The world's heat is growing more dangerous, and the biggest increase in temperatures is taking place not under the hot midday sun but during overnight darkness.
Using climate records and population estimates spanning seven decades, researchers concluded in a new study that harmful heat stress is increasing across every continent. Interestingly, they also found that nighttime "feels-like" temperatures have been climbing faster than daytime levels.
What's happening?
Researchers in a study published in Nature Climate Change analyzed the ERA5-HEAT dataset, which contains heat-stress records dating back to 1940, and used population figures to estimate global exposure to dangerous heat.
As Mongabay reported, the study found that roughly 70% of the global population now experiences 90 or more days of strong heat stress each year. In the 1970s, that figure was 55%.
The researchers also discovered that, over the past 50 years, dangerous heat stress has become more intense even in places not historically accustomed to it, including Europe. Recent heat waves there have proved deadly, with one event linked to more than 10,000 excess deaths, per Mongabay.
Rebecca Emerton, the study's lead author and a senior scientist at the European Centre for Medium-Range Weather Forecasts, said the danger was no longer hypothetical.
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"Heat stress is already affecting lives worldwide and is no longer a future threat," Emerton told Mongabay.
Why does it matter?
Heat stress is not just about how high the temperature climbs. It also reflects how hot conditions actually feel to the human body, including the effect of humidity, which makes it harder to cool down through sweating. When nights stay hot, people lose an important window for recovery.
In public health terms, high overnight temperatures can worsen symptoms for people with asthma, diabetes, and cardiovascular disease. Children and older adults are especially vulnerable, and the effects can build during prolonged heat waves.
Additionally, hot nights can disrupt sleep, strain household cooling units, imperil worker safety, and put pressure on hospitals and emergency systems. Communities with few resources, limited tree cover, or unreliable access to cooling often face the greatest risks.
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Christopher Callahan, an Earth system scientist at Indiana University who was not involved in the study, said the researchers' methods were reliable but cautioned that the dataset they used may actually undercount the most extreme temperatures in some parts of the "global south," or countries that are generally economically undeveloped, where vulnerability is high and long-term climate records are limited.
What's being done?
The study's authors said governments should expand heat-health action plans and early warning systems based around heat stress, not just air temperature. The researchers said this change could help officials identify where health threats are escalating fastest and apply interventions where they are most needed.
The research also points to the broader challenge of inequity, with the most vulnerable populations often lacking the resources necessary to address increasing heat stress.
"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," said Nasiru Idris, director of the Ibrahim Usman Jibril Institute for the Built Environment at Nigeria's Nasaraw State University, Keffi. "We cannot adapt our way out of extreme heat without immediate and equitable climate finance."
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