Climate change

Carbon dioxide (CO₂) emissions originate from burning fossil fuels. CO₂ emissions can remain in the atmosphere for hundreds to thousands of years¹⁹ and so it is the cumulative emissions that matter as they accumulate in the atmosphere thereby increasing CO₂ concentrations.

While aviation’s sectoral share of global CO₂ emissions remains at around 2.5% on an annual basis, both global emissions and aviation emissions have dramatically increased in recent years (

), with 47% of global aviation CO₂ emissions between 1940²⁰ and 2019 having occurred since 2000 .

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¹⁹At its simplest, carbon dioxide cycles between the atmosphere, oceans and land biosphere. Its removal from the atmosphere involves a range of processes with different time scales. About 50% of a CO₂ increase will be removed from the atmosphere within 30 years, and a further 30% will be removed within a few centuries. The remaining 20% may stay in the atmosphere for many thousands of years (IPCC Fourth Asessment Report, 2007). This is further elaborated in the fifth and sixth assessment reports of the IPCC.
²⁰Due to limited commercial aviation activities prior to 1940, and the subsequent signature of the ICAO Chicago Convention in 1944, this date is often used to mark the beginning of the modern aviation industry.

Unlike CO₂, the effect from aviation non-CO₂ emissions depends on where they are emitted. They are also termed ‘short-lived climate forcers’, since their effect operates on a timescale of hours to decades.

Nitrogen oxides (NO_X)

NO_X are gases that react with other chemical species within the atmosphere, resulting in both warming and cooling that is called the ‘net NO_X’ effect. In the short term (hours to days), NO_X leads to the formation of ozone (O₃) which is a greenhouse gas and thus has a warming effect. NO_X also leads to the formation of highly reactive species, including the hydroxy radical (OH). OH is the primary means by which ambient methane (CH₄) from other sources (e.g. agriculture, coal mining and combustion) is broken down, resulting in a cooling effect (decades). While there are additional small cooling effects associated with the methane reduction, and the formation of O₃ is influenced by the amount of surface emissions precursors, the overall balance between all these effects is currently considered to be a net warming.

Contrail-cirrus clouds

Contrails form behind aircraft, typically at cruising altitudes of 8 to 12 km, from the condensation of water vapour on soot particles to form ice crystals under conditions of temperature and humidity that leads to the saturation of air with water. While the formation of short-lived linear contrails is easy to predict, the formation of ‘persistent contrails’ that last from a few minutes to a few hours and can form cirrus cloud coverage are much harder to predict for purposes of avoidance

. The interaction of contrails and contrail cirrus with solar and infrared radiation results in a net warming that occurs mainly under night time conditions, as shown in .

Other non-CO₂ emissions

The emissions of soot particles and sulphate aerosols (primarily as SO₂ from sulphur present in the fuel) have a short-term (weeks) direct warming and cooling climate effect respectively, as well as a potential indirect effect through their interaction with clouds. The short- term direct climate effect of water vapour emissions is very small at subsonic cruise flight altitudes in the troposphere. However, water vapour emissions from an increasing number of flights above the tropopause, such as supersonic aircraft and certain subsonic business jets, can have a warming effect as they fly in the drier stratosphere.

Since the 1999 IPCC Special Report, a number of scientific assessments of aviation’s climate impacts have been conducted

. Despite these advances in the details of the underlying science, uncertainties on non-CO₂ effects remain. The climate effects of emissions from aviation were reassessed in 2020 and are presented as Effective Radiative Forcing (ERF) in . Red bars indicate warming effects and blue bars indicate cooling effects. Numerical best estimate ERF and RF values are given in the columns with 5–95% confidence intervals along with ERF/RF ratios²¹ and confidence levels.

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²¹The change in metric from RF to ERF resulted in a 50% reduction in the overall estimated climate change effect from contrail-cirrus.

suggests that non-CO₂ emissions represent the largest fraction of the total ERF of aviation, at present, although the level of uncertainties from the non-CO₂ effects is 8 times larger than that from CO₂, and the overall confidence levels of the largest non- CO₂ effects are ‘low’. While no best estimates of ERF have been provided for the aerosol-cloud interactions from sulphur and soot emissions, these should not be ignored since they could potentially be important.

The effects of CO₂ on climate are well understood and well-quantified, and so measures put in place to reduce CO₂ will mitigate the contribution of aviation emissions to climate change. A similar level of understanding should be sought on the effects of non-CO₂ emissions. Based on the precautionary principle, cost-effective actions should be considered in order to reduce the overall climate impact from all aviation emissions, taking into account the prevailing uncertainties in non- CO₂ effects as part of a risk-based assessment in order to ensure confidence in robust mitigation gains. 

A recent study considered a scenario where global aviation CO₂ emissions were reduced by 2.5% every year from 2025 until 2050, and air traffic was reduced to about 50% compared to pre-COVID levels (similar to air traffic in summer 2020). It was concluded that the impacts of the continued rise in accumulated long- term CO₂ emissions and the fall in short-term non-CO₂ emissions would balance each other out, thereby leading to no further increase in current aviation-induced warming

. As such, the non-CO₂ share of total aviation climate forcing is not a constant and depends entirely on the rate of change of CO₂ emissions.

Environmental certification standards already exist for various aircraft engine non-CO₂ emissions, including NO_X and nvPM. In developing and implementing further EU policies to reduce aviation non-CO₂ emissions, the evolving scientific uncertainty of their precise climate change impact is important and needs to be taken into account. In addition, there is a need to assess possible trade-offs between the CO₂ and non-CO₂ climate impacts. A common scale known as a ‘net CO₂- equivalent emissions metric’ is often used, although it is important to note that this comparison will vary depending on the metric and time horizon used²².

One possible policy option being considered is to lower the concentration of aromatics (and sulphur) in fuels in order to obtain a cleaner burn and potentially less contrail cloudiness, while another potential option is the mitigation of contrail-cirrus clouds through the re- routing of flights around ice-supersaturated air regions

. Win-win policy options that deliver reductions in both CO₂ and non-CO₂ emissions, such as the rapid uptake in Sustainable Aviation Fuels, could ensure ‘no regret’ actions.

Where trade-offs occur, a robust policy assessment methodology is essential to ensure the proposed policy leads to a reduction in the overall climate impact from aviation. Specific research to address knowledge gaps has been identified in order to inform potential policy options to abate the climate impact of non-CO₂ emissions

.

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²²For example, one such metric is the Global Warming Potential for aviation effects over a time-horizon of 100 years (GWP100), where a multiplier of 1.7 is applied to the CO₂ emissions in order to account for the impact of non-CO₂ emissions. In comparison, the Global Temperature Potential metric (GTP100) has a multiplier of 1.1.