North Sea marine heatwaves now last three times longer than in the 1980s, UEA study finds

Marine heatwaves in the North Sea have tripled in length since the 1980s and now occur in most years rather than roughly once a decade, according to the most comprehensive analysis yet of British coastal waters, published on Monday by the University of East Anglia.

Researchers at UEA’s school of environmental sciences combined satellite sea surface temperature records with readings from 23 buoys and lightships going back to 1981 to identify periods when the water was warmer than 90% of readings for that date. In the 1980s such events lasted an average of nine days and occurred in two years out of ten. Between 2016 and 2025 they lasted an average of 28 days and occurred in nine years out of ten. The longest, in the summer of 2023, lasted 61 days.

The study, in the journal Global Change Biology, links the heatwaves to measurable ecological shifts. Cod recruitment in the southern North Sea fell by an estimated 40% in years following a major heatwave; sandeel abundance in the Dogger Bank, a key food source for puffins and kittiwakes, fell by about a quarter. Warm-water species including anchovy, red mullet and the Portuguese man o’ war have become routine where they were once rare.

“The North Sea is shallow and semi-enclosed, which means it heats up fast and does not have a deep cold reservoir to draw on,” said Dr Olumide Fashola, the lead author. “What used to be a once-a-decade anomaly is now the normal summer. The ecosystem has not had time to catch up, and some of it will not.”

Sea surface temperatures off East Anglia reached 20.6C on 23 June this year, 3.1C above the 1991-2020 average for the date, and remained above 19C for five weeks. The Met Office classified the event as a category 3 marine heatwave, the second highest grade. Fishers in Lowestoft reported catching bluefin tuna, a Mediterranean species, within sight of the shore in August.

The study found that the frequency and length of heatwaves tracked the global rise in ocean heat content but were amplified by weaker summer winds over the North Sea, which reduce the mixing that normally cools surface water. Whether the calmer summers are themselves a product of climate change or natural variability is, the authors say, an open question.

The findings have implications for the offshore wind industry, which is building foundations and cables designed around historical water temperatures, and for the oyster and mussel farms of the Essex and Lincolnshire coasts, which lost an estimated £4m of stock in 2023. The Shellfish Association of Great Britain said its members were already moving to deeper beds.

“People think of sea temperature as something that matters to coral reefs,” Fashola said. “It matters to Cromer crab.”