Met Office warns of extremely strong marine heatwave in the English Channel north of Brittany

Brest – 09.07.2026: The UK Met Office reports a persistent marine heatwave in parts of the waters around the United Kingdom, which in sections of the English Channel north of Brittany is already classified as ‘extreme’. Sea surface temperatures are well above the seasonal average, with values usually only seen in August. The most affected areas are shallower and lower-current sectors where warm surface water remains for longer.

According to current analyses, temperatures deviate by about 1.5 to 4 °C from the 1982-2012 reference mean. Temperatures of around 15 to 19 °C were measured in the mentioned zones—exceptional for early July at this latitude. The assessment is based on combined evaluations from satellite observations and buoy measurements. The Met Office cites long-lasting high-pressure conditions, weak mixing of the upper water layers, and the aftereffects of earlier heatwaves on land as contributing factors. Added to this are the longer-term warming trend of the oceans and a recently active El-Niño signal that affects global marine conditions.

Experts see ecological and economic risks. Prolonged warm surface water can increase stress on seagrass beds, plankton and fish spawn. Near the coast, periods of low oxygen content may occur, which can lead to shifts in fish stocks or losses in mussel and oyster farms. For fisheries, there are uncertainties about fishing areas and times; aquaculture operators must closely monitor water quality, feed intake and mortalities. Additionally, higher temperatures can promote algal growth and bacterial contamination, which may result in local bathing water warnings.

French authorities, including Météo France and maritime administrations, are reportedly in contact with British authorities, according to media reports. Observers recommend running monitoring networks closely along the Breton and Norman coasts, taking additional water samples and, if necessary, considering short-term protective measures—such as temporary rest zones in sensitive bays or adjustments to harvesting and fishing windows. The Royal National Lifeboat Institution reminds people of basic safety rules independently, since changed currents and temperature gradients can catch swimmers and water sport enthusiasts off guard.

The Met Office assessment was published in early July 2026; the situation is being continuously updated. Coastal communities and businesses are advised to follow guidance from local prefectures, health authorities and port authorities. Scientific institutes on both sides of the Channel plan to document further developments to detect potential ecological damage early and to facilitate coordinated responses between France and the United Kingdom.