The impact of marine heatwaves on global coastal carbon sink

Marine heatwaves (MHWs) disrupt marine ecosystems and drive significant socioeconomic impacts. In biologically productive, human-dominated coastal waters, MHWs can cause disproportionately large effects relative to the open ocean. In such systems, MHW-induced changes in physical and biological processes have the potential to strongly modulate air–sea CO₂ exchange. However, despite growing evidence that MHWs perturb regional CO₂ fluxes, their integrated impact on the global coastal carbon sink has not yet been quantified. Using four observation-based CO₂ flux datasets, prioritizing a global coastal product, we find that MHWs enhance net CO₂ uptake in global shelf seas by 4.3 ± 0.2% during 1985–2020, while uptake declines in the open ocean. The enhancement is driven primarily by polar and subpolar shelf seas, where frequent MHWs coincide with sea-ice loss and reductions in non-thermal dissolved inorganic carbon (DIC), outweighing reduced uptake or enhanced outgassing in lower-latitude, warming-dominated regions. Simulations with a global ocean biogeochemical model indicate that the observed DIC reductions are primarily driven by enhanced biological carbon fixation. Our results reveal region-specific MHW impacts on coastal carbon dynamics and underscore the critical role of high-latitude shelf systems in the global carbon budget under ongoing climate change.