A new study in Nature Communications finds that marine heat waves can raise CO2 uptake in polar and subpolar coastal seas by about 11%. The reason is counterintuitive: warmer water normally holds less gas, but in these regions heat waves shrink sea ice and expose more surface water to the atmosphere. More open water also lets sunlight reach phytoplankton, which pull carbon out of surface waters. Researchers analyzed global coastal and shelf sea CO2 exchange from 1985 to 2020 using ocean models. Shelf seas cover only about 7% of the world's ocean area, yet in polar regions heat waves can increase open-water area by up to 30%. The authors caution that it is unclear whether this extra uptake will last as marine heat waves become more frequent and intense. The study adds nuance to how the ocean responds to warming. It also matters for anyone working on ocean carbon accounting or climate projections, because regional effects can run opposite to the open-ocean pattern.
