Global heat stress now reaches 22% of the world’s population, study finds
Extreme heat stress now affects 22% of the world population, up from 16% in the 1970s, about a billion more people, a Nature Climate Change study finds.
Photo by Dominika P on Pexels
Global heat stress has intensified across every continent since the 1970s, and the share of people exposed to extreme heat has climbed from 16% to 22%, according to a study published in “Nature Climate Change” on June 22, 2026. The work, led by Rebecca Emerton at the European Centre for Medium-Range Weather Forecasts (ECMWF), estimates that about one billion more people now face at least one day of extreme heat stress each year than would have in the 1970s.
The team measured heat with the Universal Thermal Climate Index (UTCI), a “feels-like” scale that combines air temperature, humidity, wind, and radiation, so it can differ from the reading on a thermometer. Using the ERA5-HEAT dataset, the researchers tracked daytime extremes, nocturnal heat, and compound day-night events from 1950 to 2024, comparing recent years (2015 to 2024) with the 1970s.
What the global heat stress study found
The headline numbers describe both how many people are affected and how the extremes are shifting. The table below summarises the main verified figures.
| Heat stress measure | Value |
|---|---|
| Population facing at least one extreme heat stress day (1970s) | 16% |
| Population facing at least one extreme heat stress day (recent) | 22% |
| Additional people exposed each year vs the 1970s | about 1 billion |
| Warming rate, hottest nights of the year | 0.32 °C per decade |
| Warming rate, hottest days of the year | 0.27 °C per decade |
| Additional strong-to-extreme heat stress days per year, subtropics | up to 50 |
Source: Emerton et al., “Global heat stress intensification and its expanding footprint on the human population,” Nature Climate Change (June 22, 2026); ECMWF.
On the UTCI scale, “strong” heat stress begins at 32 °C and “extreme” heat stress at 46 °C. The study reports that hazardous heat has reached regions that rarely saw it before, expanding the map of exposure rather than only raising it where heat was already common.
Nights are warming faster than days
One of the study’s clearest signals is that the hottest nights are heating faster than the hottest days. The ten warmest nights of each year warmed at 0.32 °C per decade, against 0.27 °C per decade for the ten warmest days. Hot nights matter for health because the body relies on cooler overnight hours to recover from daytime heat.
Regional gains are uneven. In parts of southern Europe, northern Africa, and the Arabian Peninsula, the peak UTCI on the ten hottest days is now up to 4 °C warmer than in the 1970s, and up to 5 °C warmer in parts of southwestern Europe, according to ECMWF. In Europe, extreme heat stress now occurs about 2.5 times more often. Some subtropical regions see up to 50 additional strong-to-extreme heat stress days each year.
“Compared to the 1970s, heat stress is now more severe, more frequent and longer-lasting,” said Rebecca Emerton of ECMWF. The rise in hazardous heat adds to the pressure on health systems already tested by hotter summers, a strain examined in Is the Global Healthcare System Ready for Climate Change?.
Why the feels-like measure matters
Because the UTCI blends humidity, wind, and radiation with temperature, it can read well above or below the air temperature, which is why the study frames heat as a “feels-like” hazard rather than a single thermometer value. That approach helps explain why humid regions can reach dangerous conditions at air temperatures that sound moderate. The findings build on hard lessons from recent extreme summers, including the toll set out in Europe’s 2025 Heatwave: Billions Lost and Lives at Risk, and they strengthen the case for cooling measures such as those in How Trees Reduce Urban Heat: Cooling Benefits Backed by Data.
About ECMWF
The European Centre for Medium-Range Weather Forecasts is an intergovernmental organisation established in 1975 and supported by a group of European member and cooperating states. Its headquarters are in Reading, United Kingdom, with additional sites in Bonn, Germany, and a data centre in Bologna, Italy. ECMWF produces global weather forecasts and the widely used ERA5 reanalysis, and it operates two European Union Copernicus services, the Climate Change Service and the Atmosphere Monitoring Service. The ERA5-HEAT dataset used in this study is derived from that reanalysis. By applying a feels-like index across 75 years of data, the ECMWF-led team has quantified how far extreme heat has spread across the human population, giving governments and health agencies a clearer measure of a hazard that the study reports is now more severe, more frequent, and longer-lasting than in the 1970s.
Sources: Nature Climate Change; EurekAlert / ECMWF; Phys.org
Featured image: photo by Dominika P on Pexels (free Pexels license).
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I specialize in sustainability education, curriculum co-creation, and early-stage project strategy. At WINSS, I craft articles on sustainability, transformative AI, and related topics. When I’m not writing, you’ll find me chasing the perfect sushi roll, exploring cities around the globe, or unwinding with my dog Puffy — the world’s most loyal sidekick.
