Extreme feels-like temperatures, heat stress days and tropical nights have all become dramatically more frequent, long and severe over the past six decades as the planet’s warming intensifies — a result of the burning of fossil fuels coal, oil and gas — according to a new study published in the journal Nature Climate Change on Monday.

The researchers went beyond just temperature, which is frequently studied, and used feels-like temperatures, to understand more of the impact on people. They assessed heat stress on individual humans, influenced by temperature, humidity, wind speed and more. They used what’s called the Universal Thermal Climate Index to analyze those factors and model the human body’s response to the environment.

The combination of heat and humidity can be dangerous for humans, because humidity impacts how sweat evaporates, and that’s a cooling mechanism. Heat waves that are humid can be more fatal than dry heat waves as humans don’t cool down as easily.

Heat stress is worsening in already-warm regions, and beyond

Past studies have looked at the extent to which human-driven climate change has sent temperatures soaring, especially in recent years. One study says people globally suffered an average of 41 extra days of dangerous heat in 2024. Some research says that the world is on track to add nearly two months of superhot days each year by the end of the century.

Here, researchers looked at heat stress at three levels: strong (index temperatures of greater than or equal to 32 degrees Celsius, or 89.6 degrees Fahrenheit); very strong (index temperatures of greater than or equal to 38 degrees Celsius, or 100.4 degrees Fahrenheit); and extreme (index temperatures of greater than or equal to 46 degrees Celsius, or 114.8 degrees Fahrenheit).

Places that might see around 50 more days per year of at least strong heat stress compared with the 1970s include parts of Southern Africa, such as in Namibia and Angola; Eastern Africa, including parts of Tanzania, Kenya and Uganda; and parts of Mexico and Central America.

In Southern Spain, Italy, Greece and Turkey, some areas will see up to 40 additional days with strong heat stress compared with the 1970s. Much of Southern Europe is seeing almost a full month of additional strong heat stress days from decades ago.

In the U.S., much of the country sees 15 or more days of at least strong heat stress, and southern parts, including Texas and Florida, are seeing close to 25 or more days with very strong heat stress.

Those heat stress seasons are also lasting longer.

The study’s lead author Rebecca Emerton, also a senior scientist at the European Centre for Medium-Range Weather Forecasts in the United Kingdom, said it was 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.”

According to the study, the feels-like temperatures on the ten warmest nights of each year have also increased faster — 0.32 degrees Celsius (0.58 degrees Fahrenheit) per decade — than the ten warmest days, 0.27 degrees Celsius (0.49 degrees Fahrenheit) per decade.

For tropical nights, the researchers considered minimum temperature of 20 degrees Celsius (68 degrees Fahrenheit). This means people might not be recovering properly from daytime heat in the overnight hours.

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

The future impact depends on action

The world has known that adding heat-trapping gases to the atmosphere by burning fossil fuels and cutting down forests will warm the globe, said Jennifer Francis, a climate scientist at the Woodwell Climate Research Center on Cape Cod, who was not involved in the research.

“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.”

Emerton says the work highlights the urgent need to mitigate future warming and ensure adaptation strategies, heat health action plans, early warning systems and climate risk assessments are in place.

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Alexa St. John is an Associated Press climate reporter. Follow her on X: @alexa_stjohn. Reach her at ast.john@ap.org.

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