In this May 4, 2016, file photo, Indian women walk home after collecting drinking water from a well at Mengal Pada in Thane district in Maharashtra state, India.

(AP Photo/Rajanish Kakade, File)

At a Glance

Up to a third of the 1.5 billion people who live in the Ganges and Indus river basins could be affected unless climate-warming carbon emissions are reduced.South Asia is particularly vulnerable because of three mechanisms: the yearly monsoon, topography and irrigation systems.

By the end of the century, just venturing outdoors in South Asia could become a deadly endeavor, a new study says.

Wide areas of India, Pakistan, Bangladesh, Nepal and Sri Lanka are already areas hard hit by extreme weather events, with flooding and deadly droughts occurring regularly, but that will only worsen as climate change drives heat and humidity to new levels, the scientists say.

According to the in the journal Science Advances, up to a third of the 1.5 billion people who live in the densely populated agricultural regions of the Ganges and Indus river basins could be affected unless climate-warming carbon emissions are reduced.

"Climate change, without mitigation, presents a serious and unique risk in South Asia, a region inhabited by about one-fifth of the global human population, due to an unprecedented combination of severe natural hazard and acute vulnerability," lead author Eun-Soon Im, a former MIT research scientist and current assistant professor at Hong Kong University of Science and Technology, wrote.

While previous studies have focused primarily on temperature projections, this study also took into account humidity and the body's ability to cool off.

The researchers took these three variables to come up with a measurement known as"wet-bulb temperature," which is taken while a wet cloth is wrapped around a thermometer.

Because the web-bulb temperature varies from a dry-bulb temperature and is usually lowerdependingon humidity levels, it can help determine how easy it is for water to evaporate. It can also offer a glimpse into when climate change will become dangerous, the researchers say.

In this May 17, 2016, file photo, Indian vendors who sell sun shades for car windows take rest by a roadside on a hot summer afternoon in Jammu, India.

(AP Photo/Channi Anand, File)

A human can survive a wet-bulb temperature of up to about 95 degrees Fahrenheit, the scientists say. Wet-bulb temperatures above this "human survivability threshold"make it difficult for humans to sweat, which reduces the body's ability to cool off. At this point, heat-related illnesses including heat stroke can lead to death within a few hours, even in the shade.

While wet-bulb temperatures have rarely surpassed 90 degrees F, scientists fear it will become all too commonin the future if human-caused climate change continues unabated.

People living in South Asia are particularly vulnerable becauseof the sheer number of people who work outdoors regularly. There is also limited access to electricity, and thanks to deforestation, very little shade remains in some areas.

"What we see in this study is a convergence of intense weather projections and acute vulnerability," co-author and MIT environmental engineering professor Elfatih A.B. Eltahir .

In scenarios played out by the scientists, whichlooked at a future world where efforts to curb carbon emissions kept the planet's global temperature below the 2 degrees C above pre-industrial levels and another where emissions continued "business as usual," the danger facing the people of South Asia became apparent.

While both scenarios proved dangerous, the scenario where emissions continued unabated showed that 30 percent of people in the region, or half a billion people at current population levels, could see wet-bulb temperatures above 88 degrees on a regular basis within just a few decades.

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By the turn of the century, about 4 percent,or 60 million,would face deadly wet-bulb temperatures of 95 degrees F regularly if emissions continue as they do today.

"This is an avoidable, preventable problem," Eltahir told the AP. "There is a significant difference between these two scenarios, which people need to understand."

The scientists also note that there are three mechanisms that make South Asia particularly vulnerable to more deadly heat waves in the future.

"First, the monsoon system transports warm and humid air masses into the Indus and Ganges valleys from the surrounding warm Arabian Sea and Bay of Bengal," Im wrote. "Second, surface elevations in these valleys are generally lower than 100 meters above sea level, and hence, surface air is generally warmer than surrounding higher-elevation areas. Third, much of the valleys are irrigated, which tends to enhance (temperatures) over irrigated areas because of modifications in the surface energy balance."

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A man douses himself in water at a railway station to cool himself down in Jammu, India, Monday, June 9, 2014. (AP Photo/Channi Anand)