UK coastal waters are on the verge of reaching an “extreme” marine heatwave classification rarely recorded in British seas, with forecasters warning that the English Channel and North Sea are experiencing the kind of conditions not normally expected until August.
The Met Office has confirmed that surface waters across north-west European seas are currently ranging from moderate to severe, with widespread conditions classified as “strong” and some areas reaching “severe.” Sea temperatures are already running on average 2C above normal, with some offshore areas along the English and Welsh coasts more than 4-5C warmer than usual. If warm and calm weather materialises again next week as forecast — with parts of south-east England potentially seeing temperatures above 30C — there will be little opportunity for the ocean to shed its excess heat, pushing conditions toward the “extreme” threshold.
Dr Segolène Berthou, air-sea interaction specialist at the Met Office, said: “Marine heatwaves around the UK have developed rapidly following the recent heat dome, and we are now seeing widespread strong to locally severe conditions. With further sunny and calm weather likely next week, there will be little opportunity for the ocean to release this excess heat. Such conditions would be highly unusual for UK waters.”
The current spike follows last week’s exceptional land heatwave, which broke the UK’s all-time June temperature record — set during the notorious summer of 1976 — by more than 1C. The English Channel had already been experiencing marine heatwave conditions for much of 2026, making it particularly susceptible to further warming, while anomalies in parts of the North Sea are forecast to peak at around 4-5C above average in the coming days.
The situation is not confined to UK waters. The EU’s Copernicus Climate Change Service confirmed earlier this week that global sea surface temperatures have broken seasonal records set in both 2023 and 2024. The development of El Niño conditions — formally announced by the UN’s World Meteorological Organisation last month — has contributed to the pattern, with the warming of tropical Pacific waters pushing up temperatures across multiple ocean regions globally.
Scientists are warning that the consequences for marine ecosystems could be severe and long-lasting. John K Pinnegar, principal scientist and lead adviser for climate change at the Government’s Centre for Environment, Fisheries and Aquaculture Science, said prolonged marine heatwaves can fundamentally reshape what lives in UK waters. “Marine heatwaves can have significant impacts on ecosystems and wildlife. Prolonged periods of unusually warm sea temperatures can lead to shifts in fish populations, damage important habitats such as seagrass and kelp, and increase the likelihood of harmful algal blooms. These changes can result in mass-mortality events for some marine species and alter the distribution of commercially-important fish and shellfish.”
He cited an ongoing population bloom of common octopus as a direct example, describing it as having “serious negative consequences” for crab and lobster fisheries across south-west England — an early signal of how warmer waters can destabilise established ecosystems and disrupt the livelihoods of fishing communities.
Despite the elevated sea temperatures, the Met Office has cautioned the public not to assume the water is safe to enter. Holly Clements, the Met Office’s head of warnings and guidance, said: “With warmer weather approaching, and even with marine heatwave conditions, it’s important to remember that the water around the UK is still cold. Entering it unexpectedly can lead to cold-water shock.” The public is advised to swim only at RNLI lifeguarded beaches, between the red and yellow flags, and to call 999 for the coastguard in any coastal emergency.
