The oceans are becoming a significant source of trichlorofluoromethane (CFC-11) emissions, a finding that must be accounted for when enforcing the ban on this chemical in coming years, according to a new model (Proc. Natl. Acad. Sci. U.S.A. 2021, DOI: 10.1073/pnas.2021528118). Worldwide production of CFC-11 was banned as of 2010 under the Montreal Protocol on Substances that Deplete the Ozone Layer. After 2012, however, global emissions of the chemical actually rose, reaching about 10 Gg per year. Emissions began falling as of 2018, but these recent high-level emissions highlight the need for a deeper understanding of CFC-11 lifetimes, says Peidong Wang, a graduate student at the Massachusetts Institute of Technology who worked on the modeling study. The oceans have absorbed a significant amount of CFC-11, taking up as much as 1% of global emissions of the substance in 1994 and extending the chemical’s lifetime on Earth. When human emissions of CFC-11 were still high, this effect was negligible. But in the future, ocean emissions might be mistaken for illegal human production, which could confuse regulatory efforts. The MIT models suggest that the oceans will switch to annual net emissions of about 0.5 Gg per year of CFC-11 in the 2070s. Depending on how urgently humans address climate change, differences in ocean circulation and precipitation triggered by global warming could accelerate this timeline, Wang says. Now the team aims to incorporate other water-soluble ozone-depleting chemicals in its ocean-atmosphere model.