Marine heatwaves increase CO₂ absorption by northern coasts by 11%
An international study shows that marine heatwaves, intensified by climate change, boost the absorption of CO₂ by coastal seas by 11%, especially in the polar and subpolar regions of the Northern Hemisphere.
Marine heatwaves, which are becoming more frequent with global warming, increase the absorption of CO₂ by coastal waters by around 11%. This increase, highlighted by the Helmholtz-Zentrum Hereon, is strongest in polar and subpolar seas where sea ice is rapidly retreating.
Marine heatwaves: definition and detection
A marine heatwave (MHW) is defined as a prolonged period during which the sea surface temperature significantly exceeds seasonal norms, often by several degrees Celsius. These events are detected using satellite observations from Copernicus, in situ measurements, and ECMWF reanalyses. Artificial intelligence algorithms, notably deep neural networks, help filter data noise and quickly identify the onset, duration, and intensity of MHWs.
The phenomenon is based on several interconnected mechanisms:
Increased thermal stratification: Warming of the surface layer creates a thermal barrier that limits mixing with cooler waters below. This stratification traps CO₂ dissolved in the surface layer, where it is most available for phytoplankton photosynthesis.
Nutrient release: Ice melt releases iron, phosphorus, and other micronutrients previously trapped in ice. These nutrients stimulate phytoplankton growth, increasing biological carbon fixation.
Modification of CO₂ solubility: Although warm water holds less gas, the increase in biological productivity largely compensates for this loss of solubility, resulting in a net positive flux of CO₂ to the ocean.
Combined in situ measurements and atmospheric-ocean model outputs confirm that during MHWs, the net flux of CO₂ to the ocean increases measurably, although uncertainties remain regarding the persistence of this effect after the event ends.
The phenomenon is particularly pronounced in the polar and subpolar seas of the Northern Hemisphere: the continental shelves of the Barents Sea, Bering Sea, and the Bay of Biscay. In these areas, rapid sea ice loss exposes large expanses of warm water, promoting MHWs that can last several weeks. Local current dynamics are then modified, enhancing vertical mixing and nutrient distribution.
