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In 2023, the Earth experienced an unprecedented surge in marine heatwaves across its oceans.
They set new records in intensity, geographic extent, and duration, with many lasting well over a year and 96 per cent of the ocean surface affected.
Following new research, scientists now warn that these prolonged temperature spikes might herald a tipping point for the world’s oceans with grave consequences for the planet.
Earth’s oceans may have undergone a fundamental shift
In 2023, heatwaves resulted in both the North Atlantic and Southwest Pacific oceans experiencing record-breaking sea surface temperatures.
“We know that marine heatwaves have become increasingly common and more intense over time because of global warming. We also know that the El Niño that kicked off in 2023 allowed more heat to enter the ocean,” says climate research scientist Alex Sen Gupta from the University of New South Wales.
“But these factors alone can’t explain the incredible scale of the jump that began in 2023.”
Scientists from China, the USA and Thailand decided to investigate what was behind the extreme warming and what ramifications it has and could have in the future.
The research found that reduced cloud cover, which allows more solar radiation to reach the water, was a key driver, alongside weaker winds that diminish cooling from evaporation, and changing ocean currents.
While the paper doesn’t explain why these influences coincided to smash temperature records, it highlights why it is critical to dedicate more research to the mechanics of ocean warming.
The study voices scientists’ fears that the Earth’s oceans have undergone a fundamental shift, transitioning to a new, hotter state that they say is now the ‘new normal’.
Author Zhenzhong Zeng, from the Southern University of Science and Technology in China, said figures suggest heat in the world’s oceans is accumulating exponentially.
If this is indeed the case, it is a trend that would go against what current climate models have projected.
Ocean warming has devastating effects on marine ecosystems and life on land
The study also warns that the oceans’ shift to a permanently warmer state could have devastating effects for life on Earth.
This is because they play a central role in regulating global temperatures by storing and slowly releasing large amounts of heat.
Because the oceans take more time to react to changes than the atmosphere, the effects of heatwaves can be both delayed and dramatic.
This includes hampering the ability to predict short-term extreme weather events, such as hurricanes, and longer-term climatic changes.
Prolonged increased water temperatures would also be catastrophic for marine ecosystems, triggering mass die-offs or migrations of species.
It also increases the chances of coral reef collapse – a concern not least because the loss of coral hinders the ocean’s ability to sequester carbon, leading to more heating.
On land, it would mean accelerated warming, as sea breezes carry hot air inland. This can cause more intense and widespread droughts, heatwaves, wildfires and storms.
This was already evidenced by Storm Daniel in 2023, which killed nearly 6,000 people. Attribution studies found that it was made 50 times more likely and 50 per cent more intense by high sea surface temperatures in the Mediterranean.
Are marine heatwaves becoming the new normal?
The findings are particularly worrying given the subsequent heatwaves in 2024 and 2025 that boiled oceans around the world.
This year, sea surface temperatures in the Mediterranean reached their highest level ever recorded for June.
On 29 June, sea surface temperatures hit 26.01°C, according to data collected by Copernicus and analysed by Météo-France. Overall temperatures were 3-4°C higher than average.
It sparked fresh warnings from marine scientists of the devastating impacts on biodiversity, fisheries, aquaculture, and weather patterns across southern Europe and North Africa.
In May, a marine heatwave hit the UK, a place where surges in sea surface temperatures are still a relatively new phenomenon.
Parts of the North Sea, English Channel and Irish coast were as much as 4°C warmer than average.
Scientists say an exceptionally warm, dry spring coupled with weak winds allowed heat to build on the ocean’s surface.
They warned the flare-up could disrupt marine ecosystems, altering breeding cycles, enabling blooms of harmful algae or attracting jellyfish that thrive in warmer waters.
‘It is critical that we continue to measure, monitor and model the future of our Earth’
A fundamental shift in ocean dynamics that defies current climate models is an alarming prospect.
Some researchers have responded that the warning is premature.
“We don’t know what’s going to happen next year, and it [ocean temperatures] might just come back to something that’s much more, let’s say, normal,” Neil Holbrook, climate scientist at the University of Tasmania in Australia, told the New Scientist, adding that current research can only draw on a few years of data.
But even so, scientists back the paper’s exhortation to study the drivers of ocean warming.
“While we urgently need to reduce our GHG emissions, it is critical that we also continue to measure, monitor and model what our future Earth is going to be like,” says Jaci Brown, Climate Lead at Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO).
“If we don’t, we can’t prepare, and we are walking into the unknown with dire consequences for our future food, health and security.”
