Aviation may have contributed as much as 4.9% to global radiative forcing in 2005 and its carbon dioxide emissions could grow by up to 360% between 2000 and 2050 1 . In 2016, the International Civil Aviation Organization adopted a global scheme requiring airline operators to offset increases in carbon dioxide emissions from international flights above 2020 levels 2,3 . Here we show that the scheme will only compensate for the emissions increase if robust criteria for the eligibility of offset credits are adopted. Offset supply from already implemented greenhouse gas abatement projects registered under the Clean Development Mechanism alone could exceed demand from International Civil Aviation Organization’s scheme. Most of these projects continue abatement even if they cannot sell offset credits. If the scheme allows airline operators the unlimited use of offset credits from already implemented projects, it will result in no notable emissions reductions beyond those that would occur anyway and neither offer incentives for new investments nor reward previous investments in offset projects. We recommend limiting the eligibility to new projects or projects that are at risk of discontinuing greenhouse gas abatement without further support. The findings are critical for negotiations under both the International Civil Aviation Organization and the Paris Agreement.