We evaluate the expected impact of the International Maritime Organization’s 2023 regulatory regime that will cap CO2 emissions from global maritime shipping. Focusing on U.S. imports— for which we compile granular vessel, route, emission, and trade data— we structurally estimate a model featuring substitution between maritime and air transportation. We demonstrate that the cap will instigate a shift towards much more carbon-intensive air transportation, causing a

short-term increase in total transportation-related CO2 emissions! Additionally, the cap will in- duce substantial welfare losses in both the short and long run. We conclude by advocating for alternative, more efficient policy options.