Guest post: France’s June heatwave caused more than 2,700 heat-related deaths

In June 2026, a record-breaking heatwave swept across Europe, with France among the first and hardest hit countries.  In a new analysis, we estimate that the extreme conditions caused more than 2,700 heat-related deaths in France. We also show how France’s extreme temperatures in June exceeded projections from climate models. Our findings illustrate the human toll of extreme weather as the world warms. We also highlight the challenges in projecting the magnitude of future heatwaves and their impacts on people. This is illustrated in the figure below, which shows how France’s average maximum daily high temperature for June recorded in 2026 (black line) compares to climate model projections (blue and orange lines). The downstream impacts of these extreme temperatures are lethal.  Scientists are able to estimate the death toll of high temperatures in many locations, depending on the availability of mortality and climate data.  There are several ways to do this.  One option is to examine death certificates to see which deaths have been directly recorded . However, there is strong evidence that this method significantly undercounts heat-related deaths, as most death certificates do not consider environmental factors such as heat when diagnosing the cause of death. Alternatively, it is possible to calculate the rate of total (“all-cause”) mortality in a given time period relative to previous time periods – for example, 2026 compared to the average of previous Junes. This “excess deaths” figure can be used as an estimate of the deaths from a heat wave.  Using this approach, Public Health France attributed around 2,000 deaths in France to the extreme heat in the week of 22-28 June. Finally, scientists can use long-term data on overall mortality and correlate changes in mortality with changes in temperature to understand the statistical relationship between the two.  Research published in Proceedings of the National Academy of Sciences in 2025 that used this third approach found that mortality rates in France increase rapidly in cold or hot conditions as daily maximum temperatures depart further from approximately 20C. This pattern of a U-shaped response of mortality to temperature – shown in the figure below – is very consistent across time periods and regions around the world.  To calculate the death toll of the June 2026 heatwave in France, we compared observed temperatures over 12-29 June to their baseline average over 1980-2025.  The difference between these two temperatures helps us understand how many more people died than they would have in the absence of such extreme conditions.