Commercial aviation accounts for less than 3% of global carbon dioxide (CO2) emissions. The relatively low share of global emissions is due to the fact that only a small fraction of the global population travels by air.
An estimated 80% of the world population has never set foot on an airplane and 1% of the global population accounts for 50% of emissions from commercial aviation. According to The International Civil Aviation Organization (ICAO) forecasts, commercial aviation will grow at 4.2% per year globally from 2018 to 2038, and above-global-average growth is projected in the Middle East, Africa, and the Asia/Pacific regions.
Between 2000 and 2015 air travel doubled, and – prior to the COVID-19 pandemic – the number of passenger-trips was projected to grow from 4 billion to over 8 billion by 2035 (IATA 2018). Pictured below is the growth in passenger-trips up to 2018. The graph reveals a resilient growth rate of ~5% with temporary declines due to economic disruptions.
The COVID-19 pandemic caused a global downturn in air travel, but as shown in the graph below, air travel has returned to pre-pandemic levels. The growth trajectory of aviation is expected to continue as previously forecast (IATA, 2024).
Notwithstanding its low share of total emissions, air travel is not a low impact activity. Air travel can contribute a significant proportion of an individual’s or a company’s climate footprint. For example, the average European emits about 9 tons of CO2 per year. If a European takes one transatlantic round-trip economy flight, say from Frankfurt to New York, they will add a bit less than 1 metric tonne of CO2 to their climate footprint due to their share of the fossil fuel combusted by the aircraft. Moreover, the warming effect of CO2 is amplified when emitted at high altitudes. Some studies suggest that high-altitude emissions could amplify radiative forcing by an amount equivalent to 1.7 to 4.3 tonnes of CO2 per passenger (IPCC 1992, Lee et al., 2021, and Bramwell et al., 2023). These figures, however, are subject to high uncertainty.
Accounting for fossil fuel combustion alone, the high relative impact of air travel has led to growing calls for people to voluntarily limit their flying, and to offset emissions from the air travel they cannot avoid. Over time, use of alternative fuels and new technologies may reduce CO2 emissions from air travel, but it may be difficult to avoid the need for offsetting entirely.
Because there will be limited room for continuing to emit greenhouse gases in a net zero world, choosing low-carbon alternatives to air travel such as video conferencing or train travel should be prioritized over the purchasing of carbon credit credits. The Stockholm Environment Institute’s TR2AIL project provides practical information on air travel and strategies for avoiding it.
If flying is a necessity, then to be conservative, some travelers may wish to apply a multiplier (e.g., between 1.7 and 4.3) to determine the number of carbon credits they need to retire, to align with the evidence that high altitude aviation has added short term radiative forcing impacts.
Climate impacts from aviation
In terms of total impact, air travel results in atmospheric warming from carbon dioxide and soot emissions, nitrogen oxide induced ozone formation, and night-time contrail and cirrus cloud formation. To a lesser extent, air travel also results in cooling due to emissions of sulfate aerosols, day-time contrail formation, and methane destruction due to nitrogen oxide emissions.
It is not easy to combine these different effects. Some influences are regional and only last for a few weeks. Others are global and last for centuries. Cloud formation (e.g., cirrus and contrail), which is poorly understood but may have a large impact, is particularly difficult to quantify. In addition, short-lived, regional effects can have enhanced impacts.
The total climate effects of flying are estimated to be 2 to 3 times larger than carbon dioxide emissions alone (EU Commission 2019, UBA 2019). These estimates can vary broadly based upon how the effects are weighted, and the time horizon applied. This explains the wide range of results when assessing the overall climate impact of aviation and when using air travel calculators created by different organizations.
Digging into the science behind these models, the uncertainty exists because parameters used to determine effective radiative forcing (ERF) often do not account for the variability in aviation altitudes and latitudes. For example, the model parameters do not account for the strong regional sensitivity of ozone (which varies at different latitudes), or the ways that methane affects nitrogen oxide, the strongest effects are at low latitudes (nitrogen oxide in stratosphere vs. troposphere creates different chemical reactions).
The climate impacts of jet aircraft emissions are summarized in the graphic below, with values identifying the warming or cooling effect of the pollutant produced by air travel.
Note that the non-carbon dioxide emission-climate forcers identified in the graphic above, are highly uncertain and variable based on many factors relating to when and where fuel combustion occurs (e.g., latitude, region, altitude) and relating to metrics that are not tracked or reported (e.g., water vapor and soot).
There are four main ways in which aviation affects the climate:
- Carbon dioxide emissions from jet fuel combustion
- Indirect impacts from nitrogen oxide emissions
- Particulate emissions from aviation: sulfates and soot
- Formation of contrails and cirrus clouds (water vapor)
Our recommendation:
When looking to offset air travel, we recommend using a multiplier of 3 to account for all climate effects (Lee et al, 2020). Applying a multiplier should go hand in hand with purchasing high-quality offset credits.
Further, choosing low-carbon alternatives to air travel such as video conferencing or train travel should be prioritized over purchasing offset credits. The Stockholm Environment Institute’s TR2AIL project provides practical information on air travel and strategies for avoiding it.
If you still must fly, carbon offset purchases should be incorporated within organizational or company level strategies to achieve emission reductions. These strategies should prioritize internal reductions and supply chain or product emission reductions over carbon offset purchases.
Carbon dioxide emissions from jet fuel combustion
CO2 is emitted during the combustion of kerosene jet fuel (referred to as ‘jet fuel’): 3.16 kg CO2 are emitted per kilogram of jet fuel combusted (ICAO, 2017).
The CO2 emissions during the production of kerosene (including transport and refinery processes) adds approximately another 0.5 kg CO2 per kg of jet fuel (myclimate 2014). A round-trip flight from Frankfurt to New York burns about 156,500 kg of jet fuel. Including upstream production emissions, this results in about 570 tonnes of CO2 for a round-trip voyage, or an average of 870 kg CO2 per economy-class passenger, taking into account that some emissions are attributed to cargo (calculated based on ICAO calculator).
Carbon dioxide stays in the atmosphere for a long time. About half of the emissions are absorbed by oceans and forests within 30 years, another 30% is removed within a few hundred years and the remaining 20% will typically stay in the atmosphere for many thousands of years.
Carbon dioxide also spreads globally and affects climate independent of where and at which altitude the emissions originated. CO2 is the main anthropogenic (human-induced) GHG and its warming effects are well understood; it is therefore often used as the basis for comparison of all other emission effects.
Indirect Impacts from Nitrogen Oxide Emissions
During jet kerosene combustion airplanes also emit nitrogen oxides (NOx). NOx affects the atmospheric concentrations of two GHGs:
- Methane (CH4) is a GHG that remains in the atmosphere for at least 10 years. Over a 100-year timeframe, methane is about 34 times as powerful as CO2 (IPCC, 2013).
- Ozone (O3) is a GHG that remains in the atmosphere for 2-8 weeks. The formation of O3 by aircraft is similar to the formation of smog by road traffic. But due to increased UV radiation at high altitudes, O3 is formed more effectively than on the ground.
The atmospheric chemistry involved is complex. To put it simply: NOx emissions from air travel lead to an initial increase in CH4 (warming) that persists over a couple of months. It is followed by a longer-term (decadal timescale) decrease in CH4 (cooling) and O3 (cooling). The later decrease in CH4 and O3 does not outweigh the initial increase in O3. Therefore, NOx emissions produce a net warming effect (Lee, 2018).
NOx causes climate effects that are relatively short-lived. Short-term effects like these are relevant when the activities that cause them to persist or increase. Although there is no accumulation, like for longer-lived GHGs.
Particulate emissions from aviation: sulfates and soot
Aircraft also emit soot (black carbon) and sulfate aerosols. Dark soot particles absorb solar radiation and therefore have a warming effect. This effect is particularly strong when soot is deposited on snow and ice, thus darkening the light surface and decreasing its albedo effect (dark surfaces absorb more radiation than light surfaces).
Sulfate aerosols lead to direct cooling, as they reflect sunlight. On the other hand, sulfate particles can also lead to cloud formation. Water vapor in saturated air can condense on these particles, resulting in contrails and cirrus clouds. This in turn leads to warming because additional heat is stored in this way.
The direct effect of particles from aircraft is short (hours to days), fairly well understood, and is estimated to be small. Indirect effects on clouds, however, are still poorly understood (Lee, 2018).
Formation of contrails and cirrus clouds (water vapor) from aviation
Clouds can have either a cooling or a warming effect:
- They can cause warming by trapping long-wave (infrared) radiation from the Earth, and
- They can cause cooling by reflecting short-wave (visible and ultraviolet) solar radiation back into space.
Overall, however, clouds caused by air travel emissions have a significant net warming effect (Lee, 2018).
Contrails
Contrails are linear ice clouds formed in the wake of aircraft. Contrails are formed by soot aerosol particles and water vapor emissions, that allow background water vapor to condense on the soot aerosol particles to form ice crystals. Whether contrails form depends on the flight altitude as well as on the temperature and humidity of the atmosphere. About 10 to 20% of all flights cause contrails, and they are short-lived.
The warming effects of contrails are different during the day than at night. During the day, contrails trap infrared radiation (a warming effect) and reflect solar radiation (a cooling effect). At night, they only trap infrared radiation and there is no cooling effect. Because contrails are short-lived, formed in areas of high air traffic density, and can affect existing cirrus clouds, they may cause local or regional climate responses. Globally, contrails have been found to have an overall warming effect (Lee, 2018). The effect is so large today that it exceeds the total warming influence of all of the CO2 emitted by aircraft since the beginning of powered flight (EESI Fact Sheet, 2019).
Cirrus clouds
Under certain conditions, contrails can result in the formation of cirrus cloud cover. Cirrus clouds are composed of ice crystals and occur above 6 km, covering approximately 30% of the Earth’s surface. On balance, they absorb solar radiation and lead to warming. Air travel-induced cirrus clouds therefore significantly increase the climate effect of flying. Research shows that the influence of contrails and cirrus clouds on the climate is growing significantly, but that it is still difficult to model the overall effect well (Bock et al 2019, Tesche et al 2016).
Airline passenger footprints
It is relatively straightforward to calculate the carbon dioxide emissions of a flight, as it is directly related to the amount of kerosene jet fuel (referred to as ‘jet fuel’) burnt. Yet, assigning the emissions to a single passenger requires taking into account several factors, the most important are:
- Flight distance
- Aircraft and engine type
- Cargo versus passengers
- Seat occupancy rate
- Seat class
Flight Distance
Flight distance is an essential factor in determining jet fuel consumption. Generally speaking, the farther the route, the more fuel burned. However, since takeoff and landing demand higher fuel burn rates than level flight, shorter routes where takeoff and landing comprise a larger portion of the overall flight tend to be less efficient (i.e., require more fuel per kilometer). Takeoff and landing are smaller portions of the overall flight for medium range routes, so they are generally more efficient. But, over very long distances the fuel use per mile increases because of the greater amount of fuel that has to be carried during the early stages of flight.
To calculate route length the great circle distance (the shortest distance between two points on the globe) between two airports can be used. Since an aircraft’s route is normally not an exact match with the great circle calculation method due to flight path routing, detours around weather, and delays due to air traffic, it is good practice for calculators to add an extra amount to the overall distance.
Aircraft and engine type
Jet fuel consumption varies considerably by aircraft model and engine type. To calculate per passenger emissions it is possible to distinguish between aircraft models or to use average numbers.
Cargo vs. passengers
The majority of a flight’s total weight is the aircraft itself and the fuel it carries. ‘Payload’ is defined as the weight of the people and items that are being transported, including passengers, their luggage, and cargo.
Passenger aircrafts often also transport considerable amounts of cargo, in particular in wide-body aircrafts on long-haul flights. Cargo consists of freight and mail.
Usually emissions are assigned to cargo and passengers based on their percentage of total cargo weight. For example, if 80% of the payload weight is passengers and 20% is cargo, then 20% of total flight emissions are subtracted from the passengers’ carbon footprint.
For each passenger an industry-wide standard of 220lbs (100kg) is assumed including their luggage. An additional 110lbs (50kg) per passenger is added for passenger infrastructure such as toilets, galleys, and crew.
Passenger to cargo ratio can vary greatly, on intra-European flights passengers make up more than 96% of the payload, on Europe-Asia flights they make up 64 – 80% (ICAO 2017). It is good practice for calculators to account for cargo load based upon historical data.
Seat occupancy rate (passenger load factor)
Emissions per passenger depends on how many passengers there are. Longer flights tend to be more efficient (fewer emissions per passenger-kilometer) because these flights use larger aircraft with higher seating capacities (BDL 2015). However, not all flights are fully occupied. Seat occupancy rate (also called passenger load factor) is the ratio of passengers to available seats onboard a given flight.
On a flight with low occupancy rates, the total emissions have to be divided by fewer passengers resulting in a higher per-person carbon dioxide footprint. Usually, emissions calculators use average occupancy rates by the air carrier.
Occupancy rates vary significantly depending on the route. They are highest for flights between Central America and Europe (83%) and lowest for flights between Africa & the Middle East and South America (60%) (ICAO 2017).
Seat class
Seat class is a key factor in determining the emissions an individual is accountable for on a given flight. A plane that is configured with all economy-class seats accommodates the highest number of passengers. First- and business-class seats take up more space and therefore fit fewer passengers. This means first- and business-class seats are assigned a greater proportion of the overall plane’s weight. Emissions should therefore be allocated to passengers according to their seat class. These cabin class weighting factors can be calculated for each aircraft type. A multiplier of about 1.5 for business class seats and of 2 to 3 for first-class should be used. Calculators may distinguish between up to three tiers of seat class.
Results
Taking all of the above factors into account, jet fuel consumption per passenger tends to vary according to distance and occupancy rate. For illustrative purposes, average fuel consumption is about (BDL 2015):
- 2 to 6.8 liters per 100 passenger-kilometers over short distances (<800 km),
- 6 to 4.2 liters per 100 passenger-kilometers over medium distances (800 – 3,000 km), and
- 9 to 3.5 liters per 100 passenger-kilometers over long distances (> 3,000 km)
…before applying any multiplier for seat class.
Don’t airlines already offset their GHG emissions?
Prospective carbon credit buyers may have heard that the aviation industry is already taking steps to reduce their GHG emissions. While this is true, the reality is that regulatory coverage of aviation emissions is highly uneven, and even where it exists it does not fully address their climate impacts.
Aviation GHG emissions have been a subject of international climate negotiations for decades, but reducing these emissions has proven challenging. The Kyoto Protocol called for richer countries to limit emissions from international aviation, working through the International Civil Aviation Organization (ICAO). In 2008, seeing little progress under ICAO, the European Union (EU) adopted legislation to include emissions from both domestic flights (within the EU) and international flights (to or from the EU) in its Emissions Trading System (the EU ETS). However, the international aviation industry objected to the inclusion of international flights, and in 2012 the EU granted an exemption for them, allowing ICAO time to develop its own emissions reduction plan.
ICAO subsequently established a goal of “carbon-neutral growth” in international aviation emissions after 2020. Under this framework, emissions may continue to rise overall, but participating airlines agree to reduce what they can (through improved efficiency and use of alternative jet fuels) and offset any remaining emissions. In 2016, ICAO adopted the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) to help achieve this offsetting goal.
While the EU and ICAO regulatory efforts are laudable as far as they go, individual flyers concerned about their carbon footprints should keep in mind several qualifications:
- Not all airlines will initially participate in CORSIA. As of April 2020, 83 countries – representing just over ¾ of international aviation activity – plan to participate in CORSIA’s pilot phase (2021-2023) and first formal phase (2024-2026). Notable exceptions include China, India, and Brazil. For international flights on airlines based in non-participating countries, emissions will not be offset by CORSIA.
- CORSIA will only address a portion of emissions. As noted, the goal of CORSIA is to offset any growth in emissions from international aviation after 2020. The baseline for measuring this growth is set at 85% of the 2019 annual emissions (this was initially proposed to be the average annual emissions from 2019-2020, but because of the downturn in air travel caused by the COVID-19 pandemic in 2020 the baseline was adjusted). This decision means that net emissions from international aviation will likely continue to be around 600-700 million metric tonnes of CO2 per year, leaving plenty of emissions for individual travelers to voluntarily offset (CORSIA is set to run through 2035).
- Outside the EU, there is little – if any – regulation of GHG emissions from domestic flights. There are no parallel restrictions in other major countries for emissions from domestic flights, which leaves a large percentage of aviation emissions uncovered – about 2/3rds of global GHG emissions from commercial aviation are from domestic flights (ICCT, 2018).
- Regulation by the EU ETS will not eliminate emissions from domestic flights. Under the EU ETS, all airlines operating in Europe are required to monitor, report, and verify their emissions, and to surrender an equal amount of allowances (emission permits). The EU ETS aims to make CO2 intensive activities, such as flying, less cost-effective. But the price pressure has been limited so far, and domestic flights still result in net emissions.
- None of these regulatory systems account for the knock-on effects of combusting jet fuel at high altitudes. As noted in the preceding pages, aviation emissions have a greater impact on climate than the simple radiative forcing effect of CO2. Yet regulatory approaches so far have addressed aviation emissions as if they were equivalent to emitting CO2 at ground level. Thus, even with complete regulatory coverage, it would still be advisable to avoid air travel and – where avoiding it is not possible – offset its (remaining) climate impact.