Heat stress expands: billions face more extreme nights

New research says heat stress—driven by rising heat and humidity—has become longer, more frequent and more widespread since the 1970s, with some regions seeing up to dozens of additional dangerous days each year and a growing number of people facing extreme he

For many people, the most dangerous part of a heat wave isn’t the peak daytime temperature—it’s whether the body gets any real chance to recover overnight.

A new study published Monday in the journal Nature Climate Change puts that risk into sharper focus. Researchers say heat stress has intensified across the world over the past six decades. becoming more frequent. longer and more severe as the planet warms—an outcome they tie to the burning of fossil fuels like coal. oil and gas.

The findings go beyond measuring temperature alone. The research uses what’s called the Universal Thermal Climate Index to examine feels-like temperatures and heat stress on individual humans. That index incorporates temperature, humidity, wind speed and other factors to model how the human body responds to its environment.

That choice matters because humidity changes the danger. Humid heat can be more fatal than dry heat because humidity interferes with how sweat evaporates—the body’s cooling mechanism. When the air holds onto moisture. the body can’t cool down as easily. leaving people exposed to heat for longer stretches.

The study says some regions that were previously less affected are now feeling the strain. Heat stress isn’t only worsening in places already known for intense heat waves; its footprint is expanding into areas where it was historically rare or nonexistent.

The researchers examined three levels of heat stress using index temperatures measured in degrees Celsius and converted to degrees Fahrenheit: strong heat stress at index temperatures of greater than or equal to 32 degrees Celsius (89.6 degrees Fahrenheit). very strong at greater than or equal to 38 degrees Celsius (100.4 degrees Fahrenheit). and extreme at greater than or equal to 46 degrees Celsius (114.8 degrees Fahrenheit).

In parts of Southern Africa, including Namibia and Angola, the study projects as many as 50 more days per year of at least strong heat stress compared with the 1970s. Eastern Africa also shows major increases, including parts of Tanzania, Kenya and Uganda.

Mexico and Central America are included among the regions that could see similarly steep changes.

Across Southern Europe—spanning Southern Spain. Italy. Greece and Turkey—some areas may experience up to 40 additional days with strong heat stress compared with the 1970s. The study describes much of Southern Europe as seeing almost a full month of additional strong heat stress days compared with decades ago.

In the United States, much of the country is described as seeing 15 or more days of at least strong heat stress, with southern areas such as Texas and Florida close to 25 or more days with very strong heat stress.

Not only are these heat-stress days increasing in number, the study says heat stress seasons are lasting longer as well.

One striking detail is how night-time danger is changing. The researchers report that the feels-like temperatures on the ten warmest nights of each year have risen faster than daytime peaks. Those ten warmest nights increased by 0.32 degrees Celsius (0.58 degrees Fahrenheit) per decade. while the ten warmest days increased by 0.27 degrees Celsius (0.49 degrees Fahrenheit) per decade.

The study also looks at tropical nights—defined using minimum temperature of 20 degrees Celsius (68 degrees Fahrenheit). That definition points to a key human impact: it’s about whether people can recover properly after daytime heat, not just how hot it feels during the day.

The researchers conclude that now, one billion more people face at least one day of extreme heat stress each year than they did in the 1970s.

Rebecca Emerton. the study’s lead author and also a senior scientist at the European Centre for Medium-Range Weather Forecasts in the United Kingdom. described the expansion of risk as the most unsettling part of the results. She said it is “striking” to see heat stress intensifying not only in places already considered hot or accustomed to heat waves. but also expanding into regions where it has historically been rare or non-existent.

Jennifer Francis. a climate scientist at the Woodwell Climate Research Center on Cape Cod. who was not involved in the research. said the study provides “stark details” about increasing dangers to billions of humans. She emphasized that the analysis shows temperature is rising and humidity is rising too. making high temperatures more deadly because the body’s sweating-based cooling system struggles to keep up.

For Emerton, the findings underscore the urgent need to both reduce future warming and prepare for what is coming. The study says the work highlights the need for mitigation of future warming and for adaptation strategies. including heat health action plans. early warning systems and climate risk assessments.

In a world already accustomed to heat headlines, the new research lands on a different fear: that the most punishing conditions aren’t staying limited to the places people expect—and that the nights are getting less merciful, too.

heat stress humidity Nature Climate Change Universal Thermal Climate Index tropical nights extreme heat climate change United States Texas Florida Southern Europe