When the human body is deprived of this natural cooling system, organs can strain and begin to fail, resulting in lethargy, sickness, and even death.
Humidity therefore makes hot weather feel much, much worse — which is the guiding principle behind the measurement known as the Heat Index. According to the US National Oceanic and Atmospheric Administration, a temperature of 92 degrees Fahrenheit will feel like 94 degrees, at a relative humidity level of 40 percent. But it will feel like a scorching 131 degrees if relative humidity is 90 percent.
The standard way to measure the combined effect of heat and humidity is known as “wet-bulb” temperature, which involves draping a water-saturated cloth over the bulb of a conventional thermometer.
During the 1995 Chicago heat wave, at noon on July 13, the actual temperature was 99°F — but the maximum wet-bulb temperature reached 85°F. That moment was actually more dangerous than conditions later on at 5:00 p.m., when the temperature reached 106°F but the wet-bulb temperature fell to 83°F.
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Lab experiments have shown wet-bulb readings of 89.6°F (32° Celsius) are the threshold above which many people would have trouble carrying out normal, outdoor activities.
“Lots of people would crumble well before you reach wet-bulb temperatures of 32°C, or anything close,” said Coffel’s coauthor Radley Horton, a climate scientist at Lamont-Doherty. “They'd run into terrible problems.”
The wet-bulb 32°C level is rarely reached anywhere on earth today.
But the new study projects that roughly a half-century from now, by the 2070s or 2080s, that 32°C mark could be commonly reached one or two days a year in the American southeast, and three to five days in parts of South America, Africa, India, and China.
The hardest-hit area in terms of human impact, the researchers say, will probably be densely populated northeastern India.
The study projects that some parts of the southeast and northern India may even sometimes hit 35 wet-bulb degrees Celsius by late century.
That threshold is actually equal to the human skin temperature, and the theoretical limit at which humans die within just hours without artificial cooling systems.
The result of these changes could be "transformative" for all areas of human endeavor, including the economy, agriculture, the military, and recreation, Horton said.
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In the southeast United States, wet-bulb temperatures reach 29 or 30°C only occasionally. By the 2070s or 2080s, such conditions could occur 25 to 40 days each year, according to the researchers.
But whether these forecasts are borne out or not depends on future emissions levels, Coffel said.
“Exceeding these thresholds can mostly be avoided through substantial emissions reductions,” Coffel said. “If we can hold global temperatures rise to 2°C, those 35°C wet bulb thresholds will probably not be crossed this century.”