In recent decades, the oceans have warmed. Marine heat waves, once rare events, have become more common. One particularly intense event known as “the Blob” lasted years and devastated plankton populations, starving millions of fish and seabirds, and damaging commercial fishing.
In January 2024, the share of the ocean surface experiencing a heat wave topped 40 per cent. Unusual heat waves have occurred in all of the major ocean basins around the planet in recent years. And some of these events have become so intense that scientists have coined a new term: super marine heat waves.
“The marine ecosystems where the super marine heat waves occur have never experienced such a high sea surface temperature in the past,” Boyin Huang, an oceanographer at the National Oceanic and Atmospheric Administration in the US, said in an email.
The seas off the coasts of the UK and Ireland experienced an unusually intense marine heat wave, one of the longest on record, starting in April, and the temperature rise happened much earlier in the year than usual. Australia and its iconic coral reefs were recently struck by heat waves on two coasts.
As the planet’s climate changes, the oceans are being fundamentally altered as they absorb excess heat trapped in the atmosphere. Hotter oceans are causing drastic changes to marine life, sea levels and weather patterns.
Some of the most visible casualties of ocean warming have been the coral reefs. When ocean temperatures rise too much, corals can bleach and die. About 84 per cent 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 came from ocean water expanding as it warms, not from melting glaciers and ice sheets.
Excess heat in the oceans can also affect weather patterns, making hurricanes more likely to 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, we can 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.
The Losses
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 corals 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.”
In 2012, a marine heat wave in the Gulf of Maine, US, 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.
“This disappearance of the shrimp was just shocking,” said Anne Richards, a retired research fisheries biologist who worked at the Northeast Fisheries Science Center in US at the time.
The fishery has not yet reopened. By 2023, the northern shrimp population was estimated to have dropped to 200 million.
Complicating matters is the fact that much of the research on marine heat waves comes from just a few countries, including Australia, the US, China, Canada, Spain and the UK.
“There are lots of regions around the world where monitoring isn’t as good so we don’t really know what’s happening,” said Dan Smale, a community ecologist at the UK’s Marine Biological Association.
The Future
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.
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 a species.”
After three months, 18 fish were returned to the ocean. Three had died, and four were enrolled in a captive breeding programme.
Scientists recognise that temporary fixes can only do so much. “It’s 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, can think of all kinds of things to do differently.”
NYTNS