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; highlighting that extreme marine heatwaves such as this have tripled over the past 80 years.

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 in which the sea surface temperatures rise well above normal for an extended time.

The study, published by a team of scientists from the Mediterranean Institute for Advanced Studies, the University of Reading, 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, co-author of the study at the National Centre for Atmospheric Science at the University of Reading, 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 to become 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.”

Published this week in the journal PNAS, 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 events. Without global warming, 47% of marine heatwaves observed between 2000 and 2020 would not have happened at all.

On top of this, the study found that marine heatwaves are becoming more intense. Global warming has added around 1°C to the maximum temperature of these events, with some regions seeing even higher increases.

The study also found that 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 University of Reading, 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.”

Last month, a separate study revealed the extent of the economic damage caused by the fallout from marine heatwaves, including flooding and landslides to impacted ecosystems and fisheries around the world.

The report – published at the time in Nature Climate Change – found there were nearly 3.5 times the number of marine heatwave days in the summers of both 2023 and 2024 compared to any other year on record. 

It stated that those suffered over the past two years – exacerbated by the onset of El Niño – have brought with them “billions of dollars in damage” around the world through events including severe coral bleaching, flooding, and landslides, as well as the reverberating impact on marine species, ecosystems, and coastal communities.

A marine heatwave is defined as at least five consecutive days when sea temperatures are in the top 10% of temperatures for that day of the year. While scientists expected to see some upward trend in average sea temperatures of the last two years, the kind of jumps recorded came as a shock to many.

It was a marine heatwave that fuelled Cyclone Gabrielle across New Zealand in 2023 in which 11 people were killed and over $8bn in damages was caused. Climate change increased the intensity of the rainfall by at least 10%.

Meanwhile, marine heatwaves caused Peruvian anchovies to move away from their usual waters, leading to the closure of commercial fisheries in 2023 and 2024 with estimated losses of $1.4bn. Furthermore, nearly 6,000 people died in Libya in 2023 when heavy rains from Storm Daniel caused the collapse of the Derna Dam, an event reported as the ‘deadliest single flood event on record in Africa.’

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