Climate change

The Intergovernmental Panel on Climate Change (IPCC) is the international body that provides policy makers with regular assessments of the science related to climate change (Working Group I), its impacts and future risks (Working Group II), and options for adaptation and mitigation (Working Group III). In 2018, the IPCC produced a Special Report on the impact from global warming of a 1.5⁰C temperature increase compared to pre-industrial levels [24] . It concluded that reaching and sustaining net-zero global CO2 emissions from human activities, and declining net non-CO2 radiative forcing, would halt global warming on multi- decadal time scales.

The IPCC subsequently released its 6th Assessment Report of Working Group I in 2021, which showed that emissions of greenhouse gases from human activities are responsible for approximately 1.1°C of warming since 1850-1900 and that immediate, rapid and large-scale reductions in greenhouse gas emissions are needed to limit warming to 1.5°C. The Report of Working Group II was subsequently released in February 2022 identifying that if 1.5°C is reached in the near-term, it would cause unavoidable increases in multiple climate hazards. The magnitude and rate of these hazards and associated risks would depend strongly on near-term mitigation and adaptation actions. Finally, the Report of Working Group III was published in April 2022 highlighting that many transport sector climate change mitigation strategies have co-benefits, including air quality improvements. It was also recognised that international cooperation is a critical enabler for achieving ambitious mitigation goals [25] . These findings of the IPCC are reinforced by the World Meteorological Organization’s report on the State of the Global Climate 2021 [26] .

Seascape, mountains on the horizon

Since the publication of the IPCC Special Report on ‘Aviation and the Global Atmosphere’ in 1999 [27] , the effects of aviation on climate from both its CO2 and non-CO2 emissions have been well established and continuously assessed. In order to halt aviation’s contribution to global warming, the sector needs to achieve net-zero CO2 emissions while also reducing the warming effect from non-CO2 emissions.

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 ( Figure 2.3. ), with 47% of global aviation CO2 emissions between 194020 and 2019 having occurred since 2000 [28] .

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 (NOX)

NOX are gases that react with other chemical species within the atmosphere, resulting in both warming and cooling that is called the ‘net NOX’ effect. In the short term (hours to days), NOX leads to the formation of ozone (O3) which is a greenhouse gas and thus has a warming effect. NOX 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 [29] . 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 Figure 2.4. [30] .

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 [31] [33] . 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 Figure 2.5. . 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.

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

Radiative Forcing Metrics

Radiative Forcing (RF) is a term used to describe when the amount of energy that enters the Earth’s atmosphere is different from the amount of energy that leaves it. Energy travels in the form of radiation: solar radiation entering the atmosphere from the sun, and infrared radiation exiting as heat. If more radiation is entering Earth than leaving, then the atmosphere will warm up thereby forcing changes in the Earth’s climate. The metric Effective Radiative Forcing (ERF) was introduced in the IPCC Fifth Assessment Report in 2013 as a better predictor of the change in global mean surface temperature due to historic emissions by also accounting for rapid adjustments in the atmosphere (e.g. thermal structure, clouds, aerosols etc.).

Figure 2.5. 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. 

Landscape with big white airplane is flying in the sky over the clouds and sea at colorful sunset

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 [28] . 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 NOX 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-COclimate 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 [32] [34] [35] [36] [37] . 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 [13] .

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.