Copernicus says 52 percent of Europe hit very strong heat stress this summer

Just over half of Europe, 52 percent of the continent, was exposed to very strong heat stress at some point this summer, the EU's Copernicus Climate Change Service said on Friday. Very strong, in the service's scale, means conditions that feel like at least 38 degrees Celsius. Western Europe had its warmest summer on record, at 21.7 degrees, 2.5 degrees above the 1991-2020 average, passing the mark set in 2003. Rivers ran at their lowest summer flows in more than 30 years.

The figures were published in Bonn on 2 October, from the ERA5 and ERA5-HEAT datasets. They describe June to August, with the service noting that record daily heat also fell in May and September. The health threshold, not the average temperature, is the number that changes how cities plan the next season.

What 52 percent counts

Copernicus counts land area that experienced the threshold at least once, not the share of days and not the share of people. A mountain grid cell and a city grid cell weigh the same in the 52 percent. The service said this was the highest such share on record. Western Europe, taken alone, recorded very strong heat stress on 13 days. Averaged across that region, strong heat stress occurred on 41 days by the end of the season, five more than in 2003.

The persistence figure is separate. In 2026, western Europe had 17 days on which at least 60 percent of the region was above 30 degrees. The same measure was 11 days in 2003 and 7 days in 2025. A hotter average can come from a few extreme afternoons. Seventeen days above that area threshold means the heat sat on most of the region for more than two weeks of the season, not for a single spell.

Rivers, and who the threshold hits

Low river flow is the other half of the Bonn release. Europe's rivers recorded their lowest summer flows in more than three decades. That is a navigation fact, a power fact and a farm fact. Barges on the Rhine carry fuel and grain on a draft that falls as the river falls. Nuclear and gas plants that use river water for cooling face temperature limits on discharge. Neither constraint appears in the 21.7 degree average. Both appear in a season that combined that average with the lowest flows in 30 years.

Heat stress is not the air temperature. It is the combination that stops sweat from cooling the body. Copernicus lists a faster heart rate, nausea, dizziness and heatstroke. Outdoor workers, older people, and people with heart disease or diabetes carry more of that risk. The 38 degree feels-like line is the service's cut for very strong. It is not a legal work limit. Several European states set their own stop-work rules at different temperatures, which is why a single continental share does not produce a single labour rule.

How this summer sits against 2003

The summer of 2003 is the benchmark because excess deaths in France, Italy and Spain made it the last European heat season that public health agencies still use as a reference. Western Europe's 2026 average passed that summer by the Copernicus reckoning. The area-days above 30 degrees also passed it, 17 against 11. The strong heat-stress day count passed it, 41 against 36. Those three comparisons are the information gain in Friday's release. A headline that says hottest summer does not say whether the heat was broader. The day counts say it was.

The World Meteorological Organization has described Europe as the fastest-warming continent. Friday's note does not re-litigate that. It measures one season against the 1991-2020 baseline and against 2003. Human influence on the trend is the WMO's claim. The 52 percent, the 21.7 degrees and the river flows are Copernicus's measurements.

The next release that would change the planning number is not another summer average. It is whether national health agencies publish excess-death counts for June to August that can be set beside 2003. Until those counts exist, the operational facts are the ones Bonn published: half the continent over the 38 degree feels-like line at least once, western Europe hotter than 2003 on three different measures, and rivers lower than they have been in a summer for 30 years.