A new study using the ERA5-HEAT dataset shows hazardous heat stress is rising globally, with nighttime "feels-like" temperatures climbing faster than daytime values. About 70% of people now face 90+ days of strong heat stress per year, up from roughly 55% in the 1970s. Warm nights worsen health risks, strain infrastructure and disproportionately impact vulnerable communities. Researchers urge heat-health plans and early-warning systems based on heat-stress metrics and call for equitable climate finance.
Hazardous Heat Stress Surges Worldwide — Nights Are Warming Fastest
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Hazardous heat stress is increasing around the globe, and the fastest warming is occurring overnight rather than at the peak of the daytime sun. New research combining long-term climate records with population data shows that more people are exposed to prolonged periods of dangerous, "feels-like" heat.
Key Findings
The study, published in Nature Climate Change, analyzed the ERA5-HEAT dataset — which provides heat-stress records going back to 1940 — and integrated those data with population estimates spanning seven decades. The authors found that roughly 70% of the global population now experiences 90 or more days per year of strong heat stress, up from about 55% in the 1970s.
Importantly, nighttime "feels-like" temperatures are rising faster than daytime values. That trend reduces the body's ability to recover overnight and compounds health risks during prolonged heat waves.
"Heat stress is already affecting lives worldwide and is no longer a future threat," said Rebecca Emerton, the study's lead author and a senior scientist at the European Centre for Medium-Range Weather Forecasts.
Health, Social and Infrastructure Impacts
Warm nights worsen outcomes for people with asthma, diabetes and cardiovascular disease and place particular strain on children and older adults. Hot nights also disrupt sleep, overwork household cooling systems, endanger outdoor workers, and increase demand on hospitals and emergency services. Communities with limited resources, sparse tree cover, or unreliable access to cooling face the greatest risks.
Researchers note that dangerously high heat stress has intensified even in regions not historically accustomed to it, including parts of Europe. Recent European heat waves have been deadly — one event has been associated with more than 10,000 excess deaths, according to reporting.
Data Limitations and Equity Concerns
Experts not involved with the study say the methods are robust but caution that ERA5-HEAT may undercount the most extreme temperatures in some parts of the Global South, where long-term weather records are sparse and vulnerability is high. That limitation could mean the study underestimates exposure for some of the most at-risk populations.
"We cannot adapt our way out of extreme heat without immediate and equitable climate finance," said Nasiru Idris, director of the Ibrahim Usman Jibril Institute for the Built Environment at Nasarawa State University in Nigeria.
Policy Recommendations
The authors recommend that governments expand heat-health action plans and early-warning systems that use heat-stress metrics (which account for humidity and human heat burden), not just air temperature. Targeted warnings and interventions can help identify where health threats are escalating fastest and allocate resources to the most vulnerable communities.
As heat stress increases worldwide, coordinated public health planning, improved cooling access, urban tree cover and equitable climate finance will be critical to reduce harm and protect the most exposed populations.
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