Parts of India and Pakistan have been smoldering under a record-breaking heatwave for weeks, exposing more than a billion people to dangerously hot conditions with no relief in sight.
While temperatures in the region cooled slightly this week, sweltering heat is expected to return in the coming days, spreading to the east, where rising “wet-bulb” temperatures — an esoteric measurement previously little known outside of meteorological circles — are taking the ability could endanger human survival, experts say.
This type of concern is becoming more urgent as climate change makes extreme heat events both more frequent and more severe, said Friederike Otto, a climate scientist at Imperial College London.
“If we’re going to do one thing to adapt, it really has to be the heat because that’s where we’re seeing the biggest changes anywhere in the world,” she said.
As the intensity of heat waves increases as a result of global warming, the risk of So-called wet-bulb temperatures will also rise, pushing some heat events into “unsurpassable” territory, experts say.
Wet-bulb temperature measures the combination of heat and humidity, which if too high can affect the human body’s ability to cool itself.
Like most mammals, humans cool themselves by sweating. Body heat is used to turn sweat into water vapor, and as this evaporation process takes place, the body cools down.
“It’s a very effective means of cooling, but it’s critical that the sweat can actually evaporate,” said Tapio Schneider, a professor of environmental science and engineering at the California Institute of Technology.
When the wet-bulb temperature, or the combination of heat and humidity, exceeds the temperature of the human body—about 97 degrees Fahrenheit or 36 degrees Celsius—sweat cannot evaporate and the human cannot cool down.
“It’s really a hard limit for survivability,” Schneider said. “You can die just sitting there. You don’t have to move or do anything else. There’s just no way to cool and you overheat.”
In dry-heat areas, the wet-bulb temperature threshold for human safety is higher. But in wetter places, temperature and humidity create a potentially lethal mix at a lower point.
The name itself comes from how meteorologists sometimes calculate wet-bulb temperatures by wrapping a wet cloth around a thermometer and measuring how much the temperature cools as a result of evaporation.
Climate studies have found that as global temperatures rise, warmer air can hold more moisture. This in turn increases the humidity and increases the wet-bulb temperature.
One in May 2020 in the Science Advances Journal found that heat and humidity in certain parts of the world are already testing the limits of human survivability. The research found that parts of South Asia, including India and Pakistan, the coast and southwest of North America, and areas around the Persian Gulf have experienced conditions “approaching or exceeding prolonged human physiological tolerance.”
Temperatures in Pakistan and north-west and central India soared above 100 degrees Fahrenheit for days last month, with the region recording its temperature the highest average temperatures since records began for the month of April. As the heatwave is expected to spread to wetter coastal regions, the risk of reaching critical wet-bulb temperature thresholds will increase, Otto said.
Pakistan’s Meteorological Bureau is forecasting severe heatwave conditions for the coming week, with officials there advising people to avoid unnecessary sun exposure.
Otto said that without decisive interventions to reduce greenhouse gas emissions and slow climate change, oppressive and dangerous heatwaves will persist.
“We’ve seen heat records being broken every year around the world, and that’s what we expect to see in a warming climate,” she said. “Climate change has been a real game changer when it comes to heat waves.”
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