Climate Change

The United Nations Intergovernmental Panel on Climate Change (IPCC) is responsible for providing policymakers with regular scientific assessments. In 2023 the IPCC published the Synthesis Report which integrates the findings from Working Group reports and the Special Reports during the 6th Assessment cycle

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. It concluded that human activities, principally through emissions of greenhouse gases (GHG), have unequivocally caused global warming, with global surface temperature reaching 1.1°C above a baseline 1850-1900 level in 2011-2020. Global GHG emissions have continued to increase, with unequal historical and ongoing contributions arising from unsustainable energy use, land use and land-use change, lifestyles and patterns of consumption and production.

The impact of a rapid-warming climate can already be noted across the globe, with widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere exacerbating weather and climate extremes. The extent to which current and future generations will experience a hotter and different world depends on choices made now and in the near term (

In 2023, the WMO State of the Global Climate Report

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and the European State of the Climate Report

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both highlighted that records were once again broken for greenhouse gas levels, surface temperatures, ocean heat and acidification, sea level rise, Antarctic Sea ice cover loss and glacier retreat. A recent acceleration in the increase of atmospheric CO2 concentrations was also reported, resulting in the highest ever level in the last 800 000 years of 427 parts per million in 2024

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. Extreme weather events under climate change, such as heatwaves, floods, droughts, wildfires, and rapidly intensifying tropical cyclones have impacted millions of lives and inflicted billions of euros in economic losses

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. 2023 was confirmed as the warmest year on record with the global average near-surface temperature at 1.45°C above the pre-industrial baseline, culminating in the warmest ten-year period on record (

and

How hot is too hot for the human body?

The IPCC states that climate change adversely affects both physical health and mental health.

Our body’s ability to cool down through sweat that evaporates into the air is impeded by humidity, as such the critical zone where core body temperature starts to rise rapidly is defined by both temperature and humidity.

Once a core body temperature of 41-42 degrees centigrade is reached, this can lead to heat stroke and significant medical problems if not treated appropriately.

Aviation CO₂ emissions

Aviation accounted for approximately 2.5% of global CO₂ emissions in 2023. Absolute emissions have been growing significantly, with 47% of aviation CO₂ emissions between 1940 and 2019 having occurred since 2000

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Aviation non-CO₂ emissions

The overall climate impact from aviation emissions is a combination of both its CO₂ and non-CO₂ emissions that include Nitrogen Oxides (NO_X), Particulate Matter (soot), Sulphur Oxides (SO_X) and water vapour as well as the subsequent effects from the formation of contrail-cirrus clouds and aerosol-cloud interactions.

Aviation emissions and their climate effects

In terms of departing flights from the EU27+EFTA, EEA/ UNFCCC data on GHG emissions from aviation peaked at around 156 MtCO₂e in 2019 (an increase of 122% compared to 1990) and represented 4% of total GHG emissions in 2022. This increase was mostly driven by international aviation, including between EU27+EFTA States, that in 2019 emitted 8.7 times more than domestic aviation.

The latest assessment on the climate impact from historic air traffic emissions between 1940 and 2018

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estimated that the Effective Radiative Forcing (ERF)³ from non-CO₂ emissions accounted for more than half of the aviation net warming effect. However, it is also important to note that uncertainties associated with such estimates are up to 8 times larger than that from CO₂ and the overall confidence levels of the largest non-CO₂ effects are ‘low’

), and there are currently no best estimates for the ERF from aerosol-cloud interactions. This calls for a continuous research effort to increase the overall confidence of such calculations.

When considering the effect of mitigation measures on the climate impact of future aviation emissions, it is important to note that CO₂ represents a long-term (decades to millennia)⁴ climate forcer and non-CO₂ emissions are short- term (weeks to decades) climate forcers. This effect can be seen in

where an approximate 70% reduction in air traffic during 2020 due to the COVID pandemic led to a reduction in the annual ERF from non-CO₂ emissions, while the ERF from CO₂ emissions remained fairly constant.

Air traffic-induced changes in cloudiness are considered to be potentially significant aviation climate forcing components but also the most uncertain. Changes in cloudiness are caused by the formation of contrails and their persistence in ice supersaturated air leading to the formation and persistence of contrail cirrus, and additionally to changes in the formation of natural clouds from aviation aerosols. The initially line-shaped contrails can be detected in satellite images and can transform within a few hours, if persistent, into contrail cirrus that may be indistinguishable from naturally formed cirrus clouds (

The properties and climate impact (cooling / warming) of contrail cirrus can vary widely depending on the time of day, ambient atmospheric conditions and on the aerosols emitted by the aircraft. Further research into the aspects of aviation induced changes in cloudiness will enhance understanding of aviation non-CO₂ effects. This includes:

The formation of contrail ice crystals on remaining soot particles and emitted volatile particulate matter, in the case of lean burn engines, and on oil droplets
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, chemi-ions and ambient aerosols mixed into the aircraft plume
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The formation of contrail cirrus may lead to reductions in natural cirrus cloudiness, although the size of this effect is even more uncertain than the contrail cirrus climate impact, with only one study suggesting this feedback may reduce the contrail cirrus climate impact by half
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Natural cloudiness changes when aircraft fly through clouds, due to contrail formation within clouds
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, is an effect that is currently not included in estimates of contrail climate impact.
The impact of aerosols emitted by air traffic on the formation of natural clouds is highly uncertain. Studies disagree on the magnitude of the impact of soot emissions on cirrus cloudiness, although most suggest a cooling to near zero net effect. In contrast, the cooling effect connected with changes in mid-tropospheric cloudiness caused by sulphur emissions is clear, but the magnitude is similarly uncertain
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Introducing new technology such as lean burn engines or alternative fuels, including hydrogen, could also lead to changes in aerosol emissions and as mentioned, to the chemistry of climate forcing compounds in general. With the potential emergence of future aircraft fuelled by liquid hydrogen from renewable energy, these aircraft could support zero CO₂ emissions flights but would still emit NO_X and water vapour, although technical solutions (e.g. fuel cells) are also looking to limit these emissions. Gaseous hydrogen emissions could also affect atmospheric chemistry by impacting the lifetime of other greenhouse gases, namely methane, ozone, and water vapour, with an overall climate warming effect

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. As such, existing best practice methodologies to estimate aircraft GHG inventories may need to be reviewed and updated in order to remain fit for purpose

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Mitigation measures to reduce the overall climate impact

In-sector mitigation measures from technology, operations and fuels, which are covered in other Chapters of this report, contribute to reductions in both CO₂ and non- CO₂ emissions and the achievement of both ICAO goals and general European climate targets as part of the path towards climate neutrality.

When assessing the future effect of mitigation measures on the climate impact of aviation emissions, the impact from non-CO₂ emissions could be expressed as CO₂ equivalent emissions in order to assess trade-offs. However, this equivalence is influenced by the inherent uncertainties in the ERF terms as well as the metric (e.g. ATR, GWP)⁵ background atmospheric conditions and time horizon chosen. Cost-effective actions should continue to be considered in order to reduce the overall climate impact from all aviation emissions, taking into account the remaining uncertainties in non-CO₂ effects as part of a risk-based assessment in order to ensure confidence in robust mitigation gains.

Past efforts to mitigate the climate impact from aviation have focused on CO₂ emissions to a large extent. However, there are existing measures to mitigate the impact of non-CO₂ emissions, including aircraft engine emissions certification standards for NO_X and nvPM and the promotion of Sustainable Aviation Fuels (SAF). The large sensitivity of contrail formation and, consequently, the contrail climate impact of aerosol emissions

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indicates that introducing cleaner fossil-based fuels and Sustainable Aviation Fuels (e.g. lower aromatics, sulphur), would be a promising mitigation method in the formation of persistent contrail-cirrus clouds. Nevertheless, the introduction of SAF alone may not balance the increase in contrail cirrus climate impact due to the current and forecasted increases in air traffic

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Additional mitigation measures are being explored within Europe and internationally. This includes efforts to promote cleaner fuels through fuel standards and to demonstrate the feasibility of incorporating contrail mitigation into the routine operations of the Single European Sky Network Manager and Air Navigation Service Providers. Research to enhance weather and contrail forecasting capabilities will be critical to assess where significant mitigation benefits can be achieved and the associated costs to avoid persistent contrail formation

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. The updated EU Innovation Fund has supported mitigating these impacts since 2023.

Based on the 2023 revisions to the EU ETS Directive

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and the 2040 climate target communication

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, the European Commission has also developed a Monitoring, Reporting, and Verification (MRV) framework in line with the precautionary principle to address the non-CO₂ aviation effects on climate change

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. The revised EU ETS Directive requires aircraft operators to monitor and report once a year on the non-CO₂ aviation effects occurring from 1 January 2025. By the end of 2027, based on the results from the application of the MRV framework for non-CO₂ aviation effects, the Commission shall report and, where appropriate, act with a legislative proposal to mitigate non-CO₂ by expanding the scope of the EU ETS.

The MRV is applicable from 1 January 2025 and is based on the two guiding principles of:

Flexibility
- Use of GWP metric with multiple time horizons (20, 50 and 100 years);
- Simplified approach for small aeroplane operators with emissions below a threshold;
- MRV IT tool provided by the Commission to automate processes and minimise administrative effort (operators also allowed to use their own alternative tools); and
- Provision of default values, where needed, to fill data gaps (e.g. engine identifier, fuel properties).
Precision – requires the ‘weather-dependent’ approach to be used as a default approach.

Research and Coordination Initiatives

In 2020, EASA published a report on the latest scientific understanding of the climate impact from aviation non- CO₂ emissions and associated policy measures

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. Since then, there have been significant new research initiatives looking at how to mitigate the overall climate impact of aviation emissions (CO₂ and non-CO₂) within various EU programmes (e.g. Horizon Europe, Clean Aviation, SESAR) as well as national projects.

In order to bring greater cohesion to this research, the European Commission and EASA set-up the Aviation Non- CO₂ Experts Network (ANCEN) in 2024. The network, fully funded by Horizon Europe, serves as a forum to facilitate a coordinated approach among key European experts from various stakeholder groups, including policymakers, scientific/research community, industry and civil society. It aims to reach a common understanding on issues in order to provide objective, timely and credible technical advice to inform discussions across various areas (e.g. research, analysis, policy, technology, operations, fuels). While not a regulatory body, ANCEN seeks to support the development of capabilities (e.g. data, modelling) that can inform robust decision-making on the implementation of mitigation measures. Information on the terms of reference, membership, future work and an overview of ongoing non-CO₂ research initiatives in Europe can be found on the ANCEN website

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³ 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.).
⁴ At 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 4^th Assessment Report, 2007). This is further elaborated in the 5^th and 6^th assessment reports.
⁵ The metric Average Temperature Response (ATR) include ‘efficacy’, which is particularly useful for accounting for the impact of contrails as it represents the change in global mean temperature per unit climate forcing exerted by the forcing agent, relative to the response generated by a standard CO₂ forcing starting from the same initial climate state. Global Warming Potential (GWP) is commonly used in international climate policy and agreements, including in the Paris Agreement, and can be adjusted with efficacy.

Published in collaboration with