By Katy Daigle
Venturing outdoors may become deadly across wide swaths of India, Pakistan and Bangladesh 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, this one - published in the journal Science Advances - is somewhat unique in also considering 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 up to a wet-bulb temperature of 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 31C, a level that is already extremely hazardous.
"It is hard to imagine conditions that are too hot for people to survive for a more than a few minutes, but that is exactly what is being discussed in this paper," said Stanford University climate scientist Chris Field, who was not involved in the study. "And of course, the danger threshold for punishing heat and humidity is lower for people who are ill or elderly."
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 per cent in of India's population still has no access to electricity. In some areas that have been deforested, they may not even have very much shade.
"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 said.
With no limit on global warming, about 30 per cent of the region could see dangerous wet bulb temperatures above 31C 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.
And 4 per cent of the population - or 60 million in today's population - would face deadly highs at or above 35C by 2100. If the world can limit global warming, that risk exposure declines drastically.
About 2 per cent of the population would face average wet bulb temperatures of 31C or higher.