Marine heat waves usually mean bad news for the ocean's ability to absorb carbon dioxide, and in the open ocean, that holds true. CO2 uptake drops by about 8% during a heat wave, warmer water simply holds less dissolved gas. But a new global study found the opposite happening along the coast, and the reason comes down to one place.
Researchers led by Zhentao Hu at Germany's Helmholtz-Zentrum Hereon analyzed 45 coastal regions worldwide across 35 years of data, from 1985 to 2020, tracking how marine heat waves changed coastal CO2 absorption. Coastal shelf seas pulled in 11% more CO2 during the same heat wave events, and more than 90% of that boost traces back to the poles, the Arctic and Antarctic shelves. Heat wave driven sea ice loss opens more water to the atmosphere and triggers a burst of phytoplankton growth, and that biological activity pulls carbon down through photosynthesis.
The effect runs in reverse at lower latitudes. Coasts near the tropics, Florida, and upwelling zones release more CO2 during a heat wave instead of absorbing it. The direction of the effect depends on latitude, so a single "the ocean's carbon sink is failing" headline doesn't capture what's happening region by region. That's the sharper question worth asking the next time a marine heat wave headline shows up: where is it happening, and by what mechanism.
Chapters:
1:49 – CO2 Uptake Drops About 8%
2:00 – Coastal Shelf Seas Go the Other Way, Up 11%
2:11 – More Than 90% of the Coastal Boost Comes From the Poles
2:26 – The Mechanism: Ice Loss Opens Water to the Atmosphere
3:01 – Warm Coastal Water Releases More CO2
Takeaways:
Marine heat waves cut open-ocean CO2 uptake by about 8%, while coastal shelf seas see uptake rise 11% during the same events.
More than 90% of that coastal carbon boost comes from the poles, the Arctic and Antarctic shelves.
The study, led by Zhentao Hu at Germany's Helmholtz-Zentrum Hereon, analyzed 45 coastal regions worldwide across 35 years, 1985 to 2020.
Heat wave driven sea ice loss opens more open water and triggers phytoplankton blooms, which pull CO2 down through photosynthesis.
Lower-latitude coasts, including the tropics, Florida, and upwelling zones, see the opposite effect and release more CO2 during a heat wave.
The direction of the effect depends on latitude, so a single "ocean carbon sink is failing" headline doesn't capture what's happening region by region.
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