- A study published in Science Advances shows that instances of dangerous high heat and humidity doubled in frequency between 1979 and 2017.
- The study used data from nearly 8,000 weather stations across the world.
- “Wet bulb” temperatures that were previously thought to be exceedingly rare were observed nearly 80 times in the data.
Have you ever stepped outside on a hot, humid day and — instantly starting to sweat — told yourself that the temperature must be higher than what the thermometer says? That’s because at higher levels of humidity, sweat evaporates more slowly, making it harder for the human body to cool itself down. The air temperature might only be 32° Celsius (about 90° Fahrenheit), but at a high level of humidity it can feel much hotter than that and the risk of deadly heatstroke rises dramatically.
One way scientists deal with this discrepancy is by measuring what’s called the “wet bulb” temperature. Similar to the heat index (which factors in humidity to determine what the temperature “feels like”), wet-bulb readings take into account humidity and other factors to assess how weather conditions will affect human health and activity. A wet-bulb temperature (WBT) of 32°C (89.6°F), for example, is about 55.6°C (132°F) on the heat index. At 35°C (95°F) WBT, even a healthy adult in the shade will die within hours.
A new study published in the journal Science Advances says this temperature has already been exceeded for brief periods in parts of the Persian Gulf, shattering assumptions that it would take years or decades for climate change to produce such extreme conditions. Similarly high wet-bulb temperatures above 30°C (86°F) were also found to have occurred thousands of times across the world. According to the study, the number of instances per year when the combination of heat and humidity was high enough to endanger human life doubled between 1979 and 2017.
“We have every reason to expect those patterns will continue, but be potentially hotter and more humid going forward,” Colin Raymond, a climate scientist at NASA’s Jet Propulsion Laboratory and one of the study’s co-authors, said in an interview with Mongabay.
The study used data gathered from nearly 8,000 weather stations, differentiating it from prior studies that measured the average temperatures of larger areas over longer periods of time. By drilling down into the data collected from the individual stations, researchers were able to isolate wet-bulb readings that were higher than the broader averages.
Those readings revealed that dangerously high wet-bulb temperatures were most common in parts of South Asia, the Middle East, and Southwestern North America. Some were far higher than researchers expected, including nearly 80 times when wet-bulb temperatures reached 33°C (91.4°F) in conditions that were previously thought to be extraordinarily rare.
“The magnitude of the trends did surprise me and my co-authors,” Raymond said. “Just dramatic trends and large differences between what you see at a single weather station and what you see if you average a couple of weather stations and plot it out every six hours.”
An interactive map published by The Earth Institute in collaboration with the study’s authors shows that wet-bulb temperatures above 30°C were also measured across the southern coast of the United States. Overall, coastal regions where high continental temperatures collide with superheated moisture from the sea were hardest hit, along with areas where heavy monsoon rains bring that moisture further inland.
“Our findings indicate that reported occurrences of extreme [wet-bulb temperatures] have increased rapidly at weather stations and in reanalysis data over the last four decades and that parts of the subtropics are very close to the 35C survivability limit,” wrote the study’s authors.
The study showed that conditions of extreme heat and humidity had occurred throughout the 40 years of weather station data — including significant spikes during El Niño events in 1998 and 2016 — but that the overall frequency of those conditions had increased in recent years. Its authors warned that if the global climate warms by 2.5°C (4.5°F) over preindustrial levels, wet-bulb temperatures in the hottest regions of the planet would “regularly exceed” the fatal threshold of 35°C.
For people living in those regions — which include parts of Pakistan, India, Mexico and the Middle East — life could become unrecognizable long before that point, with economic activities shifting to nighttime hours and increased reliance on air-conditioning for those who can afford it. Last year, scorching temperatures killed dozens of people in India, continuing a trend of severe heat waves afflicting the region in recent years.
“We need to think more seriously about adapting to these severe events,” Raymond said. “Both changing how people live in places like Pakistan and the Persian Gulf, and places that don’t currently experience those events, like Western Europe and tropical regions, will need programs in place to learn from them.”
Raymond, C., Matthews, T., & Horton, R. M. (2020). The emergence of heat and humidity too severe for human tolerance. Science Advances, 6(19). doi:10.1126/sciadv.aaw1838