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Miami (AFP)

South Asia, home to one-fifth of the world's population, could see humid heat rise to unsurvivable levels by century's end if nothing is done to halt global warming, researchers said Wednesday.

The study in the journal Science Advances warned of "summer heat waves with levels of heat and humidity that exceed what humans can survive without protection."

The research is based on two climate models. One is a "business-as-usual" scenario in which little is done to contain climate change, and the second is aimed at limiting temperature rise to well below two degrees Celsius, as pledged by more than 190 nations under the 2015 Paris climate accord.

The study is the first of its kind to look not just at temperatures, but at the forecast of "wet-bulb temperature," which combines temperature, humidity and the human body's ability to cool down in response.

The survivability threshold is considered to be 35 degrees Celsius, or 95 degrees Fahrenheit.

Under a business-as-usual scenario, "wet-bulb temperatures are projected to approach the survivability threshold over most of South Asia, and exceed it at a few locations, by the end of the century," said the report.

About 30 percent of the population in the region would be exposed to these harmful temperatures, up from zero percent at present, said the report.

The densely populated farming regions of South Asia could fare the worst, because workers are exposed to heat with little opportunity for escape into air-conditioned environments.

"Deadly heat waves could begin within as little as a few decades to strike regions of India, Pakistan, and Bangladesh, including the fertile Indus and Ganges river basins that produce much of the region's food supply," said the report.

Indian is home to 1.25 billion people, while another 350 million live in Bangladesh and Pakistan.

Already in 2015, the fifth deadliest heat wave in modern history swept over large parts of India and Pakistan, killing some 3,500 people.

But researchers said their models gave cause for hope, too. Under the scenario in which steps are taken to limit warming over the coming decades, the population exposed to harmful wet-bulb temperatures would increase from zero to just two percent.

Temperatures would still reach dangerous levels (over 31 Celsius), but would not be quite so close to the fatal threshold.

"There is value in mitigation, as far as public health and reducing heat waves," said lead author Elfatih Eltahirhe, professor of environmental engineering at the Massachusetts Institute of Technology (MIT).

"With mitigation, we hope we will be able to avoid these severe projections. This is not something that is unavoidable."

© 2017 AFP