A landmark new study published in *Nature Climate Change* on Monday delivers stark, granular data confirming that decades of human-caused planetary warming have triggered a dramatic, widespread increase in dangerous heat stress across the globe. The research, which moves beyond standard temperature measurements to focus on “feels-like” temperatures that directly impact human health, finds that dozens of nations are now facing one to two additional months of heat stress annually compared to 60 years ago, with heat risk spreading even to regions that historically avoided such dangerous conditions.
Unlike prior climate research that has largely focused on raw air temperature increases, the study led by Rebecca Emerton, a senior scientist at the U.K.-based European Centre for Medium-Range Weather Forecasts, uses the Universal Thermal Climate Index to assess how combined environmental factors—temperature, humidity, wind speed, and more—affect the human body’s ability to regulate its internal heat. This approach provides a far more accurate picture of actual public health risk, because high humidity disrupts sweat evaporation, the human body’s primary natural cooling mechanism, making humid heat waves far more deadly than dry heat events of the same temperature.
The research categorized heat stress into three tiers of increasing risk: strong heat stress (index temperatures ≥ 32°C / 89.6°F), very strong heat stress (≥ 38°C / 100.4°F), and extreme heat stress (≥ 46°C / 114.8°F). Comparing modern data to baseline conditions from the 1970s, the team found that heat stress is intensifying in already warm regions and expanding into new geographic areas.
Hard-hit regions include parts of Southern and Eastern Africa (Namibia, Angola, Tanzania, Kenya, and Uganda), Mexico, and Central America, which could see up to 50 additional days of strong heat stress annually compared to the 1970s. Southern European nations including Southern Spain, Italy, Greece, and Turkey face up to 40 extra days of strong heat stress, with most of Southern Europe recording nearly a full month of additional heat stress days relative to 60 years ago. In the United States, most of the country experiences 15 or more extra days of strong heat stress annually, with southern states like Texas and Florida recording close to 25 or more additional days of very strong heat stress.
Beyond longer heat stress seasons, the study also found that overnight heat is warming faster than daytime heat: the 10 warmest annual nights have warmed by 0.32°C (0.58°F) per decade, compared to 0.27°C (0.49°F) per decade for the 10 warmest annual days. The rise in tropical nights—defined as nights with minimum temperatures of at least 20°C (68°F)—is particularly concerning, because it prevents people from cooling off and recovering from daytime heat exposure, raising the risk of heat-related illness and death. Overall, 1 billion more people around the world now experience at least one day of extreme heat stress annually than they did in the 1970s.
Emerton noted that the expansion of heat stress into previously unaffected regions was one of the study’s most striking findings. “It’s striking to see heat stress not only intensifying in those places that we already consider as being hot or used to experiencing heat waves … but also to see this, we call it, expanding footprint of heat stress expanding into regions where it’s historically been rare or non-existent,” she said.
Jennifer Francis, a climate scientist at the Woodwell Climate Research Center who was not involved in the study, emphasized that the new research adds critical granularity to existing understanding of climate change risks. “This study adds stark details about increasing dangers to billions of humans,” Francis said. “This analysis shows not only is temperature rising, but so is humidity, which makes high temperatures more deadly because our body’s air conditioning system — sweating — struggles to keep up.”
Lead researcher Emerton stressed that the severity of future heat stress increases depends entirely on near-term climate action. The study underscores the urgent need for aggressive cuts to fossil fuel emissions to mitigate future warming, alongside targeted investments in adaptation measures including heat health action plans, early warning systems, and updated climate risk assessments to protect vulnerable communities from rising heat risk.