Marine heat waves are spreading around the world

Boston Globe

09-06-2025

Scientists define marine heat waves in different ways. But it's clear that as the planet's climate changes, the oceans are being fundamentally altered as they absorb excess heat trapped in the atmosphere from greenhouse gases, which are emitted when fossil fuels are burned.
Hotter oceans are causing drastic changes to marine life, sea levels, and weather patterns.
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Some of the most visible casualties of ocean warming have been coral reefs. When ocean temperatures rise too much, corals can bleach and die. About 84 percent of reefs worldwide experienced bleaching-level heat stress at some point between January 2023 and March 2025, according to a recent report.
Last year, the warmest on record, sea levels rose faster than scientists expected. Research showed that most of that rise in sea levels came from ocean water expanding as it warms, which is known as thermal expansion, not from melting glaciers and ice sheets, which in past years were the biggest contributors to rising seas.
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Excess heat in the oceans can also affect weather patterns, making hurricanes more likely to rapidly intensify and become more destructive. In the southwest Pacific, last year's ocean heat contributed to a record-breaking streak of tropical cyclones hitting the Philippines.
'If we understand how global warming is affecting extreme events, that is essential information to try to anticipate what's going on, what's next,' said Marta Marcos, a physicist at the University of the Balearic Islands in Spain.
Marcos was the lead author of a recent study published in Proceedings of the National Academy of Sciences that found that climate change has been responsible for the overwhelming majority of marine heat waves in recent decades.
Some of the earliest research on mass die-offs associated with marine heat waves, before there was a name for them, came from the Mediterranean, which has been warming three to five times faster than the ocean at large. Joaquim Garrabou, a marine conservation ecologist at the Institut de Ciencies del Mar in Barcelona, Spain, started studying these events after witnessing a die-off of sponges and coral in 1999.
He and other scientists believed that with climate change, these die-offs would reoccur. 'The reality is moving even faster than what we thought,' he said. 'Having these mass mortality events is the new normal, instead of something infrequent, which it should be.'
In 2012, a marine heat wave in the Gulf of Maine highlighted the risk these events pose to fisheries. The Northern shrimp population went from an estimated 27.25 billion in 2010 to 2.8 billion two years later, according to modeling by the Atlantic States Marine Fisheries Commission.
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'This disappearance of the shrimp was just shocking,' said Anne Richards, a retired research fisheries biologist who worked at NOAA's Northeast Fisheries Science Center at the time.
Her research pointed to a significant culprit: Longfin squid, drawn north by warmer water, were eating the shrimp.
The fishery has not yet reopened. By 2023, the Northern shrimp population was estimated to have dropped to around 200 million.
Commercial fishing has always been difficult, but now, 'climate change is taking that to another level,' said Kathy Mills, a senior scientist at the Gulf of Maine Research Institute.
Complicating matters is the fact that much of the research on marine heat waves comes from just a few countries, including Australia, the United States, China, Canada, Spain, and the United Kingdom.
'There are lots of regions around the world where monitoring isn't as good as other places, and so we don't really know what's happening,' said Dan Smale, a community ecologist at the United Kingdom's Marine Biological Association.
Eventually, parts of the ocean might enter a constant state of marine heat wave, at least by today's common definition. Some scientists see today's shorter-term spikes as practice for this future.
Alistair Hobday, a biological oceanographer at Australia's Commonwealth Scientific and Industrial Research Organisation, has been conducting public briefings with marine heat wave forecasts months ahead of time.
People are tuning in — and responding.
The critically endangered red handfish lives off the coast of Tasmania, crawling along the seafloor on fins shaped like hands. These unusual fish have only been found within two small patches of rocky reef and sea grass meadow.
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In late 2023, Hobday's forecast predicted potentially deadly marine heat waves. Researchers from the University of Tasmania's Institute for Marine and Antarctic Studies, with support from the Australian Department of Climate Change, took a drastic step. They transferred 25 red handfish to an aquarium until the temperatures fell.
Jemina Stuart-Smith, an ecologist at the University of Tasmania, described those weeks as the most stressful time of her life. 'If it all went wrong,' she said, 'you're talking about the potential extinction of an entire species.'
After three months, 18 fish were returned to the ocean. Three had died, and four were enrolled in a captive breeding program.
Scientists recognize that temporary fixes can only do so much in the face of long-term warming.
'It's kind of putting a Band-Aid on a broken leg,' said Kathryn Smith, a marine ecologist and postdoctoral researcher at the Marine Biological Association.
But those studying today's extreme events still hope their work gives people some visibility into the future of the world's oceans, Hobday said. 'Clever people, if you tell them about the future,' he said, 'can think of all kinds of things to do differently.'
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