Marine heatwaves increased net carbon dioxide uptake in global continental shelf seas by 11.0 ± 1.6 percent between 1985 and 2020, according to a study published in Nature Communications.
Researchers recorded an integrated flux anomaly of –40.02 teragrams of carbon across coastal waters during the 35-year period. This coastal trend diverges from the open ocean, where marine heatwaves suppress net carbon dioxide uptake by roughly 8 percent on average. The net coastal anomaly accounts for approximately 0.62 percent of total coastal carbon absorption and 0.05 percent of global ocean uptake across the same timeframe.
Scientists evaluated four observational reconstruction datasets, led by the coastal-focused ULB–SOM–FFN–coastalv2 product. Three other datasets confirmed the net uptake increase: CMEMS-LSCE-FFNN showed a 41.57 teragram carbon uptake increase, SeaFlux recorded a 35.99 teragram increase from 1990 to 2019, and OceanSODA-ETHZ recorded an 18.93 teragram increase.
Regional drivers
Polar and subpolar shelf seas generated the global uptake surge, countering reduced absorption and enhanced outgassing in warmer latitudes. Tropical shelf waters and western boundary current systems—including the United States East Coast, the Yellow Sea, the East China Sea, and Shark Bay in Western Australia—exhibited positive flux anomalies during heatwave months. In contrast, shelf waters in the Siberian Shelves, the Barents and Kara Seas, the Antarctic Shelves, and the Northwestern Pacific posted large negative flux anomalies that drove total coastal carbon intake.
Higher water temperatures promoted outgassing by lowering carbon dioxide solubility during heatwave months. However, non-thermal dissolved inorganic carbon reductions contributed 78.1 percent and sea-ice loss contributed 21.9 percent toward overcoming this thermal effect globally. Wind speed, atmospheric pressure variations, and residual terms exerted negligible influence on the global flux response.
Biological fixation
Biogeochemical simulations using the ICON-Coast model linked the non-thermal carbon drop to surges in net primary production across high-latitude shelves. Melting sea ice expanded open-water surface areas, freshened surface layers, and increased sunlight penetration into the upper ocean. These conditions stimulated phytoplankton photosynthesis, reducing dissolved inorganic carbon concentrations across the top 50 meters of the water column.
