Air travel is a major source of carbon pollution, and currently, there’s no real way to stop it (short of grounding flights). Jet fuel made from oil is extremely energy-dense, and while many attempts have been made to blend jet fuel with biofuel, that solution is often prohibitively expensive and hardly carbon-neutral, to boot.

The Paris Agreement didn’t set limits for carbon emissions from the aviation industry, but the International Civil Aviation Organization (ICAO)—an agency of the United Nations that works with the aviation industry in 144 countries—attempted to take up that mantle. The ICAO agreed in 2016 that airlines would be required to buy carbon offsets for every ton of carbon that they emit over and above 2020 emissions projections.

A letter published in Nature Climate Change today suggests that this requirement from the ICAO is not sufficient to mitigate the effects of the aviation industry’s increased carbon dioxide in the atmosphere. Instead, the authors of the letter say the ICAO needs to mandate that the credits that airlines buy meet a specific set of criteria, to ensure that the aviation industry is actually reducing greenhouse gas emissions.