Climate impact of air transport

The climate impact of air transport is quite important, but difficult to evaluate precisely. In fact, in addition to carbon dioxide (CO2), a relatively easy-to-account greenhouse gas with emissions accounting for 2-3% of global emissions , aircraft are responsible for other emissions whose contribution to the greenhouse effect is not assessed with as much precision. In particular, this concerns emissions of nitrogen oxides (NOx) which indirectly cause the warming of the climate and especially contrails and cirrus that form under certain conditions, which also cause a warming. On the other hand, the very short lifetimes (from a few minutes to a few days) contrails, cirrus and ozone produced by the degradation of (NOx) do not allow to simply aggregate their effects to that of CO2 which has a lifespan of 100 years. However, they must be counted because their impact is important and will be felt as long as there are planes in the sky.

To consolidate the effects of all anthropogenic emissions, the IPCC uses radiative forcing, which measures the impact of past and present activities on global temperature. It estimated that aviation radiative forcing accounted for 4.9% of the total radiative forcing from 1790 to 2005, which is about three times more than the impact of CO2 alone.. With the rapid and continuous growth of air transport (about 5% per year), and the inability of the airline industry to compensate for it at the same rate with technical improvements, its climate impact continues to grow.

After more than 15 years of negotiations, a global agreement to reduce the climate impact of air transport was concluded on 6 October 2016 under the auspices of the International Civil Aviation Organization (ICAO). It aims to address the lack of air transport measures in the 2015 Paris Agreement and to achieve the goals set by the organization in 2010: to improve energy efficiency by 2% per year and to stabilize CO2 emissionsthe level they will have achieved in 2020. It establishes for this purpose a system for offsetting CO2for the fraction of emissions that would exceed the level reached in 2020 despite a “basket of technical measures” adopted at the same time. This system will result in the purchase of carbon credits by airlines from other sectors through a stock exchange, on voluntary from 2021, and then mandatory from 2027. Many voices, especially those of environmental non-governmental organizations (ENGOs) denounced the lack of ambition of this agreement.

Air Traffic Impact
The combustion of kerosene in jet engines produces mainly carbon dioxide (CO2) and water vapor, as well as gaseous pollutants such as nitrogen oxides (NOx), or particulate like soot or sulphates .

CO2which has a very long life (100 years) mixes homogeneously with the lower atmosphere and accumulates there, contributing to the continuous increase of the greenhouse effect .

Water vapor and aerosols also contribute strongly, but transiently. Depending on the flight altitude and weather conditions, the water vapor condenses or not to form condensation trails that disappear in seconds or minutes or can spread and form cirrus that may last longer. This water quickly joins the water cycle , except when it is emitted into the stratosphere .

Nitrogen oxides are degraded by photochemical reactions that consume methane (CH 4) and produce ozone (O 3). The destruction of methane, a powerful greenhouse gas , partly offsets the radiative forcing of CO2. Ozone is a greenhouse gas, but because of its short life span, it is generally not counted as CO2 equivalent.

Jet aircraft therefore have a long-term cumulative impact related to their CO2 emissions, which will last for a hundred years, and a very short – term impact on the radiative balance of the atmosphere which would disappear in a few days if the air traffic ceased.

Propeller planes that use gasoline, kerosene or diesel do not form contrails but emit CO2, nitrogen oxides and particles.

CO2 emissions
The combustion of 1 liter of kerosene releases 2.52 kg of CO2, plus 0.52 kg for extraction , transportation and refining , for a total emission factor of 3.04 kg CO2 per liter of kerosene (or 3.81 kg of CO2 per kg of kerosene, or 0,312 kg per kWh, or 3,642 kg per toe ).

In 1992, according to a special IPCC report , CO2 emissionsaircraft accounted for 2% of total anthropogenic emissions and 2.4% of fossil fuel emissions . But since air transport only developed from the 1950s, the concentration of CO2in the atmosphere attributable to it was in 1992 only a little more than 1% 2 .

In 2015, according to ATAG ( Air Transport Action Group (in) ), a group of experts from the airline industry, the flights were responsible for the emission of 781 Mt CO2out of a total of 36 Gt CO23 , or 2.2%. But according to statistics from the International Energy Agency (IEA), aviation has consumed 288 Mtoe worldwideof petroleum fuels, inducing emissions of 1,049 Mt CO2, 3.2% of global CO2 emissionsrelated to fossil fuels.

Short-lived gases and aerosols
Next to CO2 with a very long life span (100 years) and accumulates in the atmosphere, the aircraft emit water vapor, gases and aerosols with a very short lifespan on Earth’s radiation budget only lasts as long as there are planes in the air. Nevertheless, the radiative forcing for which they are responsible is important and even, today (in 2010), higher than that of CO2 accumulated since the beginning of aviation.

Radiative Forcing (RF) expresses in W/m2 the variation of the resulting radiation flux at the tropopause (or at the top of the atmosphere) related to a disturbance factor. The resulting radiation flux is the difference between the received radiative power and the re-transmitted power. Positive radiative forcing tends to heat up the system (more energy received than emitted), while negative radiative forcing goes in the direction of cooling (more energy lost than received). The IPCC takes as a reference the year 1750 and its 2014 report provides data on radiative forcing in 2011 compared to 1750.

NOx emissions
Nitrogen oxides (NOx) are not greenhouse gases, but by reacting with other chemical species present in the atmosphere, they cause, at the flight altitude of subsonic aircraft (9 to 13 km):

the production of ozone, a powerful but short-lived greenhouse gas, thus a warming of surface temperatures. At these altitudes, NOx emissionsproduce more ozone than near the ground and this ozone causes a greater warming 2 . The ozone thus produced is mainly confined to the northern hemisphere, where air traffic is more important.

Ozone FR: 0.0219 W / m 2 (IPCC 2000-2005 Assessment)
the destruction of methane, a powerful greenhouse gas with a lifespan of 12 years, so a cooling. In 1992, the share of air traffic in the atmospheric concentration of methane was estimated at 2% 6 .

FR of methane: -0.0104 W / m 2 (IPCC Assessment for 2000-2005)
At the altitude of supersonic flights, NOx emissionsdestroy the stratospheric ozone layer.

Effect of contrails and induced cirrus
Jet engines emit water vapor which may form persistent condensation trails when the atmosphere is supersaturated in ice and the temperature is less than -40 ° C . These trails consist of ice crystals whose size is generally smaller than that of the crystals constituting the natural cirrus . Their presence tends to warm the Earth. Although they reflect part of the incident sunlight and therefore tend to cool the greenhouse effect they cause, which tends to heat up, is predominant 8 .

The radiative forcing of contrails depends on their overall extent and optical thickness , which is difficult to assess accurately. In 1992, the average extent was estimated at 0.1% of the land surface, with higher proportions in regions with high air traffic (0.5% in Central Europe). It depends on the intensity of the air traffic and the extent of the areas of supersaturation which can vary with the evolution of the climate. Moreover, the optical thickness depends on the size and shape of the ice particles, which themselves depend on the nature and quantity of aerosols emitted by the reactor, these aerosols acting as condensation nuclei 6 , 9 .

EN contrails: 0.01 W / m 2 (0.005 to 0.03). Medium Confidence (IPCC Assessment for 2011) 10
Sometimes the contrails may spread to form cirrus that can persist for several hours. It has been established that these artificial cirrus also cause positive radiative forcing, the estimation of which is very uncertain because it is impossible to distinguish between natural and artificial cirrus. About 30% of the Earth’s surface is covered by cirrus clouds and studies have shown that in Europe this cloud cover has increased by 1 to 2% per decade over the last two decades, but without being able to determine with certainty the cause (s).

EN combined contrails and induced cirrus: 0.05 W / m 2 (0.02 to 0.15). Low confidence (IPCC Assessment for 2011) 10

Steam emissions
Most of the water vapor emissions from subsonic airplanes occur in the troposphere where they are discharged as rain within one to two weeks. A small fraction is however emitted in the lower stratosphere , where it can accumulate. Radiative forcing from stratospheric water vapor is very low.

FR of stratospheric water vapor: 0.002 W / m 2 (IPCC Assessment for 2000-2005)

Aerosol emissions
The reactors emit soot resulting from the incomplete combustion of kerosene as well as sulphates resulting from the combustion of the sulfur which it contains in small quantities. These solid aerosols have a direct effect on the surface temperature of the earth, the soot tends to warm it, the sulphates to cool it. The quantities emitted are, however, low compared with other anthropogenic sources.

Direct FR of aerosols: -0.001 W / m 2 (Sulfates: -0.0035 W / m 2 , soots: 0.0025 W / m 2 ) (IPCC Assessment for 2000-2005)
These aerosols are also involved in the formation of condensation trails, cirrus clouds and other clouds, but as their contribution is insufficiently known, it is not evaluated separately. It is in fact included in the radiative forcing of contrails and induced cirrus.

Total radiative forcing
According to the assessment made by the IPCC in its fourth report , aviation radiative forcing in 2005 was 78 mW / m 2 (from 38 to 139, with a probability of 90%) and represented 4.9% of the total anthropogenic radiative forcing 13 , that is, about three times more than the single impact of CO2issued by aircraft. This assessment was not updated by the IPCC in its fifth report except for contrails and cirrus clouds.

Radiative forcing (FR) due to air transport from 1750 to today(mW / m 2 )

Emissions weighting
Radiative forcing is a measure of the variation of the solar radiation power received by the Earth as a result of human activities since the beginning of the industrial revolution. It reflects the consequences of past and present activities.

In order to evaluate policies to mitigate global warming , it is necessary to integrate to the same extent the future effects of all the factors that contribute to it, both the long-term effects of CO2the very short-term effects of other emissions related to aviation activity. For this, weighting factors have been proposed to aggregate all emissions. These factors are the values by which we must multiply CO2 emissionsto take into account other emissions. Five factors were developed based on physical criteria (increase in radiative forcing, temperature) or economic criteria. Depending on the criteria used, their values range from 1.3 to 2.9.

In their communication, the air transport industry, the International Civil Aviation Organization (ICAO, a UN agency) and the public authorities, in particular France, report only CO2, claiming a 2% share of global emissions of this gas, thus implicitly referring to the IPCC estimate for the year 1992.

Air Transport Emission Factors

Pollutant emissions from passenger transport are generally reported per passenger-kilometer, obtained by dividing total emissions on a given trip by the average number of passengers and the distance traveled. CO2 emissions per passenger-kilometer depend on several parameters:

The type of plane and its consumption

Its filling rate and its freight
The covered distance. On a short flight, take-off and landing phases are proportionally more intensive fuel.
Flight altitude
The Carbon Base , “public database of emission factors required for carbon accounting exercises ” , administered by Ademe (France), provides emission factors according to the distance traveled and the number of seats from the plane. For example, a Paris-New York route (5,863 km ) in an aircraft with more than 250 seats induces an average emission of 223 g CO2eq / passenger-km, of which 101 g were combustion-related, 101 g fugitive (short-lived) and 21 g upstream 19 , for a total of 1.3 t CO2eq / passenger. The uncertainty is evaluated at 50%. A round-trip Paris-New York and is about 1/4 of total annual emissions of a French.

The Civil Aviation Directorate (DGAC) calculator , France, which supplies CO2 emissionstotal (production and distribution of kerosene + combustion during flight) for a given route, does not take into account other emissions contributing to the greenhouse effect.

By way of comparison, the average emission factor for passenger cars in France in 2010 was 168 g CO2/ km. Since their average filling rate was 1.4 persons per car, the average emission rate per passenger was 120 g CO2/ passenger-km. For comparison also, the emission factor of a TGV in France is 4 g CO2eq / passenger-km.

Emission factor according to class
According to a 2013 World Bank study, CO2 contentof air transport strongly depends on the class chosen. Thus first-class and business-class passengers have a carbon footprint 9 times or 3 times higher than economy-class passengers . This is related to the fact that there are fewer seats per m2 in these classes and their occupancy rate is also lower. Passengers also have more luggage.

Other impacts of the airline industry
A comprehensive carbon footprint of air transport should also include related activities, such as production, maintenance and disposal of aircraft and airports . The ADP Group achieves an annual report since 2011 greenhouse gas emissions at airports it manages in Paris. They were evaluated at 82 Mt CO2eq in 2015.

Evolution and Prospects

Air traffic growth and its contribution to global warming
The volume of global air traffic has doubled every 15 years since the mid-1970s, 28 which equates to a growth rate of 5% per year, well above that of world GDP .

The growth of air traffic is favored by the development of low-cost airlines and the absence of taxation of kerosene for international 29 and national flights in many countries, including France.

Passenger transport
In 2016, scheduled flights carried 3.7 billion passengers (or 10 million passengers a day), which averaged 1,896 kilometers . The number of revenue passenger-kilometers (PKP) reached 7015 billion, an increase of 6.3% compared to 2015. growth slightly less than that of 7.1% recorded the previous year.

For the 2017-2036 period, aircraft manufacturers expect that growth in passenger traffic will continue at a brisk pace, 4.4% per year for Airbus and 4.7% for Boeing, slightly down from strong growth in 2015 and 2016.

Freight
Freight is an important part of air transport (applying the principle of “one passenger + luggage = 100 kg”, we can estimate its share of 22% of air transport in 2015), but its growth is lower than that of passenger traffic. In 2015, 51 Mt were transported, covering an average of 3,678 km , or a transported amount of 187.6 billion tonne-km, an increase of 1.7% over the previous year. In 2016, growth was 2.6%.

Climatic impact on the rise
CO2 emissionsand the other factors contributing to the greenhouse effect have continued to increase and continue to increase as technological improvements in aircraft and the optimization of operational procedures are far from sufficient to offset the strong growth in traffic. While the International Civil Aviation Organization (ICAO, a UN agency) aims for a 2% annual improvement in the energy efficiency of the air fleet, the airline industry has committed to an improvement of 1.5% per year between 2009 and 2020.

The IPCC Special Report published in 1999 shows that aviation’s contribution to the greenhouse effect would increase in all the scenarios studied, while other industries should be able to reduce their share significantly.

International agreements
The Chicago Convention of 1944, which established the International Civil Aviation Organization (ICAO) has banned any tax on kerosene for international flights.

2016 Agreement under the aegis of ICAO
After more than 15 years of negotiations, the first global agreement to reduce the climate impact of air transport was concluded onOctober 6, 2016within ICAO . It aims to achieve the goals set for the organization in 2010: improve energy efficiency by 2% per year and stabilize CO2 emissionsat the level they will have reached in 2020. It also aims to fill the gap in air transport measures in the Paris Agreement 38 . It sets up a system for offsetting CO2 emissionsfor the fraction of emissions that would exceed the level reached in 2020 despite the “basket of measures” adopted at the same time :

Modernization of air traffic management
accelerating the introduction of new technologies to reduce aircraft consumption
development and implementation of sustainable alternative fuels
The system endorsed by Resolution A39-3 is known as CORSIA ( Carbon Offsetting and Reduction Scheme for International Aviation, a program (or regime) for compensation and abatement for international aviation) 39 . It will result in the purchase of carbon credits by airlines from other sectors via a stock exchange from 2021, first on voluntary, then mandatory after 2026. August 23, 201772 states representing 88% of international air activity had volunteered. Only international flights between non-exempt countries are concerned. Domestic flights are not affected, but the actions on can be included in the action plans submitted by States in the context of the Paris agreement. It only takes into account CO2 emissionswhose share in global emissions is estimated to be less than 2%.

International agreements on reducing GHG emissions from air transport

The agreement should not cost more than 1.8% of sales to airlines by 2035 .

Reviews
Several countries, including Russia and India have criticized the agreement and are not brought to candidates for voluntary implementation phases, because according to them, he bear an unfair burden to emerging countries. On the other hand, many voices and in particular those of ENGO denounced the lack of ambition of the agreement:

it is insufficient to achieve the Paris Agreement’s goal of limiting warming to 2 ° C or even 1 ° C 38 and does not require the aviation sector to assess its share to achieve this. It allows for virtually unlimited growth in the aviation sector;
by setting up a compensation mechanism, it puts much of the effort on other sectors of the economy and sends the “irresponsible message that air transport will reach zero emissions”;
it will not weigh enough on ticket prices. According to the NGO Transport et Environnement , “barely more than the price of a coffee”;
it will only cover 25% of emissions: it concerns only international flights and provides for many exemptions. On the other hand, it does not affect emissions below the level reached in 2020;
it will not take effect until 2021 and will be on a voluntary basis until 2027;
it does not include requirements on the quality of offsets. On the other hand, carbon credits linked to forests will be difficult to use and in any case insufficient;
the exchange of carbon credits was chosen because it is not very transparent and cheap. It would have been preferable to introduce a carbon tax , clearer and easier to implement or to join a system of exchange of quotas to stick to the European system.

European regulations
In Europe, the Community system of exchange of emission allowances (EU ETS) applies since 2012 in the emissions of CO2 Civil Aviation pursuant to Directive 2008/101 / EC of 19 November 2008. However, faced with the challenge of twenty-six states outside the European Union, the European Commission proposed inNovember 2012postpone the application of the regime to flights to and from the European Economic Area (EEA) until a global solution is found under the aegis of ICAO. However, the Directive has continued to apply to all flights within and between the 31 European countries applying the EU ETS.

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