Climate change is causing frequent, well-studied, extreme heat events. Less well understood is the additional heat stress experienced by billions of people as a result of those extreme heat events. However, a new study published in Nature Climate Change aims to change that.
The study analyzed more than 70 years of climate data stored in the ERA5-HEAT data set, which includes heat stress data going back to 1940. The researchers combined that climate information with population data to estimate human exposure to hazardous heat worldwide.
The study found that nighttime “feels-like” temperatures have increased faster than daytime temperatures. Around 70% of the world’s population now experiences at least 90 days of strong heat stress annually, compared with 55% in the 1970s.
They found that hazardous heat stress has intensified across every continent over the last five decades, including in places historically unaccustomed to such heat like parts of Europe. A recent heat wave in Europe is said to have caused more than 10,000 excess deaths.
“Heat stress is already affecting lives worldwide and is no longer a future threat,” Rebecca Emerton, lead author of the study, and a senior scientist with the European Centre for Medium-Range Weather Forecasts, told Mongabay in a video call. Extreme heat causes more human deaths than any other extreme weather event; children and the elderly are particularly vulnerable. Underlying health conditions including asthma, diabetes and cardiovascular disease can become deadly with heat.
“This study provides strong evidence that heat stress is increasing globally and that more people are being exposed to dangerous conditions, offering a stronger basis for assessing climate-related health risks,” Nasiru Idris, director of the Ibrahim Usman Jibril Institute for the Built Environment at Nigeria’s Nasarawa State University, Keffi, told Mongabay in an email. “We cannot adapt our way out of extreme heat without immediate and equitable climate finance,” added Idris, who was not involved with the study.
Describing the methods used for the study as robust, Christopher Callahan, an Earth system scientist at Indiana University in the US who was not involved with the work, cautioned that reliance on the ERA5 data set may still underestimate extreme temperatures in some in the Global South, where vulnerability is highest but historical climate records remain limited.
The study highlights the need for governments to strengthen heat-health action plans, improve early warning systems and incorporate heat stress, rather than air temperature alone, into climate adaptation and planning, the researchers noted.
Banner image: Grace Chyuwei pours water on Joe Chyuwei to help with the heat in Death Valley National Park, California, in 2025. Image by AP Photo/John Locher, File.
