The study, published in the Proceedings of the National Academy of Sciences, built a model of sea surface temperatures since 1940 that removed the heating the climate crisis has caused.
The analysis revealed there were about 15 days of extreme heat a year at the ocean surface in the 1940s, but the figure had jumped to a global average of nearly 50 days a year. Some regions, including the Indian Ocean, the tropical Atlantic and the western Pacific have 80 heatwave days a year, i.e one day in every five.
Half of the marine heatwaves since 2000 would not have happened without global heating, which is caused by burning fossil fuels. The heatwaves have not only become more frequent but also more intense: 1˚C warmer on average, but much hotter in some places, the scientists said.
“Here in the Mediterranean, we have some marine heatwaves that are 5˚C hotter,” said Dr Marta Marcos at the Mediterranean Institute for Advanced Studies in Mallorca, Spain, who led the study. “It’s horrible when you go swimming. It looks like soup.”
As well as devastating underwater ecosystems such as sea grass meadows, Marcos said: “Warmer oceans provide more energy to the strong storms that affect people at the coast and inland.”
One disastrous example was the intense rainfall that caused catastrophic flooding in Libya in 2023, which killed 11,000 people. It was made up to 50 times more likely by global heating, which had raised temperatures in the Mediterranean by as much as 5.5˚C. That resulted in more water vapour and therefore more rain.
Recent major marine heatwaves include an exceptionally long event in the Pacific in 2014-15, which caused mass mortality among marine life. Intense heat hit the Tasman Sea in 2015-16 and record sea temperatures around the UK and in the Mediterranean Sea in 2023. Scientists had warned in 2019 that ocean heatwaves were increasing sharply, killing swathes of sea-life like “wildfires that take out huge areas of forest”.