CNN
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Large parts of the world, including China and the Midwestern United States, are on track to become too hot for humans to cope with, as accelerating global temperatures expose billions to heat and humidity so extreme that their bodies will not be able to now cope, according to a new study.
The researchers used temperature and humidity data along with climate models to analyze humanity's exposure to potentially deadly heat as the world warms, looking at a range of temperature increases from 1.5 degrees Celsius to 4 degrees Celsius above pre-industrial levels.
They found that above 2 degrees Celsius of warming, a significant portion of the world's population will be vulnerable to “wet heat stress” with devastating consequences for human health, according to the study published Monday in journal Proceedings of the National Academy of Sciences. The world has already warmed around 1.2 degrees Celsius.
“Wet heat stress is a particularly challenging problem because it directly affects the human body and causes morbidity and mortality,” said Matthew Haber, study co-author and professor of atmospheric and planetary earth sciences at Purdue University.
When heat and humidity levels are high, sweat evaporates much more slowly than usual, meaning that its cooling effect is lost and the body may not be able to regulate its temperature. This can lead to heat exhaustion and heatstroke, which can cause heart attack and organ failure. The elderly, the very young and those with pre-existing conditions are more vulnerable to heat illness.
Temperatures exceeding human tolerance have only been breached a few times in human history and for a few hours each time in the Middle East and Southeast Asia, according to the report.
But as temperatures continue to rise, many more will be exposed for much longer periods, the study found.
While countries have pledged to limit global warming to below 2 degrees Celsius, the target of 1.5 degrees Celsius is well off track. Even if global climate pledges are met – something the world is not currently on track for – temperatures are expected to rise between 2.1 and 2.9 degrees Celsius, according to a recent UN report.
Above 2 degrees of warming, 2.2 billion people living in Pakistan and India's Indus River Valley, 1 billion people in China and 800 million people in sub-Saharan Africa will experience long hours of heat and humidity per year that exceed human levels. tolerance.
People in these areas will be even more vulnerable as many do not have access to air conditioning or other ways to cool down, the report notes.
At 3 degrees – which the study authors say is the most likely level of warming by 2100 if no action is taken – there is a sharp increase in people exposed to life-threatening heat and humidity. “It's really incredibly disturbing,” Huber told CNN.
Wet heat waves will affect areas of the world that are not used to such extreme conditions.
The Midwestern United States will become a “high heat stress hotspot” at 3 degrees of warming, the report said. The Midwest is susceptible to this kind of heat stress in part because its climate straddles the line between dry and wet, Huber explained, allowing the region's heat to push into the danger zone on very humid days.
Another factor that makes the region vulnerable is its agriculture and a phenomenon called “sweat corn,” Huber said.
“The plants we eat sweat through evapotranspiration and that can add moisture above what would normally be there,” he said.
So-called “hot spots” — periods when heat and humidity are especially life-threatening — will be concentrated in the Missouri and Mississippi River valleys, but also elsewhere in the U.S., including the Gulf and Atlantic coasts. according to the study.
At 4 degrees of warming, the study's worst-case scenario, the researchers found that 1.5 billion people worldwide would experience a month of wet heat stress each year, and about 2.7 billion people would experience at least a week of these extreme conditions .
Parts of Yemen could experience heat and humidity beyond human tolerance for more than 300 days a year, rendering it virtually uninhabitable.
“All over the world, official strategies for climate adaptation focus only on temperature,” said Qinqin Kong of Purdue University and a co-author of the study. “But this research shows that humid heat will be a much bigger threat than dry heat.”
Keeping warming below 2 degrees Celsius by reducing planetary pollution will sharply reduce global exposure to life-threatening heat and humidity, according to the report.
“Every tenth degree or so plays a role down the line, and we want to reduce the warming as fast as we can,” Daniel Vecellio, the study's lead author and a postdoctoral researcher at George Mason University, told CNN. “If we get to these emissions reductions faster, here are all the people we can save, here are all the lifestyles that don't have to change.”
Kim Cobb, a climate scientist at Brown University who was not involved in the study but was lead author of the latest report by the UN Intergovernmental Panel on Climate Change, said the study's conclusions are “compelling” but not surprising. “Extreme heat is already responsible for countless deaths worldwide every year,” Cobb told CNN.
“It is important to underline, as this study does, that heat is not an equal opportunity killer. It disproportionately kills people in lower-income communities, often communities of color. This is true both here in the US and around the world.”
