The 2023 marine heatwave in the Northeast Atlantic broke all records, affecting marine ecosystems and the economy. In 2026, global ocean temperatures were even higher than in 2023. Researchers at the University of Bern have now investigated how long and intense marine heatwaves could become under today’s climate conditions. Their findings show that even slightly different weather patterns could plausibly produce much more severe and long-lasting events.
Bern, Oct 20, 2026.- In 2023, the Northeast Atlantic experienced a marine heatwave of unprecedented intensity and duration. From November 2022 onwards, sea temperatures continued to rise in the waters between the British Isles, the Iberian Peninsula and northwestern Africa. By June 2023, temperatures across a large area were more than 3°C above the long-term average. The heatwave lasted a total of 240 days. Ocean heatwaves can increase mortality among birds, fish and marine mammals, trigger harmful algal blooms, and disrupt marine food supplies, with increased fish mortality reducing fishery catches. The ocean heat wave in the northeastern Atlantic also intensified the heat wave that was affecting the European continent at the time.
Researchers at the University of Bern have now investigated whether an even more extreme marine heatwave could occur under today’s climate conditions. Their findings suggest that small changes in atmospheric conditions could substantially intensify and prolong marine heatwaves in this region. In the most extreme case, such an event could last as long as 579 days, with sea temperatures reaching up to 4.5°C above the long-term average. The results have just been published in the journal Nature Communications.
“Boosted” heatwaves
“High-pressure systems combined with weak winds and strong solar radiation favor the development of marine heatwaves,” explains lead author Catherine Gregory, a postdoctoral researcher with the Ocean Modeling research group at the Climate and Environmental Physics (CEP) division of the Physics Institute and the Oeschger Centre for Climate Change Research. “Strong solar radiation warms the ocean surface, while weak winds reduce mixing with cooler water below and limit heat loss to the atmosphere. Together, these conditions allow heat to accumulate near the surface, intensifying the marine heatwave.
To explore different possible scenarios for the heatwave, the researchers used a method known as ensemble boosting. This modelling approach simulates different possible evolutions of a selected extreme weather or climate event. To do so, the researchers used a climate model to make small, random changes to the initial atmospheric conditions seen during the 2023 marine heatwave event and then allowed the model to run freely to generate numerous different versions of the event. The simulated trajectories remain physically plausible but can differ considerably in intensity and duration of the resulting marine heatwaves. “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 Catherine Gregory.
The 2023 record still stands – for now
The 2023 heatwave set a record in the Northeast Atlantic, and that record still stands despite the extreme heat across much of Europe in the summer of 2026, says climate researcher Thomas Frölicher, the study’s senior author and head of the Ocean Modelling research group. “Ocean temperatures worldwide did indeed reach record highs in 2026. In the Northeast Atlantic, however, temperatures remained slightly below the levels recorded in 2023.” Thomas Frölicher stresses that this is no reason for complacency: “Our study shows that the 2023 marine heatwave did not represent the upper limit of what is physically possible. Even more extreme events could occur in the future, with far-reaching consequences for marine ecosystems, fisheries and heat conditions on land – particularly as human-induced climate change continues to warm the oceans.”
Global warming of 3°C could make extreme marine heatwaves more common
In the second part of the study, the researchers investigated 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 heatwaves comparable to the event in the summer of 2023 remain relatively rare. However, once global warming exceeds 3°C, such events become increasingly common, with the hottest event reaching almost 5°C above the long-term mean and the longest lasting event persisting for 1500 days. “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,” says Thomas Frölicher.
Publication details:
Gregory, C., Hofmann Elizondo, U., Guinaldo, T., Frölicher, T., (2026) The 2023 North Atlantic marine heatwave could have evolved into a far more extreme event. Nature Communications
DOI: 10.1038/s41467-026-78863-5
URL: https://www.nature.com/articles/s41467-026-78863-5