Legal analysis carried out by leading environmental law barristers concludes [1] that countries must include non-CO2 emissions - estimated to account for at least half of aviation’s climate impact - in their national climate plans to comply with their commitment to the Paris Agreement. The independent advice, commissioned by environmental NGO’s T&E and Opportunity Green, shows that countries have a legal obligation to include these emissions in their Nationally Determined Contributions (NDCs), which they need to submit ahead of COP30 (November 20, Brazil).
The Paris Agreement's temperature-based targets obliges countries to have plans in place to limit global warming. It has been proven that non-CO2 emissions - such as contrails, the white lines in the sky - emitted by planes contribute significantly to this warming. Indeed, science has shown that the impact of these effects on the planet is at least equal to the CO2 emissions of aviation. Therefore, those countries which are not including these emissions are failing in their climate commitments.
Diane Vitry, Aviation Director T&E, says: “Scientists have been warning about the warming caused by contrails for 25 years. Now legal advice shows countries must act to reduce them. It’s time to move from paper to action and include the full climate impact of aviation in national climate plans or they will be failing in their commitments signed only ten years ago in Paris.”
The legal conclusion is clear: countries are obliged by the Paris Agreement to include non-CO2 emissions from aviation. Three main arguments back this conclusion:
Carly Hicks, Chief Strategy and Impact Officer at Opportunity Green, says “The International Court of Justice rules that all countries must do more to prevent the severe consequences that every increment of warming has on our planet. This advice confirms that including the impact of airplane contrails must be part of this effort. Aviation has an emissions problem that cannot continue to be ignored. This means including all aviation emissions—both CO2 and non-CO2—in countries' climate plans, in compliance with international law.”
Avoiding contrails, the low hanging fruit to reduce aviation’s climate impact
Aviation contributes to global warming by emitting CO₂ and currently accounts for around 2-3% of annual global CO₂ emissions. But on top of that, aircraft engines emit other gases as well as particulate matter that affect the climate and our health. These are called non-CO₂ emissions and are little discussed despite their potential huge impact on the environment. The most visible non-CO2 emissions are the so-called contrails, condensation of water vapour from a plane, which cause 1-2% of global warming. Moreover, contrail warming is a highly concentrated issue — less than 3% of global flights generated 80% of contrail warming in 2019.
However,mitigating these contrails and their effect can be done more quickly and cost-effectively than other climate issues. Only a slight change in the route of a small percentage of flights would be needed, reducing the effects of contrails and providing much-needed short-term climate benefits in the race to meet the targets set out in the Paris Agreement.
International emissions, like aviation’s CO2 impact on international flights, are rarely included in NDCs even if legally, countries are obliged to, according to a prior analysis. Countries must include long-haul CO2 and non-CO2 aviation effects in the forthcoming round of NDCs submitted ahead of COP in Brazil, T&E says, and those countries which have already submitted NDCs should update them to include such effects. The EU is due to submit its NDC on behalf of all Member States by September 2025. This would be an opportunity for the EU to lead by example and reiterate its climate ambition by submitting an ambitious NDC that includes contrails, T&E says.
Notes to the editor:
[1] Legal Advice by Estelle Dehon KC and Lois Lane, Cornerstone Barristers, “In the matter of the United Nations Framework Convention on Climate Change and the Paris Agreement. Re: Inclusion of non-CO2 aviation emissions in Nationally Determined Contributions” (July 2025)
