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Humid Heat Waves Will Top Limits of Human Survivability

By the end of the century, deadly heat waves could become an annual occurrence in South Asia.
Indian villagers gather around a well to fill their pots with water at Natwargadh village.

Indian villagers gather around a well to fill their pots with water at Natwargadh village.

Blistering heat waves are becoming more frequent around the globe, thanks in large part to climate change. A 2003 heat wave in Europe killed 70,000; in 2010, at least 10,000 people died in a Moscow heat wave; and, in 2015, more than 3,500 people died in a heat wave that hit India and Pakistan. Nearly a third of the world's population is currently exposed to deadly temperatures at least 20 days per year, and even with extreme reductions in carbon emissions, nearly half of all humans could face dangerous heat waves by 2100. One of the hardest hit regions will be South Asia, including Pakistan, Nepal, India, Bangladesh, and Sri Lanka, where temperatures could exceed the limits of human survivability by the end of the century, according to a new study published today in Science Advances.

That limit, based on a combined measure of temperature and humidity also known as "wet-bulb temperature," is 35 degrees Celsius. Anything above that threshold and the body's natural cooling mechanisms falter, meaning we begin to overheat. "Not even the fittest humans would be able to survive, even in well-ventilated, shaded conditions" for more than a few hours, says Jeremy Pal, a professor of civil engineering and environmental science at Loyola Marymount University in Los Angeles, and co-author on the new study.

The researchers used high-resolution climate simulations to predict future wet-bulb temperatures under two climate scenarios: business as usual, in which emissions continue at current rates and the planet warms by more than four degrees Celsius; and mitigation, in which the globe drastically cuts emissions to comply with the Paris Agreement and temperatures only increase slightly more than two degrees Celsius.

Under the business-as-usual scenario, temperatures will near the 35-degree Celsius cutoff across most of South Asia, and exceed it in northeastern India and Bangladesh. Under the mitigation scenario, temperatures will still regularly approach 31 degrees Celsius—which, the authors note, is still dangerous for most humans.

"Parts of South Asia as well as Southwest Asia will become uninhabitable without air conditioning during short periods of extreme years."

Right now, heat waves that exceed the 31-degrees Celsius cutoff are expected to occur roughly every 25 years in South Asia; in other words, there is a 4 percent chance of a heat event in any given year. Under the business-as usual emissions scenario, by the end of the century heat events of that magnitude would be expected every year. Even under the mitigation scenario, there would be a 50 percent chance of such a heat wave in any given year.

The most extreme heat waves are projected to occur in the Indus and Ganges river valleys. "I see this as a perfect storm as the region is densely populated, very poor, lacks adaptation capacity, and is dependent on agriculture—meaning outdoor working conditions," Pal says. "Hundreds of millions of people could be at risk during extreme years."

The good news is that, under the mitigation scenario, the wet-bulb temperature was not expected to exceed the 35-degree Celsius threshold anywhere in South Asia, which means we can still avoid the worst consequences of climate change—provided that countries rapidly shift away from fossil fuels. But it will largely be up to the biggest emitters—that is, developed nations including the United States—to make a difference.

"Affluent regions such as the U.S. and Europe have the financial means adapt to these adverse consequences of climate change; however, poorer regions, where approximately 80 percent of the world's population resides, do not necessarily have the capacity to adapt," Pal says. "This is particularly unjust because per capita greenhouse gas emissions in most of these poorer regions are less than a tenth of those in industrialized nations."

However, in June, President Donald Trump announced that the U.S. would withdraw from the Paris Agreement, and several other nations, including Russia, Switzerland, and Turkey have yet to ratify it.

"If climate change continues unabated," Pal says, "parts of South Asia as well as Southwest Asia will become uninhabitable without air conditioning during short periods of extreme years."