May 13, 2026 — Seas recover. That’s the working assumption behind most marine conservation planning – heatwaves arrive, fish flee or die, then the water cools and the count resets.
A new study of 19 enclosed seas found that resets after heatwaves may stop happening. Some are on track to spend more than 330 days a year locked in heatwave conditions. Not a temporary extreme. A new permanent state.
Impact of heatwaves on Earth’s seas
The findings come from a team led by Matthias Gröger, a physical oceanographer at the Leibniz Institute for Baltic Sea Research Warnemünde (IOW) in Germany. His group ran climate model projections for 19 enclosed seas around the world.
These are stretches of saltwater hemmed in by land – the Mediterranean, the Baltic, the Red Sea, the Gulf of Mexico, and 15 others. Small and shallow compared to open ocean basins, with little water exchange beyond their narrow gates.
That geometry is the problem. Heat that would dissipate across the Pacific instead piles up in a confined space. Nowhere to go, no way to dump it.