Marine heat waves can increase coastal carbon dioxide uptake by roughly 11 percent in polar and subpolar shelf seas, according to a global analysis led by the Helmholtz-Zentrum Hereon. The phenomenon occurs because extreme warm spells shrink sea ice cover, exposing more open water to gas exchange and phytoplankton growth.
Conventional climate wisdom holds that extreme warmth is bad news for ocean carbon storage. Because gases dissolve less easily in warmer water, open-ocean studies have long shown that natural carbon uptake drops during prolonged heat waves. Yet a new global analysis published in Nature Communications turns that expectation on its head for certain marine environments.
Researchers define marine heat waves as periods when seawater temperatures stay above the local 90th-percentile baseline for at least five consecutive days. While these episodes exhaust natural defenses elsewhere, shallow coastal waters in colder northern regions react differently. Polar and subpolar shelf seas along continental margins in areas like the North Atlantic and Northwest Pacific experience accelerated ice melt during these warm spells, fundamentally altering how the ocean interacts with the atmosphere.
How Sea Ice Loss Triggers Higher Gas Exchange
Shelf seas account for only about 7 percent of the world’s total ocean area, yet their response to extreme heat packs an oversized climate punch. In polar regions, the international research team found that marine heat waves can expand open-water surface areas by as much as 30 percent.
When heat waves can shrink sea ice cover, they expose vast stretches of water directly to the atmosphere for the first time in weeks or months. This sudden exposure opens up new pathways for gas exchange. With blocks of ice removed from the equation, sunlight penetrates surface waters more deeply, stimulating the proliferation of phytoplankton.
These microscopic organisms form the foundation of marine food webs. Through photosynthesis, phytoplankton draw carbon directly out of surface waters, creating a biological vacuum that pulls additional carbon dioxide down from the air.
Global Datasets and Long-Term Modeling at Helmholtz-Zentrum Hereon
To uncover this counterintuitive dynamic, the international team used advanced ocean models to scrutinize global datasets tracking carbon dioxide exchange across coastal and shelf seas between 1985 and 2020. They then cross-referenced those computational results against broader open-ocean records.
The resulting comparison demonstrated that polar shelf seas absorbed roughly 11 percent more carbon during marine heat waves. Specialized scientific teams, such as those studying polar oceans and ice shelf stability at institutions like the British Antarctic Survey, utilize similar numerical modeling and autonomous platforms to simulate regional climate shifts and reduce long-term predictive uncertainty.
Uncertainty Ahead for Future Climate Scenarios
While the findings offer fresh precision for climate models, scientists caution against viewing the boost in carbon absorption as an unmitigated environmental win. The broader marine ecosystem faces overlapping stressors, including microplastic pollution that interferes with deep ocean carbon storage, severe heat stress threatening coral reefs, and ocean spray carrying chemical pollutants like PFAS back into the atmosphere.
The durability of this enhanced carbon sink remains an open question as global temperatures continue to climb.
