In recent years, all the major ocean basins have experienced unprecedented heat waves, with some events reaching such extreme intensity that researchers have introduced the term 'super marine heat waves' to describe them.
According to Boyin Huang, an oceanographer at the National Oceanic and Atmospheric Administration, marine ecosystems impacted by these super heat waves have never before encountered such elevated sea surface temperatures.
The coastal waters around the United Kingdom and Ireland witnessed one of the longest and most severe marine heat waves on record starting in April, with temperatures rising unusually early in the year. Similarly, Australia’s iconic coral reefs on two coasts have recently suffered from intense heat wave events.
While definitions of marine heat waves vary, the consensus is clear: as the planet warms, oceans are absorbing excess atmospheric heat trapped by greenhouse gases from fossil fuel combustion, fundamentally transforming marine environments.
Rising ocean temperatures are triggering profound changes in marine biodiversity, sea levels, and atmospheric conditions.
Coral reefs are among the most visibly affected, with excessive heat causing widespread bleaching and mortality. Recent data indicate that between January 2023 and March 2025, approximately 84 percent of coral reefs globally experienced heat stress sufficient to cause bleaching.
Last year, the warmest on record, sea levels increased at a faster pace than anticipated. Research reveals that most of this rise resulted from thermal expansion—water expanding as it heats—rather than glacier and ice sheet melt, which historically were the main contributors.
Excess ocean heat also influences weather extremes, contributing to rapid intensification of hurricanes and more destructive storms. For instance, elevated ocean temperatures in the southwest Pacific last year fueled a record number of tropical cyclones striking the Philippines.
Marta Marcos, a physicist at the University of the Balearic Islands, emphasizes the importance of understanding how global warming drives extreme events to better predict future developments.
Dr. Marcos led a recent study published in the Proceedings of the National Academy of Sciences, which concluded that climate change is responsible for the vast majority of marine heat waves observed in recent decades.
Ecological Consequences
Some of the earliest documented mass die-offs related to marine heat waves emerged from the Mediterranean Sea, which is warming three to five times faster than the global ocean average. Marine conservation ecologist Joaquim Garrabou began investigating these events after observing extensive sponge and coral mortality in 1999.
He and colleagues predicted recurring die-offs due to climate change, but the pace of these events has exceeded expectations. Garrabou notes that such mass mortality incidents are becoming the new normal rather than rare occurrences.
A 2012 marine heat wave in the Gulf of Maine exposed the vulnerability of fisheries. The Northern shrimp population plummeted from an estimated 27.25 billion in 2010 to just 2.8 billion two years later, according to models by the Atlantic States Marine Fisheries Commission.
Anne Richards, a retired fisheries biologist formerly with NOAA’s Northeast Fisheries Science Center, described the shrimp collapse as shocking. Her research implicated longfin squid, which migrated northward due to warmer waters, preying on the shrimp.
The Northern shrimp fishery remains closed, with populations estimated at around 200 million by 2023.
Kathy Mills, a senior scientist at the Gulf of Maine Research Institute, highlights that climate change is intensifying the challenges of commercial fishing beyond previous difficulties.
Research on marine heat waves is largely concentrated in a handful of countries, including Australia, the United States, China, Canada, Spain, and the United Kingdom.
Dan Smale, a community ecologist at the UK’s Marine Biological Association, points out that many regions lack adequate monitoring, leaving gaps in understanding of marine heat wave impacts worldwide.
Warming oceans can trigger cascading effects through marine food webs, starting with plankton populations at the base.
Following the end of commercial whaling in the 1970s and 1980s, North Pacific humpback whales recovered to a population peak of about 33,000 in 2012.
However, the marine heat wave known as 'the Blob' from 2014 to 2017 disrupted this recovery. The event reduced wind and wave activity, limiting nutrient upwelling and leading to declines in phytoplankton, zooplankton, fish, and other dependent species.
The Lingering Impact of 'The Blob'
Research following 'the Blob' revealed that the repercussions of severe marine heat waves can persist long after the initial event subsides.
Ted Cheeseman, a Ph.D. candidate at Southern Cross University and co-founder of the Happywhale database, identified a sharp decline in humpback whale sightings by 2021.
After multiple years of verification, his team published findings attributing a 20 percent decrease in the North Pacific humpback population from 2012 to 2021 to the loss of key food sources like krill during and after the heat wave.
Cheeseman noted that with an estimated 7,000 whales missing and no evidence of their presence elsewhere, there is no plausible alternative explanation for the decline.
Future Outlook
Scientists warn that some ocean regions may soon exist in a near-permanent state of marine heat waves, at least by current definitions. Present-day short-term events might be a preview of this future.
Alistair Hobday, a biological oceanographer with Australia’s Commonwealth Scientific and Industrial Research Organisation, has begun issuing marine heat wave forecasts months in advance to inform public response.
These warnings are gaining attention and prompting action.
For example, the critically endangered red handfish, which inhabits just two small rocky reef and seagrass areas off Tasmania, faced potential extinction in late 2023 when forecasts predicted lethal marine heat waves.
Researchers from the University of Tasmania’s Institute for Marine and Antarctic Studies, supported by the Australian Department of Climate Change, relocated 25 red handfish to captivity until ocean temperatures normalized.
Jemina Stuart-Smith, an ecologist involved in the rescue, described the period as extremely stressful, emphasizing the risk of losing an entire species.
After three months, 18 fish were returned to the wild, three died, and four were placed in a captive breeding program.
Scientists acknowledge that while such interventions offer temporary relief, they cannot counteract the broader trend of long-term ocean warming.
Marine ecologist Kathryn Smith likened these efforts to applying a Band-Aid on a broken leg, underscoring the magnitude of the challenge.
Still, researchers hope their work provides insight into the oceans’ future, inspiring proactive measures. Dr. Hobday noted that informed individuals can devise innovative solutions when presented with credible forecasts.
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