UK waters hit extreme marine heatwave

The Met Office said on July 3 that a long-running marine heatwave around the UK could intensify to “extreme” levels, a category rarely recorded in UK waters. The agency said surface waters around much of the English and Welsh coastlines, the English Channel and the central and southern North Sea were already in Category 2 “strong” marine heatwave conditions, with some locations reaching Category 3 “severe.” Sea-surface temperatures in those areas were about 1.5C to 4C above normal, the Met Office said, with some offshore parts of the Welsh and English coasts reaching anomalies of 4C to 5C. The forecaster said those waters were now as warm as temperatures usually seen in August, after a period of record-breaking atmospheric heat in late June. (metoffice.gov.uk) ### What exactly is happening in UK waters? The Met Office said the heat is most widespread around English and Welsh coasts, the English Channel, and the central and southern North Sea. In tidally mixed regions such as the Irish Sea, English Channel and southern North Sea, it said the warming is not only at the surface but also extends below the surface, with anomalies of about 2C at depth. (metoffice.gov.uk) The agency said the English Channel has already experienced marine heatwave conditions for much of 2026. It added that parts of the North Sea were forecast to peak at around 4C to 5C above average in the coming days. ### What does “extreme” mean in this context? (metoffice.gov.uk) A marine heatwave is a period of persistently unusual ocean warmth, NOAA says. A commonly used threshold defines it as temperatures above the 90th percentile of historical values for at least five consecutive days. Mercator Ocean, which operates the Copernicus Marine Service, says marine heatwaves are graded by intensity from moderate to extreme. (metoffice.gov.uk) Under that system, Category 4 is classified as “extreme,” which is the level the Met Office said parts of UK waters could reach if warm, calm conditions persist. ### Why are scientists worried about marine life? (psl.noaa.gov) The Met Office said prolonged periods of elevated seawater temperatures can trigger “mass-mortality events” for some marine species. It said the heat can alter the distribution of commercially important fish and shellfish and encourage new species to establish populations in UK waters. (mercator-ocean.eu) BBC reporting, citing scientists and the Met Office, said long periods of sea heat can cause mass die-offs among some seagrasses, shellfish and other species, while also favoring warm-water species including octopus. ### How does this connect to global ocean temperatures? Copernicus Climate Change Service and Copernicus Marine Service said on July 1 that daily global sea-surface temperatures exceeded previous records for this time of year. (metoffice.gov.uk) Copernicus said its daily data reached 20.86C on June 21, above the 20.83C seen in 2023 and 2024, while Copernicus Marine data showed 21.0C the same day. (yahoo.com) Copernicus Marine said the first half of 2026 was marked by “sustained and exceptional ocean warmth,” with average global sea-surface temperature of about 20.94C from January through June. By the end of June, it said, about 82% of the global ocean was experiencing marine heatwave conditions of varying intensity. (climate.copernicus.eu) ### What comes next for the UK forecast? The Met Office said a renewed spell of sunny, settled weather would give the sea little opportunity to cool and could push the current event into Category 4 “extreme” conditions. The agency said inland temperatures in parts of southeast England were forecast to rise into the low 30s Celsius next week, conditions that would keep pressure on already-warm coastal waters. (metoffice.gov.uk) (marine.copernicus.eu)