A study published in Nature Climate Change on Monday reveals that rising temperatures and humidity have extended the dangerous heat stress season since the 1970s, with Mexico, Kenya, and Italy among the worst-affected areas. The research shows that many regions now experience one to two additional months of heat stress compared to several decades ago, and some areas face even greater increases.
Heat Stress Measured by Feels-Like Temperatures
The researchers went beyond standard temperature measurements, using the Universal Thermal Climate Index to assess heat stress on individuals. This index accounts for temperature, humidity, wind speed, and other factors to model the human body's response to the environment. Humidity is a critical component because it affects how sweat evaporates, which is the body's cooling mechanism. Humid heat waves can be more fatal than dry ones as the body struggles to cool down.
According to the study, extreme feels-like temperatures, heat stress days, and tropical nights have become dramatically more frequent, longer, and severe over the past six decades due to global warming driven by fossil fuel combustion.
Regional Impacts and Additional Days of Heat Stress
Places that may experience around 50 more days per year of at least strong heat stress compared with the 1970s include parts of Southern Africa (such as Namibia and Angola), Eastern Africa (including 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. Much of Southern Europe now faces almost a full month of extra strong heat stress days compared to decades ago.
In the United States, much of the country experiences 15 or more days of at least strong heat stress, and southern parts, including Texas and Florida, see close to 25 or more days with very strong heat stress.
Expanding Footprint of Heat Stress
Rebecca Emerton, lead author and senior scientist at the European Centre for Medium-Range Weather Forecasts, noted 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."
Nighttime Temperatures Rising Faster
The study found that feels-like temperatures on the ten warmest nights of each year have increased faster—0.32 degrees Celsius (0.58 degrees Fahrenheit) per decade—than the ten warmest days, which rose 0.27 degrees Celsius (0.49 degrees Fahrenheit) per decade. For tropical nights, the researchers considered a minimum temperature of 20 degrees Celsius (68 degrees Fahrenheit). This means people may not recover properly from daytime heat during overnight hours.
One Billion More People Affected
Now, one billion more people face at least one day of extreme heat stress each year than they did in the 1970s. Jennifer Francis, a climate scientist at the Woodwell Climate Research Centre who was not involved in the research, said, "This study adds stark details about increasing dangers to billions of humans. 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."
Urgent Need for Adaptation
Emerton emphasized that 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.
