A new global study published in the journal nature Climate Change has revealed that heat stress is intensifying across the planet at an alarming rate, exposing billions of people to increasingly dangerous conditions. Researchers found that both daytime and nighttime heat have become more severe since the 1970s, with the world hottest nights warming faster than the hottest days and dangerous heat now affecting regions that previously escaped such extremes.
The study, led by researchers from the European Centre for Medium-Range Weather Forecasts, analyzed global heat stress trends from 1950 to 2024 using the Universal Thermal Climate Index (UTCI), a measure of “feels-like” temperature that considers humidity, wind, radiation, and the human body’s response to heat. The findings paint a troubling picture of a warming world where hazardous heat is becoming more frequent, more intense and longer lasting.
According to the research, the hottest nighttime temperatures have been increasing by 0.32°C per decade since the 1970s, outpacing the warming of the hottest daytime temperatures, which have risen by 0.27°C per decade. This trend is particularly concerning because warm nights prevent the human body from recovering from daytime heat, increasing the risks of illness and death.
The study found that the number of heat stress days and tropical nights has risen sharply worldwide. Some regions now experience up to 50 additional heat stress days each year compared to the 1970s. Areas across southern Europe, northern and southern Africa, South America, and parts of North America have seen especially dramatic increases. Researchers also observed that the geographical footprint of dangerous heat has expanded, bringing severe heat stress to regions where it was once rare or absent.
Europe, North America, Africa, Asia, South America, and Oceania all recorded significant increases in extreme heat stress. Europe and South America experienced the largest relative increases, with extreme heat stress occurring 2.5 times more often than it did in the 1970s. In Africa, where heat stress is already widespread, the frequency of extreme heat conditions rose from 4.1 percent to 7.4 percent, the highest level among all continents.
The duration of the heat stress season has also lengthened. Across the Northern Hemisphere, periods of moderate heat stress now last an average of 15 days longer than they did in the 1970s. Europe has seen some of the most dramatic changes, with moderate heat stress now beginning weeks earlier in spring and continuing later into autumn.
Nighttime heat is becoming a growing concern. The researchers found that tropical nights when temperatures do not fall below 20°C are not only becoming more common but are increasingly accompanied by significant heat stress. In 2024, the proportion of tropical nights associated with moderate heat stress reached a record high of 11.4 percent, compared with just 2.2 percent in the mid-1960s.
The study also highlighted a sharp increase in compound heat events, where intense daytime heat is followed by unusually warm nights. These prolonged periods of heat are particularly dangerous because they deprive people of relief and recovery time. In Europe, compound heat events lasting 15 to 30 consecutive days are now more than three times as common as they were in the 1970s.
Researchers estimate that the number of people exposed to dangerous heat has risen dramatically. In the 1970s, around 55 percent of the global population experienced at least 90 days of strong heat stress each year. Today, that figure has climbed to 70 percent. Exposure to at least one day of extreme heat stress annually has increased from 16 percent to 22 percent of the world’s population, representing roughly one billion additional people.
Importantly, the study found that climate-driven increases in heat stress are contributing as much as, or more than, population growth to rising exposure levels. The greatest concentrations of heat exposure were identified in sub Saharan Africa, South and Southeast Asia, the Arabian Peninsula, and the Mediterranean region.
The researchers warn that the findings have major implications for public health, economic productivity, and climate adaptation planning. Heat is already the leading cause of weather related deaths worldwide, and the growing intensity and duration of heat stress could place billions more people at risk in the coming decades. They argue that adaptation measures such as heat health warning systems, urban cooling strategies, and climate resilient infrastructure will be essential to protect communities as global temperatures continue to rise.
The study concludes that without rapid action to reduce greenhouse gas emissions and strengthen adaptation efforts, the escalating burden of heat stress will become one of the defining public health challenges of the 21st century.