India’s Heat Wave Is a Grim Warning for Deadly ‘Wet Bulb’ Temperatures

Temperatures up to 45 degrees celsius are engulfing northern India right now.

An abnormally early heat wave has brought India the highest temperatures it’s seen in 122 years this month—and climatologists are concerned. For starters, heat waves are the deadliest form of natural disaster, they typically place socially vulnerable populations at disproportionate risk, and, in this part of the world, are only expected to worsen. 

But another factor could prove lethal for India as the severity and frequency of extreme heat worsens: Wet bulb conditions. 

Wet bulb temperature is a metric that accounts for both heat and humidity—it’s technically measured by the read of a thermometer covered in a water soaked cloth, but can perhaps most intuitively be thought of as a representation of “how effectively a person sheds heat by sweating.” 

It’s the reason that dry heat is commonly thought of as more comfortable than humid heat. 

When there’s enough moisture in the air, the body can’t perform one of its primary cooling functions: Sweating. Our bodies regulate heat by eliminating it in the form of water, which evaporates on the surface of the skin; this water must move in a gradient from high to low vapor pressure for it to evaporate into the atmosphere. Sweating alone does not cool the body off: It must evaporate in order for heat to be released from the body. When there’s enough water vapor in the atmosphere, the pressure differential that would normally allow the evaporation of sweat isn’t there. Without a conduit, sweat lingers on the body and heats up infinitely, increasing the likelihood that the body overheats and its essential functions cease to, well, function. 

There is a point at which wet bulb temperatures become, as Radley Horton, Lamont Research Professor at the Columbia Climate School put it in a 2020 paper in the peer-reviewed journal Science Advances, “too severe for human tolerance.” A wet bulb temperature of 35 degrees celsius (95 degrees fahrenheit) is the upper physiological limit, his paper notes. An exceedance of 35 degrees celsius would be deadly. 

“Even if they're in perfect health, even if they're sitting in the shade, even if they're wearing clothes that make it easy in principle to sweat, even if they have an endless supply of water,” Horton previously told Motherboard. “If there's enough moisture in the air, it's thermodynamically impossible to prevent the body from overheating.”

Climate models once anticipated that these temperatures would not occur until the mid-21st century, but they’re already here, and they’re concentrated in South Asia, among a few other localities (the coastal Middle East and coastal southwest North America), Horton’s paper found. 

In fact, under a business-as-usual emissions scenario, he and his colleagues write, South Asia could see wet bulb temperatures regularly exceed 35 degrees celsius by the third quarter of this century. 

India has already seen wet bulb temperatures exceed 25 degrees celsius in southeast regions, which is hospitable, but uncomfortable. Recent projections show this is likely to get worse across South Asia: A 2021 study published in the peer-reviewed Geophysical Research Letters projected that more Bangladesh, India and Pakistan has already seen over 3.3-billion person-days of wet bulb exceedance (that is, 3.3-billion people experienced these temperatures over a single day) and these rates are only going to compound under current warming goals. 

“South Asia is one of those regions where vulnerability to extreme heat is particularly  elevated  due  to  the  coexistence  of  both  natural  and  human  stressors,  such  as  high  population  density,  poverty-driven  low  adaptive  capacity  and  background  hot  and  humid  climate  conditions,” the paper reads, noting that the risk of extreme heat is particularly high for approximately 60 percent of the region’s working population, who are engaged in outdoor agricultural labor. 

“An addition of half a degree to the current global warming levels would further expose substantially large population to more prevalent deadly hot extremes in many new geographical areas in South Asia,” the paper says