Photo by Sebastien Gabriel

Global sea surface temperatures reached their highest level ever recorded for the month of July, according to new data from the Copernicus Climate Change Service (C3S), as a prolonged marine heatwave around the UK edges towards rare “extreme” territory and much of England and Wales is declared officially in drought. The average sea surface temperature across the extra-polar ocean (60°S–60°N) hit 20.96°C in July, surpassing the previous July record of 20.89°C set in 2023, a milestone also reported by the BBC and the Guardian.

A record fuelled by heat domes and El Niño

The record forms part of a wider picture of exceptional heat across western Europe. Copernicus, implemented by the European Centre for Medium-Range Weather Forecasts (ECMWF), reported that persistent high-pressure systems trapped heat over the region through June and July, with average June-July temperatures for western Europe reaching a new record of 21.62°C, 2.79°C above the seasonal norm and surpassing the previous high set in 2022. Around Europe, sea surface temperatures reached record highs for July along the Atlantic coast and western Mediterranean, associated with widespread strong or severe marine heatwave conditions, while temperatures in the tropical Pacific remain exceptionally high as developing El Niño conditions are forecast to strengthen further in the coming months.

Samantha Burgess, Strategic Lead for Climate at ECMWF, described the summer as “a clear example” of climate breakdown intensifying extremes that reinforce each other, telling the Guardian that persistent high-pressure systems trapped heat over western Europe and amplified widespread drought. “As soils dry out, they lose their ability to provide natural cooling, allowing heat to build more readily,” she said. “This is a clear example of how climate change is intensifying heat extremes, with heat and drought increasingly reinforcing one another.”

Drought bites in England and Wales

The dry conditions underpinning the marine heat are now translating into a domestic water crisis. England and Wales recorded their driest July since records began 190 years ago, according to the Met Office, with large parts of England and the whole of Wales already officially declared in drought. The Environment Agency has warned further areas of England could follow if dry conditions persist, adding pressure to water supplies, abstraction and, by extension, the freshwater inputs that shape estuarine and coastal ecosystems.

The economic toll of this pattern is starting to be quantified. The Guardian reports that early estimates put the cost of June’s heat to European grain farmers at €2bn in lost crops, while a new working paper led by the CMCC European Institute on Economics found the 2015-18 mega-drought cost Europe €439bn overall, with damage compounding across sectors for years after the event itself. Marta Mastropietro, a researcher at CMCC and the Polytechnic University of Milan and the paper’s lead author, said conventional impact estimates tend to miss this longer tail. “Even a single dry year causes real economic damage, but a persistent drought is different: its effects can compound across sectors and keep accumulating for years afterward,” she said.

Mike Kendon, a Met Office climate information scientist, said the scale of change now being observed marked a genuine departure from historical norms. “We are right now living in a time of historic and unprecedented change,” he told the BBC, adding that “evidence shows the climate of the 20th century has now gone.” The Met Office’s latest climate assessment found the hottest day of the year in southern England is now 4.5°C warmer than it was between 1961 and 1990.

UK waters approach category four conditions

For UK marine stakeholders, the more immediate story is playing out closer to home. According to the National Oceanography Centre’s rolling monitoring of the marine heatwave that has built around the UK and Ireland since February, the southern half of UK waters remained in category I-III conditions as of 9 August, with a small area south of Weymouth entering category IV, or “severe”, conditions, and the English Channel running 2-3°C warmer than usual for the time of year. Waters around the northern half of the UK remain close to average. The BBC reports the UK is braced for its fifth heatwave of the year this week, with persistent dry weather likely to push more areas into official drought.

Cefas’ WaveNet buoy network, which monitors sea temperatures at 72 sites around the UK, has recorded some striking anomalies. Dr Bryony Townhill, principal scientist and deputy director of the international marine climate change centre at Cefas, said the South Knock buoy in the Thames Estuary had recorded its highest ever temperature this week, at 21.6°C. “We are not yet at the annual peak for sea temperatures, which normally occurs next week,” she said. “It is quite possible that this could be exceeded given the high levels of sunshine and weak winds forecasted over the coming week.”

Eloise Matthews, an Applied Climate Scientist at the Met Office, said the coming weeks were unlikely to bring relief. “Climatologically, August is the month of peak sea surface temperatures around the UK, and the outlook is for marine heatwave conditions to persist around their current levels throughout the month, although this will be dependent on weather conditions,” she said.

Early signs of ecological disruption

Dr Townhill pointed to mounting anecdotal evidence of the heat’s ecological knock-on effects, including continued reports of large numbers of octopus in UK waters, increased sightings of sharks and ocean sunfish, and a fluorescent Noctiluca algal bloom in the Irish Sea. “Scientists across the UK are working to determine the exact cause of these events and other potential impacts of marine heatwaves,” she said.

The pattern is consistent with the wider damage marine heatwaves are known to cause. Hotter seas are driving longer and fiercer marine heatwaves, which have bleached coral reefs on a mass scale and decimated kelp forests and fisherieselsewhere in the world, the BBC notes, while sustained ocean heat can trigger mass die-offs in nursery habitats and threaten shellfish and pen-reared aquaculture that cannot escape warming waters or the associated drops in dissolved oxygen, as Oceanographic Magazine reports.

What comes next

With the seasonal SST peak still ahead, El Niño conditions forecast to strengthen further, and a fifth UK heatwave of the year already forecast this week, both Copernicus and UK monitoring bodies expect the current run of heat, on land and at sea, to persist into the autumn. NOC’s ongoing subsurface monitoring, including CTD data from the RRS James Cook and readings from the Porcupine Abyssal Plain Sustained Observatory, is tracking how deep the warm anomaly is being mixed through the water column, a factor scientists say will shape how long the heatwave persists and how severely it disrupts marine food webs, fisheries and coastal water supplies into 2027.