Planet’s heat bill comes due as one billion more people face extreme heat stress than in the 1970s

Mexico, Kenya, Italy and other nations around the world are experiencing one to two more months of heat stress than they were several decades ago, new research published Monday says, and some areas even more so. Regions previously untouched by heat stress are now feeling it, too. 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