Polar and subpolar shelf seas dominate enhanced coastal CO2 uptake during marine heatwaves.

Marine heatwaves (MHWs) perturb air-sea CO2 fluxes, but their cumulative impact on the global coastal carbon sink remains poorly understood. Using four observation-based CO2 flux datasets, with priority given to a global coastal product, we find that MHWs enhance net CO2 uptake in global shelf seas by 11.0 ± 1.6% from 1985 to 2020, in contrast to reduced uptake in the open ocean. The enhancement is largely attributable to polar and subpolar shelf seas, where frequent MHWs coincide with sea-ice loss and non-thermal reductions in dissolved inorganic carbon (DIC), outweighing reduced uptake or enhanced outgassing in lower-latitude, warming-dominated regions. Analysis with a global ocean biogeochemical model suggests that the observed DIC reductions are largely linked to enhanced biological carbon fixation. Our findings reveal region-specific MHW impacts on coastal carbon dynamics, highlighting a critical role of high-latitude shelf systems in the global carbon budget under ongoing climate change.