In the most extreme case, a marine heatwave could last as long as 579 days, with sea temperatures reaching up to 4.5°C above the long-term average
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In 2023, the northeast Atlantic experienced a record-breaking marine heatwave that impacted marine ecosystems and the economy alike over 240 days. Such heatwaves can increase mortality among animals such as fish, birds and marine mammals, trigger algal blooms and disrupt marine food supplies.
Three years on, researchers from the University of Bern investigated whether an even more extreme marine heatwave could occur under today’s climate conditions, considering that global ocean temperatures were higher in 2026 than in 2023.
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They found that even slightly different weather patterns could produce much more severe, longer-lasting marine heatwaves in this region. In the most extreme case, a marine heatwave could last up to 579 days and raise sea temperatures by 4.5°C above the long-term average.
Published in Nature Communications, the research used a method known as ‘ensemble boosting’ to gather its findings. This modelling simulates different possible evolutions of an extreme weather or climate event.
‘With this method, we can investigate which weather conditions make a marine heatwave particularly intense and long-lasting, and how extreme it could become in the worst case,’ explains lead author Catherine Gregory.
The researchers also looked at how the duration and intensity of marine heatwaves in the northeast Atlantic could change under different climate change scenarios. With global warming of less than 2°C, severe marine heatwaves like the 2023 one remain ‘relatively rare’. However, under scenarios of global warming exceeding 3°C, such events become increasingly common.
‘The 2023 marine heatwave would no longer be exceptionally intense in a world with 3°C of warming. This shows that we need to prepare for events that go beyond anything we have experienced historically,’ said climate researcher and the study’s senior author Thomas Frölicher.
What is a marine heatwave?
A marine heatwave can be declared after at least five consecutive days when sea temperatures exceed the threshold for unusually warm conditions at that specific location and time of year, based on observations made historically. As such, marine heatwaves are relative – what may be considered very warm in the North Atlantic could be entirely normal in the Mediterranean.
Marine heatwaves can occur for several reasons, such as changes in ocean currents or persistent high pressure that brings sun and lighter winds to the area.
In the summer of 2026, Europe experienced marine heatwaves. Some regions saw sea temperatures rise as much as 6°C above normal. Marine heatwaves are also on the rise, with the number of events doubling since 1982.
