Venturing outdoors may become deadly across wide swaths of Bangladesh, India and Pakistan by the end of the century as climate change drives heat and humidity to new extremes, according to a new study.

These conditions could affect up to a third of the people living throughout the Indo-Gangetic Plain unless the global community ramps up efforts to rein in climate-warming carbon emissions. Today, that vast region is home to some 1.5 billion people.

"The most intense hazard from extreme future heat waves is concentrated around the densely populated agricultural regions of the Ganges and Indus river basins," wrote the authors of the study, led by former MIT research scientist Eun-Soon Im, now an assistant professor at the Hong Kong University of Science and Technology.

While most climate studies have been based on temperature projections alone, this one -- published Wednesday in the journal Science Advances -- also considers humidity as well as the body's ability to cool down in response.

Those three factors together make up what is called a "wet-bulb temperature," which is the air temperature taken when a wet cloth is wrapped around the thermometer. It is always lower than the dry-bulb temperature -- how much so depends on the humidity. It can help estimate how easy it is for water to evaporate.

It can also offer a gauge for where climate change might become dangerous.

Scientists say humans can survive a wet-bulb temperature of up to about 35 degrees Celsius, beyond which the human body has difficulty sweating to cool down, or sweat doesn't evaporate, leading to heat stroke and ultimately death within just a few hours -- even in shaded, ventilated conditions.

So far, wet bulb temperatures have rarely exceeded 31 C, a level that is already considered extremely hazardous.

Most of those at risk in India, Pakistan and Bangladesh are poor farmworkers or outdoor construction labourers. They are unlikely to have air conditioners -- up to 25 percent in of India's population still has no access to electricity. In some areas that have been deforested for industry or agriculture, they may not even have very much shade.

For the study, the researchers carried out computer simulations using global atmospheric circulation models under two scenarios -- one in which the world comes close to meeting its goal of curbing emissions to limit Earth's average temperature rise to 2 degrees C above pre-industrial levels, and one if it continues emitting at current levels.

Both scenarios play out dangerously for South Asia. But with no limit on global warming, about 30 percent of the region could see dangerous wet bulb temperatures above 31 degrees C on a regular basis within just a few decades. That's nearly half a billion people by today's population levels, though the full scale could change as the population grows. Meanwhile, 4 percent of the population -- or 60 million in today's population -- would face deadly highs at or above 35 degrees C by 2100.

But if the world can limit global warming, that risk exposure declines drastically. About 2 percent of the population would face average wet bulb temperatures of 31 degrees C or higher.

Experts say countries must work toward meeting the Paris agreement goals to limit average global warming to 2 degrees C, especially since the world has already warmed by 1 degree C. That average will play out differently across the planet, and South Asia is expected to be hit harder than other regions.