Mexico, Kenya, Italy and other nations are now living through one to two more months of heat stress than they were several decades ago, according to new research published Monday in the journal Nature Climate Change. The study says some areas are seeing even more heat stress, and regions that were previously untouched are now being pulled into the same punishing conditions.
Who Pays for the Warming
The burden lands on ordinary people first. Extreme feels-like temperatures, heat stress days and tropical nights have all become dramatically more frequent, longer and more severe over the past six decades as the planet’s warming intensifies, the study says. The changes are linked to the burning of fossil fuels, including coal, oil and gas. That is the machinery behind the crisis: a system that keeps extracting and burning while everyone else is left to sweat through the consequences.
The researchers did not stop at temperature alone. They used feels-like temperatures to assess heat stress on individual humans, taking into account temperature, humidity, wind speed and other factors. They used the Universal Thermal Climate Index to analyze those factors and model the human body’s response to the environment. In other words, the study measures not just what the thermometer says, but what the body actually endures.
The study looked at heat stress at three levels: strong, defined as index temperatures of greater than or equal to 32 degrees Celsius, or 89.6 degrees Fahrenheit; very strong, defined as index temperatures of greater than or equal to 38 degrees Celsius, or 100.4 degrees Fahrenheit; and extreme, defined as index temperatures of greater than or equal to 46 degrees Celsius, or 114.8 degrees Fahrenheit. Those are the thresholds where the climate stops being abstract and starts becoming a daily threat.
The Expanding Footprint
Places that might see around 50 more days per year of at least strong heat stress compared with the 1970s include parts of Southern Africa, including Namibia and Angola; Eastern Africa, including parts of 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 compared with the 1970s, and much of Southern Europe is seeing almost a full month of additional strong heat stress days from decades ago.
In the U.S., much of the country sees 15 or more days of at least strong heat stress, and southern parts, including Texas and Florida, are seeing close to 25 or more days with very strong heat stress. The map of suffering keeps widening, and the people at the bottom are the ones forced to absorb it.
Those heat stress seasons are also lasting longer. Rebecca Emerton, the study’s lead author and a senior scientist at the European Centre for Medium-Range Weather Forecasts in the United Kingdom, said 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.”
According to the study, the feels-like temperatures on the ten warmest nights of each year have increased faster, by 0.32 degrees Celsius, or 0.58 degrees Fahrenheit, per decade, than the ten warmest days, which have increased by 0.27 degrees Celsius, or 0.49 degrees Fahrenheit, per decade. The night offers less relief than it used to, and the body gets fewer chances to recover.
What the Numbers Say
For tropical nights, the researchers considered a minimum temperature of 20 degrees Celsius, or 68 degrees Fahrenheit, meaning people might not be recovering properly from daytime heat in the overnight hours. The study says 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 Center on Cape Cod who was not involved in the research, said, “This study adds stark details about increasing dangers to billions of humans.” She said, “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.”
Emerton said 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. The study’s findings make clear who is being asked to adapt: not the fossil fuel system that caused the damage, but the people living inside it.