South Asia, where one-fifth of the world's people live, could face summer heat waves that are impossible to survive without protection, thanks to global warming, new research suggests. Hardest hit regions are in northern India, Bangladesh, and southern Pakistan, home to 1.5 billion people. These are also among the poorest regions in South Asia. Many are dependent on subsistence farming that requires long hours of hard outdoor labor.

"That makes them very vulnerable to these climatic changes,” said MIT professor of environmental engineering Elfatih Eltahir, one of the co-authors.

The study shows that on the current business-as-usual trajectory of carbon emissions these deadly heat waves could hit the region within a few decades with potentially devastating impacts on the fertile Indus and Ganges River Basins that produce much of the region's food supply.

However, cuts in carbon emissions as pledged under the 2015 Paris Climate Agreement dramatically reduces the risk to the region.

“Emission cuts will make a big difference in the lives of the most vulnerable people in the region. This is not an abstract concept,” said Eltahir.

The study, published Wednesday in Science Advances, used state-of-the-art climate models to project potential future heat and humidity in South Asia, already one of the warmest regions of the world. Hot weather's most deadly effects result from a combination of high temperature and high humidity, called a wet-bulb temperature. A temperature of 94 degrees Fahrenheit (34.4 degrees Celsius) and 80% humidity produces a wet-bulb or “feels like” temperature of 129 degrees Fahrenheit (53.9 degrees Celsius) on the NOAA National Weather Service Heat Index. This is considered extremely dangerous without some way to cool down.

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At a wet-bulb temperature of 35 degrees Celsius or 167 degrees Fahrenheit “feels like” (100 degrees Fahrenheit with 85% humidity for example), the human body cannot cool itself enough to survive more than a few hours. Such conditions are quite rare currently. However, lower temperatures are often deadly. A 2015 heat wave that killed at least 3,500 in India and Pakistan saw wet-bulb temperatures around 122 degrees Fahrenheit (50 degrees Celsius). A similar wet-bulb temperature was reached during the 1995 Chicago heat wave.

Currently about two percent of the Indian population sometimes gets exposed to extremes of 32-degree Celsius wet-bulb temperatures. According to Eltahir’s study, without carbon emissions cuts that will increase to about 70 percent of the population by 2100. And about two percent of the people will sometimes be exposed to the survivability limit of 35 degrees Celsius.

Heat Drives Human Migration

The rural poor have little capacity to cope with extreme heat and they are likely to move into cities in search of water, food, and cooling, said Robert McLeman, an expert on climate and migration at Canada’s Wilfred Laurier University.

“One study in Bangladesh showed that extreme heat is more likely to drive migration than flooding is,” McLeman said.

Future sea level rise gets far more attention and study than increases in extreme heat, which may actually have a larger impact a lot sooner. “No one’s really figured out practical solutions yet,” he said.

However, at a conference in Portland, Oregon, last year city officials from the northwest states met to discuss how to cope with the growing migration of people from California and the southwest who are moving because it’s getting hotter, said McLeman, a speaker at the meeting.

“Officials know more people are coming and they’re trying to figure out how to deal with that.”

While Florida and the southeast states will also face more extreme heat, America’s dangerous heat zone is in the middle of the country from Minnesota, along the Mississippi River Valley to New Orleans, said Richard Rood, a meteorologist and climate expert at the University of Michigan.

“Places like St Louis and Chicago often experience extended periods of hot and humid weather while extremes in coastal states are moderated by the ocean,” Rood said.

The U.S. has already experienced record heat waves with just 1C of warming from climate change. Without making carbon emission cuts the future will bring average increases of more than 4 degrees. That will be a “vastly different world,” he said.

For the past 20 to 30 years large numbers of American retirees, young professionals, and others have moved from the north to the south. In the future Rood says there will be a reverse migration back north to escape relentless heat in the southern half of the country.

People in the Middle East and parts of Africa are already moving because of extreme heat and drought, he said.

“Reducing our carbon emissions now will really make a difference in the future.”