July 20: Heat stress is becoming more frequent, intense and widespread across the world, exposing millions more people to hazardous temperatures and extending heat stress seasons, according to a new global study published in Nature Climate Change.
The study, led by researchers from the European Centre for Medium Range Weather Forecasts (ECMWF), analysed more than seven decades of data from the Copernicus Climate Change Service’s ERA5-HEAT dataset using the Universal Thermal Climate Index (UTCI), which measures “feels-like” temperature by considering air temperature, humidity, wind and solar radiation.
Researchers found that the maximum feels like temperature during the 10 hottest days of the year has increased by an average of 0.27°C per decade since the 1970s, while the warmest night time temperatures have risen even faster at 0.32°C per decade.
The analysis showed that the world now experiences about two additional days per decade with at least strong heat stress one extra day with very strong heat stress and two more tropical nights, when temperatures remain above 20°C overnight.
Lead author Rebecca Emerton said rising night time temperatures are becoming a growing concern because they reduce the body’s ability to recover from daytime heat.
Comparing 2015-2024 with the 1970s, researchers found daily feels-like temperatures during the hottest days have increased by up to 4-5°C across parts of Europe, northern Africa and the Arabian Peninsula, while many other regions recorded increases of 2-3°C.
The study also found that hazardous heat now affects a much larger geographical area. Subtropical regions, including southern Europe, southern North America, northern and southern Africa, South America and Australia, experience up to 50 additional days each year with at least strong heat stress compared to the 1970s.
Extreme heat stress now occurs 2.5 times more frequently in Europe and South America twice as often in North America and has also increased significantly across Africa, Oceania and Asia.
Researchers reported that heat stress seasons have lengthened considerably, particularly across the Northern Hemisphere. On average, the season with moderate heat stress now lasts 15 days longer, while seasons with strong, very strong and extreme heat stress have extended by 12, nine and six days, respectively.
The study also highlighted a sharp rise in compound heat events where extremely hot days are followed by tropical nights, increasing health risks due to limited overnight recovery. In Europe, single day compound heat events have increased by 73% since the 1970s, while events lasting 15-30 days have become 3.4 times more common.
Population exposure to dangerous heat has also increased significantly. During the 1970s, 55% of the global population experienced at least 90 days of strong heat stress each year. That figure has now risen to 70 per cent.
Similarly proportion of people exposed to at least one day of extreme heat stress has increased from 16 per cent to 22 per cent representing around one billion additional people experiencing extreme heat annually after accounting for population growth.
The researchers noted that climate change, rather than population growth alone, is the primary driver behind increasing heat exposure. Regions facing the highest levels of very strong heat stress include sub Saharan Africa, South and Southeast Asia, the Arabian Peninsula and the Mediterranean.
The study concludes that as global temperatures continue to rise, long term datasets such as ERA5 HEAT will play a crucial role in understanding evolving heat risks and supporting adaptation strategies to protect public health.