Over 2 billion people in India and Pakistan to experience intolerable heat if temperatures rise by 2C

Some of the world’s largest cities will be the most severely impacted as well, research says

Over 2 billion people in India, Pakistan and some of the world’s largest cities will be exposed to intolerable levels of heat within this century, new research has revealed.

Some of the world’s largest cities – from Delhi and Shanghai to Lagos and Chicago – will face the most severe impacts, according to research published in the journal PNAS on Monday.

Billions of people in regions like the US Midwest could struggle to survive under longer periods of heatwaves as the world continues to heat up, according to the study by the Penn State College of Health and Human Development, Purdue University College of Sciences and Purdue Institute for a Sustainable Future.

The situation will get particularly grim for regions like south Asia that have already faced scorching heatwaves driven by the climate crisis in recent years.

And if global temperatures increase by 2 degrees Celsius or more than current levels, the heat could get so extreme that such regions will be unable to naturally cool themselves.

“It’s very disturbing,” said study co-author Matthew Huber of Purdue University. “It’s going to send a lot of people to emergency medical care.”

Around 750 million people could experience one week per year of potentially deadly humid heat if temperatures rise 2C above preindustrial levels, the study found.

At 3C of warming, more than 1.5 billion people would face such a threat.

The world is on track for 2.8C warming by the year 2100 under current policies, according to the 2022 UN Emissions Gap report, even though the landmark Paris Agreement signed by almost 200 countries called for efforts to be made to limit warming by 1.5C.

While India, Pakistan and the Gulf have already briefly touched dangerous levels of humid heat in recent years, the study found other major cities across the world will suffer if the world keeps heating up.

“It’s coming up in places that we didn’t think about before,” said Dr Vecellio, highlighting rising risks in South America and Australia as well.

At 4C of warming, Hodeidah in Yemen would see around 300 days per year of potentially unsurvivable humid heat.

To track such moist heat, scientists use a measurement known as “wet-bulb” temperature. This is taken by covering a thermometer with a water-soaked cloth. The process of water evaporating from the cloth mirrors how the human body cools down with sweat.

The study published on Monday built on past research by Dr Huber, George Mason University climatologist Daniel Vecellio and other scientists that delved into the point at which heat and humidity combine to push the human body beyond its limits without shade or help from technologies such as air conditioning.

In a previous landmark 2010 study, Dr Huber proposed that a wet-bulb temperature of 35C persisting for six or more hours could be the conservative limit for the human body.

Beyond this, people were likely to succumb to heat stress if they could not find a way to cool down.

A decade later, a group of American scientists co-led by Dr Vecellio put Dr Huber’s theory to the test by placing young, healthy adults in environmental chambers with high wet-bulb temperatures.

They found the limit was lower at between 30C and 31C.

Dr Huber and Dr Vecellio joined forces for Monday’s study to apply this lower limit to the world under various future climate warming scenarios, ranging between 1.5C and 4C.

“This will be a critical benchmark for future studies,” said atmospheric scientist Jane Baldwin of the University of California Irvine who was not involved in the research.

“Unfortunately, it’s a somewhat grimmer picture than you would have gotten with the 35C limit,” she said.

Monday’s research added to growing concerns about dangerous wet-bulb temperatures.

Another study published last month in Sciences Advances used Dr Vecellio’s threshold alongside weather station data and climate models to reach a similar conclusion: that the geographic range and frequency of dangerous humid heat will increase rapidly under even moderate global warming.

Additional reporting by agencies