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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.

In the past two years climate change, exacerbated by El Niño, caused multiple record-breaking marine heatwaves - which did billions of dollars of damage around the world.

This study found that in 2023-24, nearly 10% of the ocean hit record-high temperatures. In this comment article, scientists summarise the devastating consequences for coral reefs, fisheries, and coastal communities. They warn that as long as the rate of human-induced climate change keeps rising, marine heatwaves will continue to worsen, and that more proactive action is needed to avert the damage that extreme ocean temperatures already cause.

A marine heatwaves is defined as at least five consecutive days when sea temperatures are in the top 10% of temperatures for that day of the year.

Prof Michael Burrows of ��ҹѰ��, a co-author on the study, said: “The increase in average sea temperatures in 2023 and 2024 was quite shocking. We expected an upward trend but those jumps were really big, and it was no surprise that marine heatwaves had such profound and widespread impacts.

“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.”

In 2023 in New Zealand a marine heatwave fuelled Cyclone Gabrielle, which killed 11 people and caused over US$8 billion of damage. Climate change increased the intensity of the rainfall by at least 10%. 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 US$1.4 billion. Nearly 6,000 people died in Libya in 2023 when heavy rains from Storm Daniel caused the collapse of the Derna Dam - the deadliest single flood event on record in Africa. 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. However, the researchers say that as marine heatwaves worsen with climate change, these functions are at risk. In 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. The impacts did not stop at the oceans; marine heatwaves have driven extreme weather such as deadly atmospheric heatwaves and flooding on land.

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

“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.”

Good forecasting and prompt action reduced the impacts of some marine heatwaves. 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. In the USA, some corals and conches were moved into deeper, cooler waters. In Peru, the government paid benefits to the fishers who could not go to sea when they were forced to close the anchovy fishery. Better forecasting and rapid response plans could have reduced impacts in other regions.

While El Niño exacerbated marine heatwaves in 2023-24, previous research showed that human-induced climate change already caused a 50% increase in marine heatwaves between 2011-2021. If we keep burning fossil fuels and cutting down forests, marine heatwaves could be 20-50 times more frequent and ten times more intense by the end of the century. Replacing oil, coal and gas with renewable energy is therefore vital to safeguard ocean life and coastal communities.

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. Off northern Spain, marine heatwaves hit shellfish, hurting the livelihoods of women who traditionally harvest them.

Seabird populations in Scotland were affected as their food sources declined, 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. Hotter oceans fuelled Storm Daniel, which brought deadly flooding to Greece, Bulgaria, and Turkey.

Prof Burrows discussed the rise of marine heatwaves in .