A new study published in the journal Nature Climate Change has found that over recent decades, people in many parts of the world have been experiencing much longer periods of dangerous heat. In some countries, the heat-stress season has expanded by one to two months, while regions that previously experienced little or no extreme heat are now increasingly feeling its effects.

The researchers found that over the past 60 years, not only have air temperatures risen sharply, but so have so-called “feels-like temperatures” — a measure that takes into account humidity, wind, and other factors affecting how the human body experiences heat. This indicator provides a more accurate picture of how difficult heat is for people to endure under real-world conditions.

To assess the risks, the scientists used the Universal Thermal Climate Index (UTCI), which models the human body's response to environmental conditions. This approach shows that the danger comes not only from high temperatures themselves but also from their interaction with humidity. When the air is saturated with moisture, sweat evaporates more slowly, reducing the body’s natural cooling capacity. As a result, humid heatwaves can often be more dangerous than dry ones.

The most significant changes were recorded in Africa, Central America, and the Mediterranean region. In parts of Namibia, Angola, Tanzania, Kenya, Uganda, Mexico, and several Central American countries, the number of days with severe heat stress has increased by approximately 50 days per year compared with the 1970s.

Southern Europe is also facing a serious situation. In some regions of Spain, Italy, Greece, and Turkey, the number of days with severe heat stress has risen by nearly 40 days annually. Across much of the Mediterranean, people now experience almost an additional month of extreme heat compared with conditions half a century ago.

In the United States, large areas of the country now endure at least two additional weeks of significant heat stress each year. In southern states such as Texas and Florida, the number of days with very severe heat has increased by more than three weeks.

The researchers emphasize that the problem is not limited to the growing number of hot days. Heat-stress seasons are becoming longer and are starting earlier. Moreover, dangerous heat is expanding into regions where such conditions were previously rare or virtually nonexistent.

“It is striking to see heat stress not only intensifying in places we traditionally consider hot, but also spreading into regions where it was once uncommon or entirely absent,” said lead author Rebecca Emerton of the European Centre for Medium-Range Weather Forecasts.

Scientists are particularly concerned about nighttime temperatures. The study found that the warmest nights are heating up faster than the hottest days. The average increase in the temperature of the ten warmest nights of the year is about 0.32 degrees Celsius per decade, compared with 0.27 degrees for the hottest days.

This means that millions of people are recovering less effectively from daytime heat. Nighttime hours that once provided relief are becoming increasingly warm, placing additional strain on the human body.

According to the study’s authors, at least one billion more people today experience at least one day of extreme heat stress per year compared with the 1970s.

The researchers stress that future developments will depend largely on how successfully humanity can reduce greenhouse gas emissions and adapt to changing climate conditions.

Among the measures considered essential are early-warning systems, heat-protection plans for vulnerable populations, and more effective assessments of climate-related risks.