 per cent of global aviation emissions,” said Smith, who lives on a canal boat in London, England. “This polluting elite either needs to pay more … or [governments should] put in regulation to stop people from flying so much.” Asher suggests that countries could introduce a tax on frequent fliers.

More broadly, many people are suggesting a change in attitude. While most COVID-19 restrictions are no longer in effect, author Emily Thomas said that the period of reduced travel gave us an opportunity to reflect on its necessity.

“I think one of the positive impacts that pandemic lockdowns had on Westerners was reminding us that travel is a privilege, not a right,” said Thomas.

Smith and Asher say that part of what needs to happen to reduce aviation’s effect on the environment is a change in narratives — to push back on the notion, as Smith said, that flying thousands of kilometres to an all-inclusive resort is “the only way to feel human.”

Thomas believes it starts with redefining “travel.”

“There is definitely a tension here, but travel does not equal flying,” she said, noting that humans “have been travelling for thousands of years” and that commercial flight is a relatively new phenomenon.

“There are many other ways to explore the world, and the advantage of travelling by foot, bus or train is that you see more of it as you go along. Even when flying seems necessary, we can consciously reduce it to a minimum.”

Top illustration: CBC News | Editing: Janet Davison

Related Stories

• climate change

It’s More Than Just Rain and Snow. Climate Change Will Hit Air Travel in Surprising Ways

A version of this story first appeared in the Climate is Everything newsletter. If you’d like sign up to receive this free once-a-week email, click here .

For those watching U.S. air travel spike as the COVID-19 pandemic fades, American Airlines’ recent announcement that it would trim its flight schedule may have come as a bit of a surprise. More and more people have been flying in recent months, and in response airlines have added flights to meet that demand, not taken them away.

American cited several operational reasons for the adjustment, including labor shortages at vendors that resulted from quickly ramping up from pandemic level staffing levels, but unsurprisingly the one I want to focus on here is “unprecedented weather.” In an email, an American spokesperson told me recent bad weather at the airline’s hubs in Miami, Chicago and Detroit had disrupted operations. The company is also monitoring extreme heat in Phoenix and thunderstorms in Dallas and Charlotte, the spokesperson said.

I won’t make the case that this recent spat of bad weather is caused by climate change, but the cancellations across American’s schedule provide a good opportunity to look at how the effects of climate change might make air travel more challenging—both in terms of industry economics and the travel experience—in the coming decades.

Much attention has been paid to airline emissions , but surprisingly the need for the industry to adapt their operations to the challenges of extreme weather has received relatively little consideration. Still, there’s much to think about. A 2016 article in the journal Carbon & Climate Law Review offers a high-level overview: increased precipitation will lead to more frequent delays, extreme heat will damage runways and storm surge could damage infrastructure at a quarter of U.S. airports even under a moderate sea-level rise scenario. And that’s really just a snapshot. The article concludes that “every sector of the air transport industry will be affected.”

Any one of those factors could make for a story on its own. But I want to home in on one area that’s probably less obvious: extreme heat. Flying is one of those experiences where you hardly think about the temperature outside. You sit in an air-conditioned airport, board an often-freezing plane and exit into another air-conditioned terminal. It can be 100°F outside at your origin and 110°F at your destination without you ever taking off your sweater.

Behind the scenes, however, your airline is watching that temperature closely. Warm weather makes the air less dense, requiring planes to move faster to get off the ground. On extremely hot days, airlines often restrict a flight’s weight by cutting the number of passengers or restricting the plane’s cargo load. On rare occasions, extreme heat can lead airlines to cancel flights altogether. In the summer of 2017, American Airlines made news for canceling flights in and out of Phoenix because temperatures were too high for some planes to operate at all.

Even if planes are operating, extreme heat creates problems on the ground, too. In May, American Airlines started operating a cart on the tarmac in Phoenix to deliver water, Gatorade and popsicles to help keep employees from overheating, according to a report from FOX10 Phoenix. In many parts of the globe, particularly in parts of the Middle East, outdoor air temperatures are already approaching a level where staying outside for long periods of time can be unsafe.

Right now, heat-related problems happen rarely enough that even a frequent flier may not notice—but as temperatures rise that will almost certainly change. A 2017 study in the journal Climatic Change found that up to 30% of flights departing at the hottest part of the day may face weight restrictions in the coming decades. It seems safe to bet that we’ll see many more cancellations, too.

Climate change-related disruptions to flight schedules are the ultimate example of a “first-world problem,” and this obviously isn’t the most pressing climate concern. But I think it’s a useful example of how climate change will seep into areas we may not expect at a glance and, without a proactive attempt to address it, make life just a little worse.

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Write to Justin Worland at [email protected]

Aviation’s contribution to global warming higher than expected

4 November 2021

Major new study reveals that aviation could consume up to one-sixth of the remaining temperature budget to limit warming to 1.5˚C.

Aviation is responsible for more global warming than implied by its carbon footprint alone.

According to new research funded published today, aviation could consume up one-sixth of the remaining temperature budget required to limit warming to 1.5˚C by 2050. The article, published in Environmental Research Letters, suggests that emissions produced by the aviation industry must be reduced each year if the sector’s emissions are not to increase warming further.

The study was part-funded by the Natural Environment Research Council (NERC) and NERC’s National Centre for Earth Observation (NCEO) with research conducted by scientists from the Science and Technology Facilities Council’s (STFC) RAL Space.

Contributing to global warming

Given that aviation is widely recognised as a sector which is challenging to decarbonise, this research will inform the discussion about aviation’s ‘fair share’ of future warming.

The researchers behind the study developed a simple technique for quantifying the temperature contribution of historical aviation emissions. This included both CO2 and non-CO2 impacts. The study also projects future warming due to aviation based on a range of possible solutions to the climate crisis. The researchers of the study are based at:

• University of Oxford
• Manchester Metropolitan University

Tackling global warming

The aviation industry has only recently begun to tackle the warming effect of flying, and this study is timely for quantifying that impact.

The solutions discussed in this study, such as moving to alternative fuels, present a clear pathway to minimising warming but these will take time to implement. In the short-term, there are actions that the industry can take right now.

Dr Simon Proud, of the NCEO and STFC RAL Space, said:

A ban on fuel tankering, where aircraft carry more fuel than they need, and hence burn extra fuel, to save the cost of refuelling at the destination, would reduce CO2 emissions in Europe alone by almost one million tonnes.

Other solutions were discussed, including more efficient air traffic control and minimising holding patterns at airports.

Download the research paper on IOP Science .

Further information

The study was part-funded by:

• NERC, grant number NE/V00946X/1
• UK Research and Innovation (UKRI)
• the Department for Transport
• the European Union’s Horizon 2020 Research and Innovation Action ACACIA, under grant agreement number 875036.

Top image:  Credit: pic4you / Getty Images

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Climate Change and Travel: Harmonizing to Abate Impact

Aisha n. khatib.

Department of Family & Community Medicine, University of Toronto, Toronto, ON Canada

Purpose of Review

With climate change being the single biggest health threat facing humanity, this review aims to identify the climate-sensitive health risks to the traveler and to recognize the role that travel plays in contributing to the detrimental effects of climate change. With this understanding, adaptations for transformational action can be made.

Recent Findings

Travel and tourism, including transportation, food consumption, and accommodation, is responsible for a large percentage of the world’s carbon emissions which is contributing to the climate change crisis at an alarming rate. Climate change is a health emergency that is resulting in a rise of significant health impacts to the traveler including increased heat illnesses; food-, water-, and vector-borne diseases; and increasing risk of exposure to emerging infectious diseases. Patterns of future travel and destination choices are likely to change due to climactic factors such as temperature and extreme weather events, forced migration, degradation, and disappearance of popular and natural tourist destinations.

Global warming is and will continue to alter the landscape of travel medicine with expansion of transmission seasons and geographic ranges of disease, increased risk of infections and harmful marine toxins, and introduction of emerging infections to naïve populations. This will have implications for pre-travel counseling in assessing risk and discussing the environmental influences on travel. Travelers and stakeholders should be engaged in a dialogue to understand their “climate footprint,” to innovate sustainable solutions, and be empowered to make immediate, conscientious, and responsible choices to abate the impact of breaching critical temperature thresholds.

Introduction

Climate change is a health emergency—a declaration that has been made by 39 countries around the world, jurisdictions amounting to a population over 1 billion citizens. Various international reports, most notably the Lancet Countdown, have exposed not only the unabated rise of health impacts from climate change but also the health consequences of delayed action, providing an imperative to immediate efforts to be taken to lessen the damage [ 1 ••, 2 ••, 3 ]. The climate-sensitive health impacts include enhanced conditions for respiratory illnesses, mental health, cardiovascular disease, and premature death [ 1 ••, 3 ]. Extreme weather patterns destabilize communities and reduce access to healthcare. Changing environmental conditions increase the suitability for the transmission of water-borne, air-borne, food-borne, and vector-borne pathogens. Currently, 40% of the world lives in tropical areas already seeded in inequities, and those at the highest risk, in underserved populations in low-income and human development index communities, will disproportionately bear the brunt of the impact. Vulnerable populations, including women, children, the elderly, members of minority groups, and those with chronic diseases and disabilities, are also at the highest risk of suffering [ 1 ••].

Innovation and resources are needed to identify, prepare for, and adapt to the harmful health impacts of climate change. How will climate alter the landscape of travel and tropical medicine? More importantly, how is the way that we travel contributing to these deleterious changes? This review aims to identify the climate-sensitive health risks to the traveler and to recognize the role that travel plays in contributing to the detrimental effects of climate change.

Climate Change 101

The last 7 years have been the warmest years on record, with mean global temperatures rising more than 1.1 °C above pre-industrial levels and edging closer to the limit laid out under the Paris Agreement. To prevent reaching a tipping point threshold that could result in catastrophic losses of human life and natural environments, the Paris Agreement calls for all countries to strive towards a limit of 1.5 °C of global warming through concerted climate action [ 4 ]. The global average temperature increase of 1.1 °C has already heralded increased frequency and magnitude of extreme weather events from heatwaves, droughts, flooding, winter storms, hurricanes, and wildfires [ 2 ••]. Future climate-related risks depend on the rate, peak, and duration of warming and will be much greater if global warming exceeds 1.5 °C [ 2 ••]. Even if global warming stabilized at 1.5 °C by the year 2100, some impacts may be long-lasting or irreversible, such as the destruction of certain vulnerable ecosystems [ 2 ••]. Warming is caused from anthropogenic emissions—to meet the Paris Agreement goals and prevent dangerous levels of global warming, global greenhouse gas emissions must reduce by half within a decade [ 1 ••]. The responsibility will lie on a collective and collaborative examination on how we are individually and as a society contributing to these emissions, and on areas that could be transformed, modified, and adapted to decrease them, thus lessening the impact of climate change.

The Impact of Travel on Climate Change

Tourism is responsible for roughly 8% of the world’s carbon emissions [ 5 ]. The carbon footprint of tourism is comprised by not only transport, but by the energy and commodities purchased by travelers including food, souvenirs, and accommodation [ 5 ]. The majority of this footprint is emitted by visitors from high-income countries, with US travelers at the top of the list, followed by China, Germany, and India. Per capita, small island destinations such as the Maldives, Mauritius, and the Seychelles hold the highest destination-based footprints from international tourism—which comprises up to 80% of their national emissions [ 5 ]. Global carbon movement, embodied in tourism and traveling, is largely a high-income affair, with travelers exerting higher carbon footprints elsewhere than their own country, and with host countries of popular destinations shouldering a higher footprint from visitors than they would exert themselves [ 5 ]. However, there is a delicate balance of economic growth and development that comes attached to tourism in low-income countries that may bear more of the burden. The UNTWO has projected that transport-related CO 2 emissions will grow 25% by 2030 and, recognizing this, has set a basis to scale up climate action to transform tourism to low emission and more efficient operations [ 6 ]. This is catalyzed in the Glasgow Declaration on Climate Action in Tourism, urging signatories about the need to accelerate urgent action, coordination, and commitment [ 7 ]. The COP27 in Egypt also recognized the cost of doing so, and established the “loss and damage” fund to compensate the developing nations most vulnerable to the climate crisis.

Transportation

Today, transportation is tourism’s main source of greenhouse gas emissions, accounting for 5% of all man-made emissions [ 6 , 8 ]. On average, planes and cars generate the most CO 2 per passenger mile, with tour buses, ferries, and trains coming well behind. In recent years prior to the pandemic lockdowns, the number of people traveling internationally skyrocketed as airfare became more affordable. Projections suggest that travel emissions will make up 12% of total greenhouse emissions by 2025 [ 6 ]. Aviation is responsible for 4% of the 1.1 °C rise in the global mean temperature already experienced since the industrial revolution [ 2 ••, 6 ]. To put into perspective, the aviation sector is responsible for 12% of transportation emissions—if it were a nation, it would be among the top 10 global emitters [ 6 ]. The total carbon impact of a single flight can be significant—for instance, it would take an acre of forest a year to absorb the same amount of CO 2 emissions of a one-way flight from London to New York and can be equivalent to going car-free for a year [ 9 ].

The impact on the coronavirus disease pandemic on global air transport was unprecedented, with up to a 74% drop in global passenger numbers in 2020, a fall of 2.7 billion compared to 4.5 billion in 2019 [ 10 ]. During this sharp decline in air travel, carbon dioxide emissions reduced by 5.4% in 2020. However, surprisingly, the amount of CO 2 in the atmosphere continued to grow at about the same rate as in preceding years, and has been attributed to a complex interplay of various factors and atmospheric components [ 11 ]. Despite these findings, the resulting reduction in anthropogenic activity did yield a glimpse into a future where emissions can be curtailed, but unfortunately, there has been a quick return back to 2019 emission levels.

The aviation industry needs to do its part to make a bigger climate impact; however, collectively, individual choices to fly responsibly should also be addressed [ 12 ]. Advice to travelers to limit their carbon footprint when flying can include choosing economy since flying business emits up to three times more carbon as it takes up more space, opting for direct flights when possible, taking a train as an alternative option to a short-haul flight, altogether skipping the flight and using virtual means of connecting, or taking daytime flights instead of the redeye as there is a heat-trapping effect of contrails and cirrus clouds at night, resulting in a higher greenhouse effect [ 13 ]. This small decision could be collectively very impactful, as in 2030, the total number of tourist trips is expected to reach 37.5 billion, of which 17.4 billion will be overnight arrivals—a 45% growth from 2016 [ 6 ]. Of note, recently, in an effort to reduce carbon emissions by planes, France has announced a plan to ban short-haul flights where travelers could alternatively take a train in under 2.5 hours—the first time a country has enacted a law prohibiting flights for environmental reasons [ 14 , 15 ].

Buying carbon offsets is another consideration—carbon offsets are a credit for emission reductions given to one party that can be sold to another stakeholder who are effecting change to compensate for its emissions [ 16 , 17 ]. Carbon offsets are typically measured in tons of CO 2 -equivalents and are bought and sold through international brokers, online retailers, and trading platforms. Buying offsets helps individuals take into account the environmental costs of air travel [ 13 ]. The price per ton of offsets, however, is far below the estimated costs of damage that a ton of carbon pollution will cause via global warming and ocean acidification [ 18 ]. Although carbon offsets are a conscious action to take to mitigate emissions when flying, reductions in flights taken, planning longer trips, and using alternative lower emitting transportation means, such as trains, are still impactful options.

Food Consumption and Wastage

Food production is responsible for roughly one-quarter of the world’s greenhouse gas emissions [ 19 ]. Getting food from farm to table means growing, processing, transporting, packaging, refrigerating, and cooking—all of which require energy and contribute to a meal’s carbon footprint. Travel often multiplies this footprint since people tend to indulge more while abroad or on vacation. Wastage of food in tourism is part of a larger global issue. If food waste were a country, it would be the world’s 3rd largest emitter of CO 2 [ 20 ]. A substantial amount of the food produced for tourism at all-you-can eat hotel buffets and in oversized restaurant portions is discarded [ 21 , 22 ]. When food is wasted, all the emissions generated in the journey of its production were unnecessary. Globally, less than half of hotels compost their food waste. When this food decomposes in landfills, methane is created which is 21 times more potent at warming than carbon dioxide—methane has accounted for up to 30% of global warming since pre-industrial times [ 23 ].

Accommodation

Accommodation accounts for a substantial part of tourism’s greenhouse gas emissions with many establishments wasting energy—intensive air-conditioning is often cited as a major culprit; or entertainment infrastructure built in areas destroying protective carbon sink ecosystems such as forests and mangroves [ 24 ]. The UNWTO concluded that the accommodation sector is responsible for 21% of CO 2 emissions, compared to other tourism contributors [ 25 ]. Accommodation-related emissions could be reduced by reducing energy consumption, increasing infrastructure energy efficiency, using greenery to cool buildings, and switching to renewable energy resources. New technologies adaptations being developed could provide hotel staff with critical data and send alerts to help them manage energy consumption and increase sustainability [ 22 ]. Traveler behavior and expectations, such as daily towel and linen changes, can also substantially contribute to energy consumption and wastage. Empowering travelers to make more sustainable choices when planning accommodation can be a powerful tool for change, but they must also be provided with consistent, reliable information and options to make these informed green decisions [ 26 , 27 ]. Due to widespread regulatory and market policies, accommodation is a sector that has the potential to become carbon–neutral by 2030 if stakeholders strategize for more transformative change [ 24 ].

The Impact of Climate Change on the Traveler

Changes in future patterns of travel and destination choices, sea level rising.

Patterns of future travel are likely to change to due climactic factors. Natural factors that tend to attract travelers to specific destinations are being diminished, such as the bleaching of coral reefs in places like the Great Barrier Reef [ 28 , 29 ]. The thought of the disappearing destination has even led to the recent trend called “last chance tourism,” which, unfortunately, can even further perpetrate the fragile conditions with an influx of climate anxious travelers [ 28 ]. With 2021 setting a new record for ocean heating, rising sea levels are threatening coastal cities, habitats, and popular tourist destinations, with a projected one foot rise by 2050 [ 30 ]. Popular cities such as Jakarta and Rotterdam are planning adaptation measures to cope with prospects of higher sea levels, such as building seawalls, planting mangroves, and rethinking roads [ 31 , 32 ]. The impact will be even more devastating to small island developing states (SIDS), such as Tuvalu and the Marshall Islands, who are already on the frontlines of climate change, facing low-lying flooding and a battering of increasing tropical storms [ 33 ]. Combined, SIDS make up approximately 65 million inhabitants, of which one third live on land less than 5 m above sea level, and where tourism accounts for a substantial economic export, over 50% in SIDS such as the Maldives, Seychelles, and Bahamas [ 34 , 35 ]. In 2019, over 44 million visitors visited SIDS with an injection of USD 55 billion into the tourism sector. A decline in beach tourism in threatened locations highlights the unique financial vulnerability of these islands to changing travel patterns while already facing an existential threat [ 36 , 37 ].

Extreme Weather from Ocean and Atmospheric Warming

Continuous warming of the ocean and atmosphere as a result of climate change is also increasing the number of natural disasters from extreme weather events—including hurricanes, cyclones, wildfires, droughts, and floods [ 36 , 38 ]. These catastrophic events modify the geographic landscape of a location, but also the ecology of pathogens, increasing the proliferation of and exposure to infectious diseases [ 38 ]. Diseases such as cholera, leptospirosis, and diarrheal diseases spread rapidly after storms and floods, putting local populations and travelers at increased risk [ 38 – 41 ]. Synergizing climactic factors intensify the level and frequency of natural disasters, which result in exacerbated outbreaks of disease. This was most recently witnessed in Pakistan after the devastating floods in June 2022, which caused an unprecedented surge of vector-borne diseases such as malaria and dengue, overwhelming the already fragile and damaged country [ 40 ]. Health practitioners and travelers must be made aware of the complex dynamic situation of regions affected by climactic hazards, to prepare for and prevent risk of infection and exposures [ 41 ].

Implications of Increasing Harmful Algal Blooms

Direct health implications can also be attributed to the fallout from ocean warming. Increasing levels of harmful algal blooms (HABS) can lead to human health consequences as well as detrimental effects on tourism and recreation. In coastal zones, swimmers and food gatherers may face closures of contaminated beaches and shellfish areas, which has already occurred in many Northern European countries [ 42 , 43 ]. HABs lead to increased toxin contamination of seafood and shellfish that can cause an array of respiratory, dermatological, neurological, and diarrheal manifestations of illness and even deaths from the various marine toxins [ 44 ]. The effects of one HAB can be wide reaching worldwide through the globalized seafood industry, masking an already underrecognized hazard for travelers [ 44 ]. Alongside the usual and increasing culprits of scombroid, ciguatera, and shellfish poisoning, recent travelers to places such as the Caribbean are for sure to have encountered the 24 million tons of sargassum encroaching onto the once pristine white beaches, an unsightly, stinky, and toxic floating brown macroalgae seaweed that can be several feet deep and cover thousands of square miles of ocean [ 45 , 46 ]. When decomposing, the sargasso releases ammonia and hydrogen sulfide gas, a broad spectrum poison that smells of rotten eggs. Breathing in these toxic gases may cause hypoxic pulmonary, dermatological, and neurocognitive symptoms. In 2018, in Guadeloupe and Martinique, 11,000 cases of suspected sargasso poisoning was reported where patients complained of heart palpitations, shortness of breath, dizziness, vertigo, headache, and skin rashes [ 46 ]. Recent NASA satellite observations revealed an unprecedented belt of the brown macroalgae stretching from West Africa to the Gulf of Mexico and south to Brazil, the biggest seaweed bloom in the world [ 47 ]. As this “Sargasso Sea” continues to chokehold once pristine beautiful beaches, travelers should be advised to not walk on these beaches, and may altogether be changing their itineraries based on Sargassum Watch Systems which have been developed [ 48 , 49 ].

Increasing Temperatures and Heatwaves

Heat waves are one of the deadliest extreme events that are increasing in frequency, intensity, and duration, however are less discussed as a travel concern as they are less visible, more widespread, and deleterious impacts harder to quantify [ 50 ]. Record temperatures in 2020 set record high heatwave exposure among people older than 65 and children younger than 1 year, populations with a higher than average risk of heat-related death [ 1 ••]. Heat is one of the largest weather-related causes of death in high-income countries, with increasing morbidity and mortality in people with cardiopulmonary, age-related vulnerability, and other chronic diseases [ 51 ]. Travel medicine must start incorporating education around risks of unrecognized heat exposures and heat-related illnesses, such as heat stroke, exhaustion, and dehydration, and incorporate heat action plans into their travel consults. With prevention and proper recognition of vulnerable populations, adaptations can be made to incorporate sustainable solutions and effective cooling interventions at the individual and community level [ 52 ]. Many travel itineraries were changed to accommodate earlier or altered locations this year due to extreme heat in various popular destinations; and subsequently, travelers discovered that cancelation policies did not allow for refunds or coverage.

Changes in Disease Epidemiology and Geographic Patterns of Transmission

Geographic expansion of vector-borne diseases.

Climate factors, such as temperature, rainfall, and meteorological events, which are being intensified by the ongoing emission of greenhouse gases, can influence disease epidemiology, thus changing the landscape of travel and tropical-related medicine [ 39 ]. Vector-borne diseases are an example of this, where climate factors can influence vector suitability of habitats for survival, breeding, and transmission [ 53 •]. With many neglected tropical diseases previously confined to subtropical and tropical areas of the world, including Chagas disease, leishmaniasis, malaria, rabies, and snakebites, predictions anticipate changes in geographic range and transmission periods, resulting in disease incursion and emergence in regions and populations previously unaffected [ 54 , 55 ]. In Europe, for example, climate change is thought to be a key factor in the spread of the Asian tiger mosquito Aedes albopictus , the vector which can transmit dengue, chikungunya, and Zika, as well as Phlebotomus sandfly species, which can transmit leishmaniasis [ 55 , 56 ].

Many temperate regions and high-income countries are now suitable for vector-borne disease transmission that were previously not present [ 56 ].

The impact of climate change on vector-borne ecosystems is complex, as it can influence the circulation and dissemination patterns of migratory birds, which can be carrier or reservoir hosts of many mosquito-borne and tick-borne zoonotic pathogens [ 57 ]. The shift in bird migration routes secondary to climate factors is thought to be a key factor in the increasing spread of mosquito-borne West-Nile virus (WNV) and tick-borne zoonotic pathogens such as Lyme disease into parts of Canada and Northern Europe, as well as tick-borne encephalitis into parts of Northern Europe [ 58 ]. With increasing suitability for habitat with climate change, trans-Saharan bird migration from Africa to Europe has also been implicated in the emergence of new zoonotic pathogens in Europe. These include the introduction of the mosquito-borne Usutu virus (USUV) and the dissemination of new tick-borne pathogens such as Crimean-Congo hemorrhagic fever virus, representing novel threats to human health [ 59 , 60 ]. As vector-borne diseases expand into new unchartered territories, surveillance and awareness of changes in disease patterns will be needed; and thorough travel histories will become more clinically relevant and important.

The Threat of Emerging Arboviruses

The Aedes aegypti mosquito, the vector for many arboviral diseases including dengue, chikungunya, yellow fever, and Zika, is sensitive to climate variability, having a higher thermal optimum for viral transmission [ 61 ]. Already in the last two decades, there has been the largest increase in global abundance, with an anticipated increase of 30% by the end of the century in a low carbon emission scenario [ 62 ]. There has been an expansion of geographical distribution of these vectors into more temperate areas, with subsequent increasing cases of travel-related arboviral infections such as chikungunya [ 63 ]. Dengue cases around the world have surged in the last few years, with the largest number of dengue cases ever reported globally in 2019, and with projections that it will impact 60% of the world’s population by 2080—Mexico alone predicted to have a possible 40% increase of annual dengue incidence [ 64 – 67 ]. With autochronous transmission reported in France, Croatia, and the USA, there is threat of ongoing outbreaks in Europe and North America [ 68 ]. Recently, an autochronous case of the Usutu virus, a Culex -borne emerging arbovirus of African origin, was also reported in France [ 69 ]. Similar to WNV, Usutu virus has the potential to become an important human pathogen of concern; with its rapidly expanding geographical distribution, it is an arbovirus on the rise [ 60 ]. Predictions of arboviral shift include further expansion risk of chikungunya into Central and South America, with an estimated increase of 7.5 million cases by mid-century with unmitigated temperature increase [ 54 ]. Over 1.3 billion new people could be exposed to Zika virus risk as thermal suitability for Zika transmission expands into East Africa, high-income North America, East Asia, Western Europe, North Africa, and the Middle East [ 70 ]. Along with the threat of travel-related introduction of these viruses into naïve populations, challenges arise around surveillance, public health management of new outbreaks in already resource-strained systems, underdiagnosis, and inappropriate management of unfamiliar infections by health professionals.

Mosquitoes Reaching New Heights: Elevation

In travel medicine, advice is often given that mosquito-borne virus transmission is considered a minimal risk over elevations of 2000 m. The changing epidemiology of dengue infections in Nepal over the last 20 years has demonstrated a stark geographical expansion of latitude and altitude. The Aedes vectors have since been found in high-altitude areas of Nepal, likely the result of vector habitat expansion due to global warming of mountain areas [ 71 ]. Of note is that Nepal only became endemic to dengue in 2006—the first reported dengue case to Nepal was in 2004, imported from India by a Japanese foreigner [ 71 ]. With over 50,000 cases reported in 2022, Nepal is a prime example of a new emerging disease increasing burden and geographic spread by latitude and elevation, introduced by a traveler, but now also affecting the landscape of novel risk for travelers to the area [ 72 ].

Temperature Variability and Malaria Transmission Risk

The effects of changing temperatures on vectors and subsequent disease transmission are not all the same, which gives variability in predictions of geographic redistribution of disease. Thermal performance curves are used to predict the way temperature can affect and shift vector and host physiology and the ability to adapt to climate-mediated ecosystem changes [ 58 ]. For example, malaria transmission by the Anopheles gambiae mosquito peaks at 25 °C, whereas dengue transmission by Aedes aegypti peaks at 29 °C [ 73 , 74 •]. The warming temperatures in the tropics, could therefore, drive a shift in climates more suitable to malaria transmission, such as in sub-Saharan Africa, to those more suitable to arboviruses. In this “Battle of the Buzz” as climate suitability increases for arboviruses, these underrecognized emerging diseases could expand and overtake the public health burden of malaria in Africa [ 73 ]. Some models of malaria vector and pathogen suitability have projected shifts depending on the region within Africa, and some have projected expansion to higher altitudes in Southern and Eastern Africa, Asia, Latin America, and the Middle East [ 41 , 75 ]. There will need to be increased research, surveillance, awareness, and education around these emerging diseases for locals, travelers, and the scientific community [ 76 ].

The Great Climate Migration

We cannot talk about travel and climate change without recognizing migration as one of the defining occurrences of our time, often redistributing the fabric of the quilt of our societies. The single greatest impact of climate change could be on human migration [ 43 ]. Migration has many drivers, including political, economic, social, cultural, and environmental factors [ 77 ••]. Climate change can be the main precipitating event, but can also interact with other factors to be an influential force [ 77 ••]. Climate disruption is expected to uproot over 200 million migrants by 2050, those forcibly displaced referred to as “climate migrants” or debatably “climate refugees,” with Sub-Saharan Africa, South Asia, and Latin America being the three regions most heavily affected [ 78 ]. But migration can also be considered to be an adaptation strategy in response to the consequences of climate change, with concerted efforts between and within nations harmonizing to abate impact [ 77 ••, 79 ]. These significant shifts in population distributions via migration and travel, whether forced or not, can also introduce novel pathogens to naïve populations, as has been witnessed for example by the importation of chikungunya into Italy; coronaviruses internationally via air travel; the Zika virus epidemic in the Americas; schistosomiasis in Corsica, France; and most recently, the outbreaks and spread of monkeypox globally [ 70 , 77 ••, 80 – 84 ]. As we progress through the shifting climate, we will continue to see an ever-increasing interplay of the role that travel, migration, and globalization have on the dispersion and introduction of emerging infectious diseases, combined with climate variability and increasing suitability, altering the geographic distribution as we know it.

The Climate Change-Inequality Nexus

Traveler privileges are tipping the scales of inequality as the number of people who can afford to travel grows alongside tourism’s environmental footprint. Between 2010 and 2020, climate-related disasters killed fifteen times more people in highly vulnerable countries, which were responsible for less than 3% of global emissions, than in the wealthiest countries [ 2 ••]. Between 2030 and 2050, climate change is expected to cause 250,000 additional deaths per year due to malaria, diarrhea, and heat stress [ 3 ]. The 46 least developed countries that make up 40% of the world’s poorest are highly vulnerable to these compounded shocks, being disproportionately affected by and at the forefront of the climate crisis [ 85 , 86 ]. There is a great opportunity to combine the overarching framework of the Sustainable Development Goals (SDG) and how they can relate to choices for responsible travel—by examining the areas of climate action, life below water, responsible consumption, decent work and economic growth, reduced inequalities, sustaining cities and communities, and developing partnerships for the goals [ 87 , 88 •]. There is a fine interplay, dependency, and domino effect of factors—even recognizing the deleterious role that high-income travelers can have on vulnerable impoverished tourist destinations must be supported with strategies to improve health and education, reduce inequalities and wastage, and spur economic growth, all while tackling climate change and working to preserve fragile ecosystems.

Practical Implications for Pre-travel Counseling?

Risk, in the context of travel medicine, refers to the possibility of harm during the course of travel. Travel medicine is based on assessing a traveler’s risk at destination and the concept of taking measures to reduce risk. As climate changes the landscape of travel risks due to global warming, understanding the major climate-associated health implications faced by travelers will be pivotal to educating and preparing them ( 41 ) Discussing air pollution linked to respiratory exacerbations, heat illnesses and associated morbidity linked to rising temperatures, and keeping up-to-date with the changing epidemiology of vector-borne diseases and emerging infectious diseases will be necessary.

Proposals to reduce the “climate footprint” of a traveler—the beneficial or detrimental influence that can be had on another person or community in relation to climate factors our actions and choices impact—should be addressed. From measuring carbon emissions, to ethical and behavioral considerations in methods and frequency of travel, to serving as sentinels and vehicles for the spread of disease, creating awareness of the “climate footprint” of a traveler, how it can be measured, and how it can be addressed should be integrated into the pre-travel consultation. By promoting education and ethical travel choices to decrease emissions, our goal should be to create a more sustainable future for destinations, communities, and travelers.

Conclusion: Harmonizing to Abate Impact

We are witnessing a climate crisis which cannot be ignored as we increasingly experience its destructive effects. As we emerge from the pandemic, we are well-suited to take advantage of this unique moment in time to address some of the greatest systemic climate challenges caused by the acceleration of travel and tourism in recent years. Now, more than ever, there is a need to examine the impact that we as travelers have on ecosystems, economies, populations, and future generations. Sustainability and responsibility need to be interwoven and embedded within the framework of travel medicine as well as developing a collaborative platform to discourse innovative and adaptive solutions. By being examples leading innovation and change, we can hope to have a positive and beneficial butterfly effect in the actions that we take. We are at a critical juncture that is beyond using incremental measures to adapt but requiring transformative change at every level of society. Although the task at hand can seem daunting, thousands of small individual choices and actions will summate to transformative outcomes and necessary change. Engaging stakeholders, industry, individuals, travelers, and the scientific community will be monumental in assessing the projected situation, establishing goalposts, discarding current habits of consumption, and replacing them with inventive, integrative, and harmonizing actions to abate the impact of climate change.

Acknowledgements

I would like to thank and acknowledge Dr. Edward Xie, Dr. Archna Gupta, and Dr. Lin Chen for their contributions in peer review, idea creation, and expert guidance.

Compliance with Ethical Standards

Aisha Khatib is Chair of the Responsible Travel Interest Group of the International Society of Travel Medicine, and a voting member of CATMAT- the Committee to Advise on Tropical Medicine and Travel, an expert advisory body to the Public Health Agency of Canada.

This article does not contain any studies with human or animal subjects performed by any of the authors.

This article is part of the Topical Collection on Tropical, Travel and Emerging Infections

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Papers of particular interest, published recently, have been highlighted as: •  Of importance •• Of major importance

Cars, planes, trains: where do CO2 emissions from transport come from?

Transport accounts for around one-fifth of global co₂ emissions. three-quarters of this is from road transport..

Transport accounts for around one-fifth of global carbon dioxide (CO 2 ) emissions [24% if we only consider CO 2 emissions from energy] . 1

How do these emissions break down? Is it cars, trucks, planes or trains that dominate?

In the chart here we see global transport emissions in 2018. This data is sourced from the International Energy Agency (IEA) .

Road travel accounts for three-quarters of transport emissions. Most of this comes from passenger vehicles – cars and buses – which contribute 45.1%. The other 29.4% comes from trucks carrying freight.

Since the entire transport sector accounts for 21% of total emissions, and road transport accounts for three-quarters of transport emissions, road transport accounts for 15% of total CO 2 emissions.

Aviation – while it often gets the most attention in discussions on action against climate change – accounts for only 11.6% of transport emissions. It emits just under one billion tonnes of CO 2 each year – around 2.5% of total global emissions [we look at the role that air travel plays in climate change in more detail in another article ] . International shipping contributes a similar amount, at 10.6%.

Rail travel and freight emits very little – only 1% of transport emissions. Other transport – which is mainly the movement of materials such as water, oil, and gas via pipelines – is responsible for 2.2%.

Towards zero-carbon transport: how can we expect the sector’s CO 2 emissions to change in the future?

Transport demand is expected to grow across the world in the coming decades as the global population increases, incomes rise, and more people can afford cars, trains, and flights. In its Energy Technology Perspectives report, the International Energy Agency (IEA) expects global transport (measured in passenger-kilometers) to double, car ownership rates to increase by 60%, and demand for passenger and freight aviation to triple by 2070. 2 Combined, these factors would result in a large increase in transport emissions.

But major technological innovations can help offset this rise in demand. As the world shifts towards lower-carbon electricity sources, the rise of electric vehicles offers a viable option to reduce emissions from passenger vehicles.

This is reflected in the IEA’s Energy Technology Perspective report. There it outlines its “Sustainable Development Scenario” for reaching net-zero CO2 emissions from global energy by 2070. The pathways for the different elements of the transport sector in this optimistic scenario are shown in the visualization.

We see that with electrification- and hydrogen- technologies some of these sub-sectors could decarbonize within decades. The IEA scenario assumes the phase-out of emissions from motorcycles by 2040; rail by 2050; and small trucks by 2060; and although emissions from cars and buses are not completely eliminated until 2070, it expects many regions, including the European Union; United States; China and Japan to have phased-out conventional vehicles as early as 2040.

Other transport sectors will be much more difficult to decarbonize.

In a paper published in Science , Steven Davis and colleagues looked at our options across sectors to reach a net-zero emissions energy system. 3 They highlighted long-distance road freight (large trucks), aviation, and shipping as particularly difficult to eliminate. The potential for hydrogen as a fuel, or battery electricity to run planes, ships, and large trucks is limited by the range and power required; the size and weight of batteries or hydrogen fuel tanks would be much larger and heavier than current combustion engines. 4

So, despite falling by three-quarters in the visualized scenario, emissions from these sub-sectors would still make transport the largest contributor to energy-related emissions in 2070. To reach net zero for the energy sector as a whole, these emissions would have to be offset by ‘negative emissions’ (e.g. the capture and storage of carbon from bioenergy or direct air capture ) from other parts of the energy system.

In the IEA’s net-zero scenario, nearly two-thirds of the emissions reductions come from technologies that are not yet commercially available. As the IEA states, “Reducing CO 2 emissions in the transport sector over the next half-century will be a formidable task.” 2

Global CO2 emissions from transport in the IEA's Sustainable Development Scenario to 2070 2

The World Resource Institute ’s Climate Data Explorer provides data from CAIT on the breakdown of emissions by sector. In 2016, global CO 2 emissions (including land use) were 36.7 billion tonnes of CO 2 ; emissions from transport were 7.9 billion tonnes of CO 2 . Transport therefore accounted for 7.9 / 36.7 = 21% of global emissions.

The IEA looks at CO 2 emissions from energy production alone – in 2018 it reported 33.5 billion tonnes of energy-related CO 2 [hence, transport accounted for 8 billion / 33.5 billion = 24% of energy-related emissions.

IEA (2020), Energy Technology Perspectives 2020 , IEA, Paris.

Davis, S. J., Lewis, N. S., Shaner, M., Aggarwal, S., Arent, D., Azevedo, I. L., ... & Clack, C. T. (2018). Net-zero emissions energy systems . Science , 360(6396).

Cecere, D., Giacomazzi, E., & Ingenito, A. (2014). A review on hydrogen industrial aerospace applications . International Journal of Hydrogen Energy , 39 (20), 10731-10747.

Fulton, L. M., Lynd, L. R., Körner, A., Greene, N., & Tonachel, L. R. (2015). The need for biofuels as part of a low carbon energy future . Biofuels, Bioproducts and Biorefining , 9(5), 476-483.

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News from the Columbia Climate School

Unraveling the Interconnections Between Air Pollutants and Climate Change

Anuradha Varanasi

In June 1991, Mount Pinatubo in the Philippines erupted for nine hours, ejecting volcanic ash, water vapor, and at least 15 to 20 million tons of noxious sulfur dioxide gas into the stratosphere. Within two hours, the gas transformed into tiny sulfate mists or aerosols that formed bright clouds. Those clouds spread across the entire Earth and persisted for a year, effectively reducing global temperatures by 0.4 to 0.5 degrees Celsius between 1992 and 1993. Once these cooling aerosols fell out of the stratosphere two years later, global temperatures rose again.

Although microscopically tiny, aerosol particles can have mighty impacts on the atmosphere and climate. Major volcanic eruptions and their resulting aerosol emissions high up in the atmosphere are infamous for altering monsoon circulations and precipitation patterns around the world, even triggering severe droughts in Eastern China and India .

Aerosols created by burning fossil fuels can also impact the climate, although the effects are somewhat different at the ground level. And as human civilizations attempt to reduce their emissions of these harmful particles, they are inadvertently generating unwelcome side effects, too.

Understanding aerosols

Ever since the first Earth Day was observed in 1970, the global average temperature has been accelerating at the rate of 1.7 degrees Celsius per century. Before 1970, the rate of warming was only 0.01 degrees C per century . At the current rate, the Intergovernmental Panel on Climate Change (IPCC) warned that the average global temperatures could rise by more than 2 degrees Celsius by 2100, which would unleash devastating impacts on the planet .

“When we talk about the causes of human-driven climate change, a lot of attention is given to greenhouse gases like carbon dioxide and methane, but the anthropogenic aerosols component is rarely mentioned,” said Scott Barrett , a vice dean at Columbia University’s School of International and Public Affairs and the Lenfest-Earth Institute Professor of Natural Resource Economics.

Aerosols (also known as particulate matter or PM) are a mix of suspended liquid and solid particles in the air with distinctive chemical compositions. The smaller the size of an aerosol, the more severe its health impacts. Particulate matter with a diameter of less than 2.5 microns (PM2.5) can easily infiltrate the lungs. PM2.5 has been associated with higher rates of respiratory , autoimmune , and neurological disorders than a comparatively bigger PM with a diameter of 10 microns or less — also known as PM10 .

Scientists estimate that 90 percent of aerosols in the atmosphere are naturally occurring, such as dust, pollen, plankton, and sea salt. On average, up to 80 percent of the particulate matter in coastal areas comes from sea salt. Waves breaking and bubbles bursting at the ocean surface make sea salt aerosols stay suspended in the air, said Faye McNeill , an atmospheric chemist and professor at Columbia University’s School of Engineering. The good news is that most natural sources of aerosols have remained at constant levels without any significant fluctuations — giving less cause for concern.

But anthropogenic or human-made aerosols are the opposite. They are constantly emitted from vehicles, coal power plants, factories, oil refineries, agricultural areas, industrial facilities, ships, and wood burning, among other activities. Since the industrial revolution began in the Global North, the presence of anthropogenic aerosols in the atmosphere had steeply increased along with greenhouse gases. As the air got more and more polluted in the U.S., by 1970, the general public and environmentalists were concerned over poor air quality.

Despite the obvious sources of air pollutants, in 1981, President Ronald Reagan claimed that trees cause more pollution than automobiles do. “This led some people to believe that cutting down all the trees will reduce air pollution. Obviously, that is not the solution,” said McNeill, who leads Columbia Climate School’s Clean Air Toolbox for Cities , a project that is working toward cleaner air in Jakarta, Indonesia, Indore, India, and Nairobi, Kenya.

“It is true that trees emit volatile organic compounds. But unhealthy levels of ozone pollution form only after these naturally occurring volatile organic compounds react with nitrogen oxides — which get emitted when coal, oil, and natural gas are burned,” added McNeill.

The majority of anthropogenic aerosols are made in the atmosphere from gas molecules. For example, during the coal burning process, the sulfur present in coal becomes oxidized and gets released into the atmosphere as sulfur dioxide gas. The gas then reacts with clouds, water vapor, and other pre-existing compounds before it transforms into sulfate aerosols that have a cooling effect on the lower atmosphere.

“Various chemical and physical transformations lead to the polluted state that we would see in an urban area,” McNeill explained.

Aerosols: A double-edged sword

In the United States, sulfur dioxide emissions gained widespread attention in the 1970s due to acid rain. When sulfur dioxide mixes with water in the air, it results in sulfuric acid raining down on those locations.

At the time, industrialized countries in the Global North were collectively emitting such high levels of sulfur dioxide from their coal power plants and vehicles that it was the equivalent of over a dozen Mount Pinatubo volcanic eruptions.

The U.S. federal government implemented the Clean Air Act during the 1990s to clamp down on the sources of sulfur dioxide pollution and prevent acid rain pollution. In Europe and Canada, governments mandated that scrubbers should be installed on all industrial smokestacks. Countries in the Global North also passed legislation that made it compulsory for vehicle owners to use exhaust emission control devices. Hefty fines were imposed on polluters. These regulations worked.

For more than three decades, the Global North witnessed a dramatic decrease in PM2.5 and ozone pollution levels. The U.S. Environmental Protection Agency reported an 80 percent decline in anthropogenic sulfur dioxide emissions between 1990 and 2014. Within the same period, deaths related to air pollution in the U.S. were halved . Forests that were damaged from acid rain started recovering .

Even though reducing aerosol emissions has immense public health and ecological benefits, researchers say it is crucial to take into account the impact of such reductions on climate change. While the Global North succeeded in cutting down aerosol pollution, they continued burning huge amounts of fossil fuels like coal. That resulted in the warming of the northern hemisphere.

“Before these policies were enforced, industrialized countries were increasing their carbon dioxide emissions at the same pace as they were increasing the levels of atmospheric aerosols,” explained McNeill. “But then they disrupted the cooling effects of short-lived aerosols by cutting down on their sulfur dioxide and nitrogen oxide emissions.”

Scientists refer to this phenomenon as uncovering global warming. Before industrialized countries collectively got rid of sulfate aerosols, warming was already occurring on a global scale — albeit at a slower pace.

The only way to simultaneously deliver benefits for public health and climate change is by transitioning to renewable energy, said McNeill. In that case, both anthropogenic aerosols and greenhouse gas emissions would be reduced.

Similar to sulfate particulate matter, other anthropogenic aerosols like nitrates and airborne microplastics also scatter and deflect solar radiation back to space, leading to atmospheric cooling. Certain forms of organic carbon could also have a net cooling influence by scattering sunlight away from the Earth’s surface.

On the other hand, black carbon and brown carbon absorb sunlight and have a warming influence on the planet — so cutting their emissions has dual benefits for public health and the planet’s temperature.

The aftermath of phasing out sulfur dioxide emissions

To better understand the complex relationship between aerosols and climate change, Columbia researchers analyzed the impacts of drastically lower levels of sulfur dioxide emissions in the northern hemisphere on other parts of the world.

Interestingly, they found that cleaner air in the Global North ended up influencing monsoon patterns in Africa’s Sahel region and South Asia in entirely different ways.

“Lower levels of sulfate aerosols in the northern hemisphere ended up changing the energy balance of the Earth’s system and affected the dynamics of how air moves around the planet. That has far-reaching impacts on the southern hemisphere,” explained Arlene Fiore , formerly an atmospheric scientist at Columbia Climate School’s Lamont-Doherty Earth Observatory, who is now a professor at the MIT Center for Global Change Science.

Dan Westervelt , an atmospheric scientist at Lamont-Doherty Earth Observatory, observed with colleagues that once the northern hemisphere started experiencing faster warming, the tropical rain belts shifted in a northward direction — thereby resulting in substantially more rainfall in Africa’s Sahel region. While local aerosol emissions might also be playing a role in these anomalies, rainfall patterns have become far more erratic than usual in most of the Sahel.

“These interconnections are concerning and fascinating all at the same time. What we do in the northern hemisphere affects other regions and can have downstream impacts,” added Fiore.

But African countries are not the only ones grappling with the side effects of lower concentrations of anthropogenic aerosols in the northern hemisphere.

In a 2022 study published in the journal Science Advances , researchers proved that the North Atlantic has also witnessed more extreme weather events with decreasing aerosol emissions. Since the implementation of the Clean Air Act more than 30 years ago, hurricane seasons became more frequent and intense in the North Atlantic region compared to prior decades.

On the other side of the globe, industrialization and economic growth have significantly increased the concentration of sulfate aerosols in India and China over the last four decades. This had a cooling influence on the land surface despite global warming. The difference between the temperatures of the land and ocean also decreased. That, in turn, drove down the intensity of monsoonal winds and resulted in fewer tropical cyclones and typhoons in South and East Asia in that time span, said Westervelt.

Westervelt’s work showed that on the Indian subcontinent, higher levels of sulfate aerosol emissions caused less rainfall over the Indo-Gangetic plain. At present, Bangladesh , Pakistan , and India have the most polluted air in the world.

Pick your poison: PM 2.5 vs. ozone pollution

In 2014, the Chinese government announced it was “ declaring war against air pollution .” Four years after allocating billions of dollars for clean air, major cities in China successfully cut down their PM2.5 concentrations by 32 percent . Unfortunately, that positive development led to a negative outcome: a spike in ozone pollution.

Ozone pollution forms when nitrogen oxides (that are emitted from burning fossil fuels) and volatile organic compounds react with each other in the presence of sunlight. Unlike the ozone layer high up in the atmosphere that protects us from harmful ultraviolet rays, on-the-ground ozone pollution is a threat to public health.

Repeated exposure to ground-level ozone can trigger chest pain and coughing, reduce lung function, and may permanently damage lung tissue. Ozone pollution has also been associated with cardiovascular disease and stroke . Westervelt and colleagues have calculated that cutting down ozone pollution by 60 percent would save 330,000 lives in China by 2050 .

Climate change has made ozone pollution a lot worse than before in many parts of the world. Warmer temperatures ramp up reactions between nitrogen oxides and volatile organic compounds that get trapped in the lower atmosphere. Researchers observed that getting rid of aerosols or fine particulate matter is another reason why Chinese cities are experiencing a spike in ozone pollution.

Previously, high levels of PM2.5 in the air acted like sponges that efficiently absorbed the radicals responsible for generating ozone pollution. The aerosols consistently inhibited ozone production. By aggressively tackling the sources of sulfur dioxide emissions, China inadvertently tinkered with the atmosphere’s chemistry. Once the sulfate-dominated PM2.5 concentrations started depleting, more sunlight and radicals were left behind to produce ground-level ozone, according to a study published in PNAS .

The need to address global inequalities

Climate scientists and innovators worldwide are grappling with the multiple challenges involved in reducing emissions of greenhouse gases and air pollutants while also promoting healthy economic development.

“This issue has not been prioritized during international climate negotiations. The focus is still on what individual countries could do for reducing their greenhouse gas emissions,” said Barrett. “Policymakers need to develop an approach that addresses both the economic interests of developing countries like India, as well as the collective interests of other nations.”

He emphasized that India — one of the world’s top three emitters of greenhouse gases — desperately needs financial and technological support from industrialized countries. This will prevent India from replicating China’s history of unsustainable growth that not only undermines India’s future development but also that of the rest of the world — thanks to greater surges in aerosol and greenhouse gas emissions.

While there are no straightforward solutions for tackling such complex and nuanced issues, experts like McNeill say it highlights how proposed geoengineering technologies (that would use aerosols to temporarily reduce global warming) can have unintended consequences.

“Every country is interconnected through the atmosphere and global trade,” added Barrett. “We need to transform the economic system for better mitigation strategies.”

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There are very many reasons to stop burning coal, especially since much of the world plants more trees not realizing the dangerous interplay with sulfur from coal. I’ve heard there is a humid haze composed of water vapour clinging to pollutants without sufficient weight to coalesce in rain. This humid haze keeps the night time radio window closed so the erth doesn’t cool down overnight. Where are studies dealing with this? If true it may be fixable,giving the tropics some relief

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The world has been its hottest on record for 10 months straight. Scientists can't fully explain why

One of the world's leading climate scientists says the world could be in "uncharted territory", with the researchers unable to fully explain why the world has been breaking heat records to such extremes for 10 months straight.

Last month was the hottest March on record, marking the 10th month in a row to reach that title, according to the European Union's key climate service Copernicus. 

In Europe, the temperature for March was 2.12 degrees Celsius above the historical average, marking the second-warmest March on record for the continent.

Around the rest of the globe, temperatures were furthest above average over parts of Antarctica, Greenland, eastern North America, eastern Russia, Central America, parts of South America, and southern Australia.

The continuation of record-breaking heat comes after 2023 was officially declared the hottest year on record, by a long way.

NASA's senior climate advisor Gavin Schmidt says while climate change and the onset of El Niño explain a significant portion of last year's heat, together with other contributing factors, there is still a margin of heat at the top that can't be explained.

He said that was concerning.

"If we can't explain what's going on, then that has real consequences for what we can say is going to happen in the future," Dr Schmidt said.

Predictions 'failed ugly'

For about a decade, he and other climate science institutes have been making predictions of global temperatures for the year ahead.

Each has a slightly different method for doing this.

Generally, it's done by looking at the baseline of global warming that the world is starting the year on, and then factoring in known climate drivers.

But all of those predictions for 2023 fell short of what occurred – the closest prediction was still almost 0.2 degrees Celsius off the mark.

It may not sound like much, but Dr Schmidt said in the context of the world's climate, it's huge.

"Those predictions, based on what was happening right at the beginning of the year failed ugly."

Dr Schmidt said there was always room for error, but usually scientists could explain what occurred upon looking back at the data.

He said this time it was not adding up. And the climate models were giving them no answers either.

"It means there's something missing in what we're thinking about here," he said.

"Either something has changed in the system and things are responding differently to how they responded in the past, or there are other elements that are happening that we didn't take into account."

What are the possible explanations?

Scientists have been investigating several different possible explanations for the higher-than-expected global heat.

Air pollution

Among them, is the theory that the amount of air pollution around the world is less than what the climate models had been accounting for, thanks to new international shipping regulations.

Many aerosols act like a "shade" to incoming sunlight, reflecting it into space. So, fewer of them would have a warming effect.

But Dr Schmidt said, while it made some difference, it didn't seem to be enough to explain just how hot it had been.

"When you put that into a model and you say, 'Is that warming effect large enough to give you this the big difference between 2022 and 2023,' the answer is no, not really as far as we can tell," he said.

The underwater volcano

Another factor that has been looked at is the underwater volcano Hunga Tonga-Hunga Ha'apai eruption in January 2022, which shot ash and other particles more than halfway into space.

Similar to pollution, volcanoes generally have a cooling effect.

But the Tonga volcano was different.

Because it was an underwater volcano, it also ejected a significant amount of water vapour – a strong greenhouse gas — into the stratosphere, and therefore is thought to have had a net warming impact.

Dr Schimdt said from what they could tell so far, this still only represented a very small change, overall.

"The magnitude of the change is in the hundredths of a degree level, so not commensurate with the size of the thing we're trying to explain," he said.

The solar cycle and other explanations

Some have looked to the solar cycle for a mean of explanation, which is reaching solar maximum – something that can also have an impact on surface temperature.

Solar maximum refers to the period of greatest solar activity during the sun's 11-year solar cycle.

But again, Dr Schmidt said it was not large enough to explain what they had seen in 2023, and it was "baked into the calculations" anyway.

"And maybe it was just random things happening in the Antarctic, and in the North Atlantic, all at once, that were unconnected and are adding up, and the reason we haven't seen it before is because we haven't had 200 years of good data," he said

"We're looking into those kinds of things as well."

A previous climate mystery

A similar climate model mystery has played out before, according to Dr Schmidt.

In the early 2000s, the trend of rising surface temperatures appeared to plateau for over a decade, despite greenhouse gas emissions in the atmosphere reaching record levels.

It was something that climate scientists couldn't fully explain at the time, becoming known as the "global warming hiatus".

It was also used heavily by climate change sceptics as evidence that the earth wasn’t getting much hotter.

However, later studies revealed there was no hiatus in global warming, rather it was being buried in the deep layers of the oceans.

Minor revisions to data inputs, uptake of heat by the oceans, natural variability and observations helped make that clear.

Dr Schmidt said it was possible something of a similar nature was happening this time too, and that the climate models were missing something, or the data wasn't quite right.

"Perhaps we haven't fully characterised the Hunga Tonga volcano, or perhaps we haven't been tracking appropriately the emissions from China, because they're not necessarily the most trustworthy of global reporters," he said.

He said it's important they work it out so they could tell whether this was simply a "blip" or the start of something different.

On this, he said the global temperatures during the northern hemisphere could give them some clues.

All eyes on northern summer

So far, the heat of 2024 has been largely in line with expectations, according to Dr Schmidt, because scientists expect a boost to global temperatures a few months after the peak of El Niño.

But he said if everything was behaving as normal, it would cool down by June.

"The key will be what happens in the next few months. If things stay super anomalous then we are looking at a systematic change, not just a blip," he said.

In the meantime, he said they will be re-examining data sets, including looking at newly available aerosol data from a recently launched NASA satellite, to try to explain the gap.

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Global Greenhouse Gas Overview

On This Page:

Global Emissions and Removals by Gas

Global emissions by economic sector, trends in global emissions, emissions by country.

At the global scale, the key greenhouse gases emitted by human activities are:

• Carbon dioxide (CO 2 ) : Fossil fuel use is the primary source of CO 2 . CO 2 can also be emitted from the landscape through deforestation, land clearance for agriculture or development, and degradation of soils. Likewise, land management can also remove additional CO 2 from the atmosphere through reforestation, improvement of soil health, and other activities.
• Methane (CH 4 ) : Agricultural activities, waste management, energy production and use, and biomass burning all contribute to CH 4 emissions.
• Nitrous oxide (N 2 O) : Agricultural activities, such as fertilizer use, are the primary source of N 2 O emissions. Chemical production and fossil fuel combustion also generates N 2 O.
• Fluorinated gases (F-gases) : Industrial processes, refrigeration, and the use of a variety of consumer products contribute to emissions of F-gases, which include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF 6 ).

Additional compounds in the atmosphere including solid and liquid aerosol and other greenhouse gases, such as water vapor and ground-level ozone can also impact the climate. Learn more about these compounds and climate change on our Basics of Climate Change page .

Global greenhouse gas emissions can also be broken down by the economic activities that lead to their atmospheric release. [1]

• Electricity and Heat Production (34% of 2019 global greenhouse gas emissions): The burning of coal, natural gas, and oil for electricity and heat is the largest single source of global greenhouse gas emissions.
• Industry (24% of 2019 global greenhouse gas emissions): Greenhouse gas emissions from industry primarily involve fossil fuels burned on site at facilities for energy. This sector also includes emissions from chemical, metallurgical, and mineral transformation processes not associated with energy consumption and emissions from waste management activities. (Note: Emissions from industrial electricity use are excluded and are instead covered in the Electricity and Heat Production sector.)
• Agriculture, Forestry, and Other Land Use (22% of 2019 global greenhouse gas emissions): Greenhouse gas emissions from this sector come mostly from agriculture (cultivation of crops and livestock) and deforestation. This estimate does not include the CO 2 that ecosystems remove from the atmosphere by sequestering carbon (e.g. in biomass, soils). [2]
• Transportation (15% of 2019 global greenhouse gas emissions): Greenhouse gas emissions from this sector primarily involve fossil fuels burned for road, rail, air, and marine transportation. Almost all (95%) of the world's transportation energy comes from petroleum-based fuels, largely gasoline and diesel. [3]
• Buildings (6% of 2019 global greenhouse gas emissions): Greenhouse gas emissions from this sector arise from onsite energy generation and burning fuels for heat in buildings or cooking in homes. Note: Emissions from this sector are 16% when electricity use in buildings is included in this sector instead of the Energy sector.

Note on emissions sector categories.

Emissions of non-CO 2 greenhouse gases (CH 4 , N 2 O, and F-gases) have also increased significantly since 1850.

• Globally, greenhouse gas emissions continued to rise across all sectors and subsectors, most rapidly in the transport and industry sectors.
• While the trend in emissions continues to rise, annual greenhouse gas growth by sector slowed in 2010 to 2019, compared to 2000 to 2009, for energy and industry, however remained roughly stable for transport.
• The trend for for AFOLU remains more uncertain, due to the multitude of drivers that affect emissions and removals for land use, land-use change and forestry.
• rising demand for construction materials and manufactured products,
• increasing floor space per capita,
• increasing building energy use,
• travel distances, and vehicle size and weight.

To learn more about past and projected global emissions of non-CO 2 gases, please see the EPA report, Global Non-CO 2 Greenhouse Gas Emission Projections & Mitigation Potential: 2015-2050 . For further insights into mitigation strategies specifically within the U.S. forestry and agriculture sectors, refer to the latest Climate Economic Analysis report on Greenhouse Gas Mitigation Potential in U.S. Forestry and Agriculture .

In 2020, the top ten greenhouse gas emitters were China, the United States, India, the European Union, Russia, Indonesia, Brazil, Japan, Iran, and Canada. These data include CO 2 , CH 4 , N 2 O, and fluorinated gas emissions from energy, agriculture, forestry and land use change, industry, and waste. Together, these top ten countries represent approximately 67% of total greenhouse gas emissions in 2020.

Emissions and sinks related to changes in land use are not included in these estimates. However, changes in land use can be important: estimates indicate that net global greenhouse gas emissions from agriculture, forestry, and other land use were approximately 12 billion metric tons of CO 2 equivalent, [2] or about 21% of total global greenhouse gas emissions. [3] In areas such as the United States and Europe, changes in land use associated with human activities have the net effect of absorbing CO 2 , partially offsetting the emissions from deforestation in other regions.

EPA resources

• Greenhouse Gas Emissions
• Sources of Greenhouse Gas Emissions (in the United States)
• Non-CO 2 Greenhouse Gases: Emissions and Trends
• Capacity Building for National GHG Inventories

Other resources

• UNFCCC GHG Data Interface
• European Commission Emission Database for Global Atmospheric Research
• World Development Indicators
• Climate Watch
• Carbon Dioxide and Information Analysis Center (CDIAC)
• Greenhouse Gas Emissions from Energy Data Explorer (IEA)

1. IPCC (2022), Emissions Trends and Drivers. In IPCC, 2022: Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK and New York, NY, USA. doi: 10.1017/9781009157926.004

2. Jia, G., E. Shevliakova, P. Artaxo, N. De Noblet-Ducoudré, R. Houghton, J. House, K. Kitajima, C. Lennard, A. Popp, A. Sirin, R. Sukumar, L. Verchot, 2019: Land–climate interactions . In: Climate Change and Land: an IPCC special report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems [P.R. Shukla, J. Skea, E. Calvo Buendia, V. Masson-Delmotte, H.-O. Pörtner, D.C. Roberts, P. Zhai, R. Slade, S. Connors, R. van Diemen, M. Ferrat, E. Haughey, S. Luz, S. Neogi, M. Pathak, J. Petzold, J. Portugal Pereira, P. Vyas, E. Huntley, K. Kissick, M, Belkacemi, J. Malley, (eds.)]. https://doi.org/10.1017/9781009157988.004

3. U.S. Energy Information Administration, Annual Energy Outlook 2021 , (February 2021), www.eia.gov/aeo

Note on emissions sector categories:

The global emission estimates described on this page are from the Intergovernmental Panel (IPCC) on Climate Change's Fifth Assessment Report. In this report, some of the sector categories are defined differently from how they are defined in the Sources of Greenhouse Gas Emissions page on this website. Transportation, Industry, Agriculture, and Land Use and Forestry are four global emission sectors that roughly correspond to the U.S. sectors. Energy Supply, Commercial and Residential Buildings, and Waste and Wastewater are categorized slightly differently. For example, the IPCC's Energy Supply sector for global emissions encompasses the burning of fossil fuel for heat and energy across all sectors. In contrast, the U.S. Sources discussion tracks emissions from the electric power separately and attributes on-site emissions for heat and power to their respective sectors (i.e., emissions from gas or oil burned in furnaces for heating buildings are assigned to the residential and commercial sector). The IPCC has defined Waste and Wastewater as a separate sector, while in the Sources of Greenhouse Gas Emissions page, waste and wastewater emissions are attributed to the Commercial and Residential sector.

• GHG Emissions and Removals Home
• Overview of Greenhouse Gases
• Sources of GHG Emissions and Removals
• Global Emissions and Removals
• National Emissions and Removals
• State and Tribal GHG Data and Resources
• Facility-Level Emissions
• Gridded Methane Emissions
• Carbon Footprint Calculator
• GHG Equivalencies Calculator
• Capacity Building for GHG Inventories

Atmospheric and economic drivers of global air pollution

Carbon monoxide emissions from industrial production have serious consequences for human health and are a strong indicator of overall air pollution levels. Many countries aim to reduce their emissions, but they cannot control air flows originating in other regions. A new study from the University of Illinois Urbana-Champaign looks at global flows of air pollution and how they relate to economic activity in the global supply chain.

"Our study is unique in combining atmospheric transport of air pollution with supply chain analysis as it tells us where the pollution is coming from and who is ultimately responsible for it," said lead author Sandy Dall'erba, professor in the Department of Agricultural and Consumer Economics (ACE) and director of the Center for Climate, Regional, Environmental and Trade Economics (CREATE), both part of the College of Agricultural, Consumer and Environmental Sciences (ACES) at Illinois.

"There is a direct link between a country's level of production and how much air pollution is emitted. But production may be driven by demand from consumers in other countries. We use supply chain analysis to quantify the links between production and consumption. This helps us to understand how production in one country is linked to domestic and foreign demand," he added.

The researchers traced the movement of pollutants through the atmosphere to understand the flow of emissions, using simulations developed by Nicole RIemer, professor in the Department of Climate, Meteorology & Atmospheric Sciences, College of Liberal Arts & Sciences at Illinois. For analytical purposes, they divided the world into five sections: the United States, Europe, China, South Korea, and the rest of the world. South Korea is located downwind of China, and it serves as an example of how a small country can be affected by pollution from a much larger upwind neighbor.

"Over recent years, South Korea has taken several measures to reduce its own pollution, yet it has experienced worsening air quality. Why? The answer is to be found in its upwind neighbor, China. Yet, a large amount of the goods manufactured in China are destined for foreign consumers in the U.S. and in Europe, among other places. As such, who is to be blamed for the increase in air pollution in South Korea? That is the challenge we embarked on with this study," Dall'erba stated.

The researchers found the amount of carbon monoxide emissions coming from China to South Korea increased from 30 teragrams (Tg) in 1990 to 42 Tg in 2014.

"To put these numbers in perspective, 5 Tg of carbon monoxide corresponds to the emissions from all of the cars in the U.S. -- roughly 274 million -- each driving 13,500 miles per year. So it's definitely not a small increase. We conclude that South Korea has, in effect, lost control of their own air quality," Dall'erba explained.

Dall'erba and his colleagues conducted a structural decomposition analysis to identify the economic drivers of carbon monoxide emissions in the five study regions. They found that while China's technological processes to reduce pollution have improved, overall carbon monoxide emissions have gone up because the country's production has increased.

Next, the researchers sought to identify where the demand that drives the increased production comes from. In China's case, some of the increase can be attributed to U.S. and European demand, but it is primarily driven by households in China. The Chinese population grew considerably between 1990 and 2014, and the country became wealthier, leading to higher consumption, Dall'erba noted.

"Our findings show that pollution is a global concern that can't be solved by individual countries. The world is connected, and we're all in this together," said co-author Yilan Xu, associate professor in ACE. "Pollution in one country can result from economic activities in neighboring countries, which in turn is influenced by who's demanding the goods produced in that country. Pollution emitted anywhere in the world is going to have consequences all over the world to varying degrees."

Dall'erba, Riemer, and Xu emphasize that everybody can play a part in reducing emissions. Producers can implement technological change; policymakers can issue regulations or provide incentives; and consumers can make choices that favor sustainable products.

• Air Quality
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• Public Health
• World Development
• Air pollution
• Automobile emissions control
• Climate change mitigation
• Environmental impact assessment
• Global warming
• Climate engineering

Story Source:

Materials provided by University of Illinois College of Agricultural, Consumer and Environmental Sciences . Original written by Marianne Stein. Note: Content may be edited for style and length.

Journal Reference :

• Sandy Dall’erba, Nicole Riemer, Yilan Xu, Ran Xu, Yu Yao. Identifying the key atmospheric and economic drivers of global carbon monoxide emission transfers . Economic Systems Research , 2024; 1 DOI: 10.1080/09535314.2023.2300787

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Gloria Dickie reports on climate and environmental issues for Reuters. She is based in London. Her interests include biodiversity loss, Arctic science, the cryosphere, international climate diplomacy, climate change and public health, and human-wildlife conflict. She previously worked as a freelance environmental journalist for 7 years, writing for publications such as the New York Times, the Guardian, Scientific American, and Wired magazine. Dickie was a 2022 finalist for the Livingston Awards for Young Journalists in the international reporting category for her climate reporting from Svalbard. She is also an author at W.W. Norton.

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Global study reveals health impacts of airborne trace elements

by Shawn Ballard, Washington University in St. Louis

As anyone with seasonal allergies knows, unseen airborne particles can really wreck a person's day. Like the tree pollen that might be plaguing you this spring, small concentrations of trace elements in the air can have significant negative impacts on human health. However, unlike pollen counts and other allergy indices, which are carefully tracked and widely available, limited knowledge exists about the ambient concentrations of cancer-causing trace elements like lead and arsenic in urban areas of developing countries.

A recent effort led by Randall Martin, the Raymond R. Tucker Distinguished Professor in the McKelvey School of Engineering at Washington University in St. Louis, analyzed global ambient particulate matter (PM) to understand two of its key components, mineral dust and trace element oxides. Trace elements—such as lead and arsenic—have well-documented associations with adverse health outcomes. While dust originates from both natural sources like deserts and human activities like construction and agriculture, trace elements are predominantly emitted by human activities like fossil fuel combustion and industrial processes.

Martin's team, including Jay Turner, the James McKelvey Professor of Engineering Education at WashU, and Xuan Liu, a graduate student working with Martin and Turner in the Department of Energy, Environmental & Chemical Engineering, examined data collected by the Surface PARTiculate mAtter Network (SPARTAN), the only global monitoring network that measures PM elemental composition.

Their work, published in ACS ES&T Air , produced a valuable dataset and methodology to identify regions with elevated trace elements. The findings also highlighted regions of concern in Bangladesh, India and Vietnam, which might benefit from interventions to reduce trace element emission from human activities.

"Reliable elemental composition data of ambient PM is crucial to understand the health risks associated with exposure to airborne trace elements," said Liu, the first author on the paper. "Our work highlights the significant health risks caused by elevated levels of airborne trace elements, particularly arsenic, in South and Southeast Asia."

"This work draws attention to the need for sustained consistent monitoring of the elemental composition of fine particulate matter in urban areas worldwide," Martin added. "Identifying potential emission sources of these elements will inform targeted interventions to mitigate exposure and safeguard public health."

Though Martin and his collaborators have found in previous studies that global air pollution from fine particulate matter fell between 1998 and 2019 and strategies like replacing traditional fuel sources with sustainable sources of energy could further curb PM pollution , their SPARTAN analysis points to ongoing concerns regarding exposure to trace elements through inhalation of PM. The team identified informal lead-acid battery recycling, e-waste recycling and coal-fired brick kilns as potential contributors to the elevated concentrations of trace elements particularly in Dhaka, Bangladesh.

More broadly, the team noted that concentrations of trace elements are particularly high in low-income and middle-income countries due to unregulated urbanization and industrialization. However, PM monitoring networks in these areas are spotty at best, hindering researchers' understanding of dust and trace element levels and their emission sources. Uniform sampling methods and reliable analyses are needed to enable comparisons across the world.

"Our growing sample collection will lead to better estimations of dust and trace element concentrations, which will allow us to perform more accurate health risk assessment and thorough investigation into emission sources," Liu said. "Certain SPARTAN sites have been selected or established as part of the Multi-Angle Imager for Aerosols (MAIA) satellite mission dedicated to studying the health impacts of various types of airborne particles. This collaboration will yield a large dataset with increased sampling frequency, helping us identify pollution sources more effectively in the future."

Provided by Washington University in St. Louis

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Global airlines are governed by strict 'freedoms of the air' dating back 80 years. They've created some funky routes.

• Global airlines are governed by nine "freedoms of the air," drafted 80 years ago in 1944.
• These dictate how airlines can operate in foreign nations, with some more complicated than others.
• The fifth freedom can give airlines a competitive edge and help capitalize on demand.

Over the past 100 years of shuttling people around the globe in metal flying machines, the world's aviation network has grown into a vast web of intersecting routes that connect nearly every corner of the globe.

Because of the complexities of crossing international borders, commercial carriers follow what is known as the "freedoms of the air" — or the right for an airline to operate within a nation other than its own.

These building blocks of aviation make international connectivity possible. 

According to the International Civil Aviation Organization, or ICAO , there are five official freedoms and four other "so-called" rights, that have been outlined in agreements between countries. ICAO is an agency of the United Nations that sets standards for the global aviation industry.

Drafted in 1944 during what is known as the Chicago Convention, the laws were written as world governments relaxed their grip on airline networks and pricing. This liberalization, however, meant countries with bigger airlines would likely dominate the skies — prompting them to implement strict route regulations.

The governing freedoms not only promote more competition and choice but also allow airlines to optimize routes and increase efficiency, according to FlightRadar24 . 

Most international carriers except for a very small few follow the basic freedoms of allowing airlines of one state to fly over or land in another, and vice versa.

Open Skies agreements simplify these international routes, like the one between the European Union and the US that allows any airline registered in either market to fly between the two.

Some freedoms are more complicated, but provide interesting and diverse route options to travelers.

The fifth and eighth freedoms of the air

Beyond the first four freedoms, there is one more officially recognized right, as well as the four "so-called" rights. The latter four were not officially drafted during the 1944 Chicago Convention but are regularly accepted and practiced worldwide.

According to ICAO, the fifth freedom gives an airline of one nation the right to fly between two other countries, so long as the one-stop routes start or end in its home country and all parties agree.

Among the most well-known fifth freedom routes are Emirates' flights from New York-JFK to Milan and Newark to Athens, both flying onward to the carrier's base in Dubai.

Similarly, Singapore Airlines flies between New York and Singapore via a stop in Frankfurt , and Australian flag carrier Qantas flies between Sydney and New York via Auckland, New Zealand, according to Google Flights.

United Airlines' delayed fifth freedom route will fly between the US mainland and Cebu, Philippines, via Tokyo starting in October, the carrier told Business Insider on Monday. It was supposed to start in July — before the FAA launched an investigation after a string of safety incidents at United.

These unique routes can be efficient for airlines trying to serve destinations that a plane can't reach nonstop, like Emirates' fifth freedom between Mexico City and Dubai via Barcelona or Latam Airlines' route between Sydney and Santiago, Chile, via Auckland. 

Still, carriers will make stops on otherwise attainable direct flights because they can capitalize on the high-demand market on both legs — filling more seats and making more money.

Customers may also view carriers like Emirates and Singapore as a more luxe offering than the competing US and EU carriers across the Atlantic.

On the other hand, an airline that wants to serve a low-demand market can better fill the plane by adding a fifth-freedom leg to a nearby city, like Dutch flag carrier KLM's flight between Amsterdam and Santiago via Buenos Aires. 

Among ICAO's most interesting "so-called" rights is the eighth freedom, which gives an airline the right to fly between two cities in a country that isn't its own— but the domestic leg seats cannot be sold as the entire journey must start or end in the foreign airline's home nation.

Related stories

Qantas used to fly an eighth freedom route between New York and Sydney via Los Angeles — but only those originating or destined for Australia could fly on the domestic cross-country leg, Forbes reported.

Also no longer flying, per Cirium data, African carrier Air Senegal, for example, launched a flight from Dakar to Baltimore with a layover in New York in 2021. The domestic leg seats couldn't be sold.

Here's a closer look at the freedoms of the air.

"Five Freedom Agreements"

First Freedom

This allows an airline of one nation to fly over another without landing.

Second Freedom

This allows an airline of one nation the right to land in another territory for a technical stop. Think refueling or an inflight mechanical issue that prompted an unplanned emergency landing.

Third Freedom

This allows an airline of one nation to carry passengers to a foreign state, and vice versa.

Fourth Freedom

This allows the airline of one nation to take on passengers originating in another. The fourth freedom is simply the reverse of the third freedom.

Fifth Freedom

This allows an airline of one nation to carry passengers between two countries other than its own so long as the route starts or ends in the carrier's home state.

"So-called" rights

Six Freedom

This allows an airline to carry passengers from one nation to another via its home state. This represents the typical hub-and-spoke network used by global airlines.

Seventh Freedom

The seventh freedom is similar to the fifth freedom but takes out the limitation of where the route must start or end. Instead, an airline has the right to fly between two nations other than its own without flying onward to its home base.

The EU's single-aviation market, for example, grants airlines the right to fly to and from any EU country, like Ireland-based Ryanair that flies between Rome and Vilnius, Lithuania.

Eighth Freedom

This allows an airline to fly between two cities in a foreign country so long as all passengers originate or are destined for the airline's home state.

Ninth Freedom

This cabotage freedom allows an airline of one nation to fly between two points in a separate single country. This does not exist in the US, but it does in the EU — like easyJet's back-and-forth nonstop between Paris and Nice, for example.

Watch: Why Singapore Changi Airport was just named best in the world

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Global warming of 3 degrees Celsius may result in 10% GDP loss: ETH study

It also found that poorer, tropical countries could see the worst effects -- up to 17 per cent gdp loss.

Photo: Bloomberg

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S&p global hikes india's fy24 gdp growth forecast by 40 bps to 6.4%, earth will likely lock into breaching key warming threshold in 2029: report, q2 gdp numbers show resilience and strength of indian economy: pm modi, india's gdp grew 7.6% in jul-sep quarter, higher than rbi mpc projection, 1.5 degrees celsius climate goal: relevance, current warming scenario, prince william returns to duties for 1st time since kate's cancer diagnosis, prince harry declares us as his new home, renounces british residency, g7 foreign ministers seek urgent defence support for desperate ukraine, vestager defends eu merger rules, says competition creates strong cos, prioritising quality, safety of boeing products over production: alaska air.

(Only the headline and picture of this report may have been reworked by the Business Standard staff; the rest of the content is auto-generated from a syndicated feed.)

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