An increase in marine heatwaves over the past two years has caused billions of dollars in damage from flooding and landslides, to impacted ecosystems and fisheries around the world, with limited actions taken to intervene or mitigate the fallout, an international team of researchers have concluded.

A new study published in Nature Climate Change has found that 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 states 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, and the reverberating impact on marine species, ecosystems, and coastal communities.

“Intensifying marine heatwaves are driving ocean temperatures to record-breaking levels, surpassing anything observed in modern history,” said Dr Karen Filbee Dexter, a researcher from the University of Western Australia and co-author on the study.

“The extreme warming events are fuelled by climate change, and are disrupting marine ecosystems, threatening fisheries, and intensifying severe weather patterns. We need urgent action to reduce global emissions and prevent further extreme temperatures in our oceans.”

Between 2023 and 2024, nearly 10% of the ocean hit record-high temperatures with a long list of consequences for coral reefs, fisheries, and coastal communities. Scientists warn that with worsening marine heatwaves driven by increasing temperatures through climate change, these effects will only continue to be felt.

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 no surprise that marine heatwaves had such profound and widespread impacts,” said Professor Michael Burrows from the Scottish Association for Marine Science and a co-author on the study.

“We need to brace ourselves for the same and worse in years to come. Building resilience by planning our preparations and responses is essential to limit the worst impacts of such events.”

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. Scientists suggest that Storm Daniel was made more intense and rainy due to sea temperatures made higher by climate change.

The ocean plays vital roles in regulating the climate, supporting marine life and providing food and jobs for billions of people. As marine heatwaves worsen with climate change, these functions are at increasing risk.

Over the past two years, marine heatwaves have forced the closure of fisheries and aquaculture, increased whale and dolphin strandings, and caused the fourth global coral bleaching event.

And the impacts didn’t stop at the oceans; marine heatwaves have been found to drive extreme weather such as deadly atmospheric heatwaves and flooding on land, too.

“The number of impacts we have seen from marine heatwaves over the past two years have been pretty crazy, with mass coral bleaching, species popping up in new locations, and the number of related extreme weather events on land,” said Dr Kathryn Smith of the Marine Biological Association in Plymouth, lead author on the study.

“The more regularly our marine ecosystems are being hit by marine heatwaves, the harder it is for them to recover from each event. As marine heatwaves continue to increase, we are likely to see further loss of marine species and ecosystems globally.”

While scientists have called out the limited actions taken to intervene and mitigate the fallout of the damage caused by marine heatwaves, some steps were successfully taken to preserve the most at-risk ecosystems.

In Australia, a quarter of the population of endangered red handfish was taken into aquariums before the marine heatwave hit, and released again when waters cooled. Over in the US, some corals and conches were moved into deeper, cooler waters while in Peru, the government paid benefits to the fishers who could not go to sea when they were forced to close the anchovy fishery within their domestic waters.

Those from the Scottish Association for Marine Sciences (SAMS) have said that better forecasting and rapid response plans ‘could have reduced some of the greater impacts felt in other regions.’

While heatwaves in 2023 and 2024 were exacerbated by El Niño, previous research has shown that human-induced climate change already caused a 50% increase in marine heatwave occurrence between 2011 and 2021.

The team at SAMS writes that if fossil fuel burning and deforestation continues at its current rate, we could be encountering heatwaves somewhere between 20 to 50 times more frequently and up to ten times more intense by the end of the century.

“In Europe, marine heatwaves contributed to record-breaking land temperatures in the British Isles, harmed fish populations, and nearly caused the extinction of the fan mussel in the Mediterranean,” the Association writes. “Off northern Spain, marine heatwaves hit shellfish, hurting the livelihoods of women who traditionally harvest them.”

Here in the UK, seabird populations in Scotland were affected by declines in food sources, while aquaculture suffered losses from harmful algal blooms. Warm water species moved north into the waters around the British Isles, leading to an increase in wildlife-watching tourism while hotter oceans fuelled Storm Daniel, which brought deadly flooding to Greece, Bulgaria, and Turkey.

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