UK waters hit with extreme heatwave as global sea temperatures reach record levels

Experts warn that some marine species are at risk of ‘mass mortality events’ in ever-warming oceans

UK waters are being hit with an “extreme” marine heatwave, the Met Office has said, as scientists warn that high ocean temperatures globally could result in “mass-mortality events” for some species.

The forecasters said these elevated temperatures have developed rapidly because of last month’s heat dome, during which most of Europe sweltered in its worst ever heatwave that scientists said would have been impossible without the climate crisis.

The third heatwave of the summer has arrived in the UK this week, with temperatures on track to exceed 30C for up to 10 consecutive days. Dr Ségolène Berthou, an air-sea interaction specialist at the Met Office, said these atmospheric conditions drive the marine heatwave.

“The ocean didn’t have enough time to cool down between the two land heatwaves,” she said.

Oceans absorb more than 90% of the excess energy in the Earth system, which is primarily caused by burning fossil fuels.

Berthou said this is the third and most intense marine heatwave the UK has seen this year, with temperatures on average 2C warmer than usual and with some locations reaching “severe” conditions where the water temperature is 4-5C warmer than usual. “There’s no sign of an end to it,” Berthou added.

The Copernicus Climate Change Service and the Copernicus Marine Service have confirmed that global sea surface temperatures have surpassed the previous records for this time of year, which were set in 2023 and 2024. This had been anticipated to follow the development of El Niño conditions , which scientists forecast to be the strongest in decades.

The UN secretary general, António Guterres, said the world must treat the arrival of El Niño conditions as the “urgent climate warning it is”.

He said: “The only effective response is climate action equal to the crisis: ending the addiction to fossil fuels, accelerating the shift to renewables, protecting the most vulnerable, and delivering early warning systems for all.”

Prof John Pinnegar, principal scientist and lead adviser at the Centre for Environment, Fisheries and Aquaculture Science, said these extreme marine heatwave conditions can result in “mass-mortality events” for some marine species and alter the distribution of commercially important fish and shellfish.

He said: “Prolonged periods of elevated sea water temperatures can also encourage new species to visit UK waters, establish new populations, potentially shaking up UK ecosystems.”

Record numbers of octopuses were found off the south-west coast of England last year, transforming the fishing industry and the marine ecosystem. A record 100 tonnes of octopus was sold in one day at Brixham market last month.