14 April 2025
Global warming has led to a threefold increase in the number of days each year that the ocean experiences extreme heat at the surface, a new study has found.
Averaged across the globe, in the 1940s, the sea surface typically experienced about 15 days of extreme heat annually. Today, that number has jumped to nearly 50 days per year. Global warming is responsible for almost half of the occurrence of marine heatwaves – periods when sea surface temperatures rise well above normal for an extended time.
The study, produced by a team of scientists from the Mediterranean Institute for Advanced Studies, the ºÚ¹Ï³ÔÁÏÍø, International Space Science Institute and the University of the Balearic Islands, also found that rising global temperatures are making extreme ocean heat events last longer and become more intense.
Dr Xiangbo Feng, a co-author of the study at the National Centre for Atmospheric Science at the ºÚ¹Ï³ÔÁÏÍø, said: “Marine heatwaves can devastate underwater ecosystems. Extended periods of unusually warm water can kill coral reefs, destroy kelp forests, and harm seagrass meadows – all critical habitats for marine life.
“As global temperatures continue to rise, marine heatwaves will become even more common and severe, putting increasing pressure on already stressed ocean ecosystems. These increased marine heatwaves could, in return, cause our atmosphere less stable leading to more frequent and powerful tropical storms in some regions. Human activities are fundamentally changing our oceans. Urgent climate action is needed to protect marine environments.”
More frequent and intense heatwaves in European Seas
Published today (Monday, 14 April) in the journal , the study explains how the research team compared observed sea surface temperatures with an estimation of how they would look if global warming had been removed since 1940. This allowed them to measure how much global warming contributes to these extreme heat events. Without global warming, 47% of marine heatwaves observed between 2000-2020 would not have happened at all.
The study also found that marine heatwaves are becoming more intense. Global warming has added about 1°C (1.8°F) to the maximum temperature of these events, with some regions seeing even higher increases.
The study also found different ocean regions respond differently to warming.
Professor Jon Robson, fellow co-author of the study from the National Centre for Atmospheric Science at the ºÚ¹Ï³ÔÁÏÍø, said: “In tropical areas, marine heatwaves mostly become more frequent rather than more intense. In contrast, areas like the Mediterranean Sea, North Sea, and parts of the Pacific Ocean see both more frequent and more intense heat events.”
https://www.pnas.org/cgi/doi/10.1073/pnas.2413505122
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