A study published in the journal Nature Climate Change indicates that the burning of fossil fuels has intensified extreme feels-like temperatures, heat stress days, and tropical nights across the globe. Mexico, Kenya, Italy, and other nations are experiencing one to two more months of heat stress annually compared to several decades ago.

The study found that one billion more people now face at least one day of extreme heat stress each year compared to the 1970s. Feels-like temperatures on the ten warmest nights of each year have increased by 0.32 degrees Celsius (0.58 degrees Fahrenheit) per decade, while feels-like temperatures on the ten warmest days of each year have increased by 0.27 degrees Celsius (0.49 degrees Fahrenheit) per decade.

Rebecca Emerton, a senior scientist at the European Centre for Medium-Range Weather Forecasts in the United Kingdom and the study's lead author, noted the expansion of heat stress. She said, "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."

Researchers used the Universal Thermal Climate Index to model the human body's response to the environment by analyzing factors such as temperature, humidity, and wind speed. The study defined strong heat stress as index temperatures at or above 32 degrees Celsius (89.6 degrees Fahrenheit), very strong heat stress as at or above 38 degrees Celsius (100.4 degrees Fahrenheit), and extreme heat stress as at or above 46 degrees Celsius (114.8 degrees Fahrenheit). Tropical nights were defined as having a minimum temperature of 20 degrees Celsius (68 degrees Fahrenheit).

Areas of Southern Africa, including Namibia and Angola, and parts of Eastern Africa, including Tanzania, Kenya, and Uganda, may see approximately 50 more days per year of at least strong heat stress compared to the 1970s. Parts of Mexico and Central America also show an increase of around 50 more days per year of at least strong heat stress. Much of Southern Europe is experiencing almost a full month of additional strong heat stress days compared to decades ago, with some areas in Southern Spain, Italy, Greece, and Turkey seeing up to 40 additional days with strong heat stress. The U.S. generally sees 15 or more days of at least strong heat stress, and southern parts, including Texas and Florida, are experiencing close to 25 or more days with very strong heat stress.

Jennifer Francis, a climate scientist at the Woodwell Climate Research Center, who was not involved in the research, commented on the study's findings. She stated, "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."