The ocean is running a fever. In 2025, the number of days of marine heatwaves – prolonged spells when the sea turns abnormally, dangerously warm – was more than triple what it was in the early 1990s.

These are not abstract statistics. A severe and persistent marine heatwave bleaches coral reefs, strips away the kelp forests that shelter young fish, empties fishing grounds and – if occurring frequently – can tip whole ecosystems past the point of recovery.

It scrambles the chemistry the ocean lives by its acidity, its oxygen, the carbon it trades with the air, and can feed fiercer weather on land. For the coastal communities whose food and livelihoods come from the sea, the harm is immediate and personal.

I have spent my career studying where the heat from climate change actually goes. The answer, overwhelmingly, is the sea. The ocean has absorbed more than 90% of the excess heat that human activity has trapped on Earth, quietly buffering those of us on land from the full force of warming. For decades, that made it our greatest and most uncomplaining ally. Ocean warming and more frequent and intense marine heatwaves are signs that the buffer is straining. The heat we have poured into the........

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