Marine heatwaves are not only devastating coral reefs and fish stocks — they are also undermining one of the planet's most important climate buffers, a new study has found.

Research published in Nature Communications shows that unusually high ocean temperatures can disrupt microscopic marine organisms, throwing food webs off balance and weakening the ocean's capacity to absorb and store carbon dioxide.

The findings add to mounting evidence that climate-driven ocean warming is triggering cascading changes in marine ecosystems and pushing the seas closer to a tipping point.

Warming oceans under strain
Marine heatwaves — prolonged spikes in sea surface temperature that can last weeks or even years — have become increasingly intense and widespread. This year, sea temperatures hit new records in the Mediterranean and along Portugal's coast.

The team of researchers monitored the Gulf of Alaska for more than a decade, focusing on two major heatwaves: one between 2013 and 2015, and another from 2019 to 2020. They found that these events significantly altered populations of microscopic phytoplankton — the foundation of the marine food chain — and weakened the ocean's "biological carbon pump."

A jammed conveyor belt
"The ocean acts like a vast conveyor belt, carrying carbon from the surface to the deep sea," explained lead author Mariana Bif of the University of Miami's Rosenstiel School. "Our research shows that marine heatwaves can jam this conveyor belt."

By disturbing plankton communities, the heatwaves reduced the amount of carbon sinking into deep waters, increasing the likelihood that more carbon will remain in or return to the atmosphere, Bif said.

The ocean currently absorbs roughly a quarter of all human-generated carbon dioxide, but this natural service could weaken as marine heatwaves intensify.

Wider climate consequences
Beyond disrupting carbon storage, marine heatwaves can worsen weather extremes on land — fuelling stronger storms, longer droughts, and more powerful hurricanes. They also threaten coral reefs and other marine habitats that support biodiversity and coastal economies.

Co-author Ken Johnson of the Monterey Bay Aquarium Research Institute said more continuous monitoring is crucial to understand these evolving risks. "To grasp how heatwaves reshape marine ecosystems, we need detailed data from before, during, and after such events," he said.

The researchers warn that unless greenhouse gas emissions are drastically reduced, the oceans' ability to protect the planet from runaway warming could steadily erode.