Updated analysis of the non-CO2 effects of aviation

The study builds on the latest scientific knowledge and presents a fresh analysis of the main climate impacts associated with air traffic, beyond carbon dioxide (CO₂) emissions. CO₂ emissions from aviation have been addressed under the EU Emission Trading System since 2012, but non-CO₂ impacts have not been specifically addressed by EU policies so far. However, the report acknowledges that a number of measures already in place also contribute to reducing the climate impacts of non-CO₂ aviation emissions.

The work was conducted by a project team assembled by EASA, comprising the most renowned experts in the field from the EU, Norway and UK. The study reflects the views of its scientific authors and does not constitute an official opinion of the Commission. A short Staff Working Document from the Commission services accompanies the study.

The significance of combined non-CO₂ climate impacts from aviation activities, previously estimated to be at least as important as those of CO₂ alone, is fully confirmed by the report.

The report calls for further reflection on how to best address non-CO₂ climate impacts, which could inform the possible development of future policies on the full impact of aviation on the climate. Doing so would contribute to the EU's climate objectives and the Paris Agreement, alongside existing actions to reduce CO₂ emissions. Such measures would also result in improvements to local air quality.

The report examines additional policy options to address the non-CO₂ climate impacts of aviation. These are divided into three categories: financial/market-related, fuel and air traffic management (ATM).

Unlike CO₂ impacts, which directly correlate to the amount of fuel burned, the complexity of measuring non-CO₂ climate impacts - and the uncertainty regarding trade-offs between the various impacts - makes targeted policy development in this area more challenging.

Further Information

• Updated analysis of the non-CO₂ climate impacts of aviation
• Full-length report
• Reducing emissions from aviation

Further background/ Q&A

Which “non-CO₂ impacts” are we talking about?

• The non-CO₂ climate impacts assessed by the experts arise from aircraft engine emissions of oxides of nitrogen (NO_x), soot particles, oxidised sulphur species, and water vapour.

What exactly are the climate impacts of non-CO₂ emissions?

• Their net impact is to warm the climate. However, individually, the respective aviation non-CO₂ emissions have different warming and cooling effects, about which there are different degrees of uncertainty.

• The authors note that the scientific understanding of the climate change effects of non-CO₂ emissions from aviation activities has advanced over the last 10 years. For example, the improved climate metric ‘Effective Radiative Forcing’ shows that the warming effect of contrail cirrus (the clouds produced by aircraft engine exhausts) is at least half that of previous estimates. Newer impacts on aerosol-cloud interactions have been identified, but no best estimates are available.

• The significance of non-CO₂ climate impacts from aviation activities, previously estimated to be at least as important in total as those of CO₂ alone is fully confirmed by the report. In terms of emissions-equivalencies (like for like, tonne-for-tonne) the report shows that the overall Global Warming Potential for aviation, with a time horizon of 100 years, is 1.7 times that of CO₂. Using a derivative metric of the Global Warming Potential (100), the GWP*, aviation emissions are currently warming the climate at approximately three times the rate of that associated with CO₂ emissions alone. This latter metric differs from the GWP100 in that it accounts for recent growth and a closer association with temperature response.

What are the findings on policies to address non-CO₂ climate impacts?

The policy options assessed in the study are divided into three categories: financial/market-related, fuel and air traffic management (ATM). The options could co-exist.

1) - The financial measures consist of a levy on aircraft NO_x emissions and/or the inclusion of such emissions under the EU ETS. This would incentivise manufacturers and airlines to reduce these emissions, while taking account of associated trade-offs.

Outstanding research obstacles to making the policies implementable include:

• the need for a better understanding the potential net cooling effect of aircraft NO_x emissions under certain future scenarios of declining emissions of tropospheric ozone precursors from surface (non-aircraft) sources;
• the possible need for an internationally recognised methodology for estimating cruise NO_x emissions;
• the lack of an appropriate CO₂-equivalent emissions metric and time horizon for NO_x emissions, taking into account the trade-off between the two;
• a methodology for setting an appropriate level for the charge.

Given these outstanding research issues, the study concludes that these measures could be implemented in the mid-term, i.e. 5 to 8 years.

2) - The fuel-related measures are about the reduction of aromatics in fuel (leading to cleaner fuel burn and reduced non-volatile Particulate Matter [nvPM] emissions) and the mandatory use of Sustainable Aviation Fuels (SAF). Both measures target the emission of soot particulates and contrail cirrus clouds. Reducing aromatics content would require fuel producers to adapt their production processes and a system to monitor the aromatics content of fuels. Given the set of research issues to be addressed according to the experts, the measure could potentially be implemented in the mid- (i.e. 5 to 8 years) to long- term (i.e. 8+ years). The mandatory use of SAFs, could be implemented through an EU blending mandate specifying that a certain percentage of the total jet fuel sold over a set time period would have to be SAF. If well designed, this should lead to simultaneous reductions in CO₂, nvPM and sulphur emissions (though not NO_x emissions).

The experts consider that if related outstanding research issues linked to this measure are resolved, the measure could potentially be implemented in the short- (i.e. 2 to 5 years) to mid-term (i.e. 5 to 8 years).

The Commission is already assessing options to boost the production and uptake of Sustainable Aviation Fuels (SAF) and plans to propose a policy initiative on this shortly. The IA will duly address the non-CO₂ dimension of SAFs.

3) - The measures in the ATM category are avoidance of ice-supersaturated areas and a climate charge. Optimizing flight trajectories to avoid climate-sensitive regions would reduce the formation of contrail-cirrus clouds, while the climate charge would address all non-CO₂ effects (NO_x, water vapour, soot, sulphates, contrails).

As there is a series of research issues that would first need to be addressed, the avoidance of ice-supersaturated areas measure could, according to experts, potentially be implemented in the midterm, i.e. 5-8 years. Regarding the climate charge, the experts deem that given the broader nature of the measure and the associated research needed, it could potentially be implemented in the long-term, i.e. 8+ years.