Different warming scenarios will bring levels of heat and humidity that human bodies cannot handle by the end of the current century, affecting billions of people across the world.
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Heat and humidity levels will reach lethal levels for hours, days, and even weeks in some parts of the world by the end of the current century, making it impossible to stay outdoors, new research has found.
The study, published Monday in Proceedings of the National Academy of Scientists, looks at how different warming scenarios – from the Paris Agreement’s 1.5C warming target through 4C – will affect temperatures and humidity levels in the atmosphere.
“Around the world, official strategies for adapting to the weather focus on temperature only,” co-author Qinqin Kong of Purdue University said in a statement. “But this research shows that humid heat is going to be a much bigger threat than dry heat.”
Excessively hot and humid conditions can indeed be deadly, as they affect humans’ thermoregulation, the process that maintains a steady internal body temperature – ideally between 36.5 to 37.5C – despite changes in external conditions. In other words, when the threshold – a theoretical upper limit of the human body’s adaptability to extreme heat, which scientists set at 35C – is breached, sweating will not be enough to cool off the body.
Some of the world’s most populated areas – typically lower-middle income countries in tropic and subtropic regions – are forecast to breach this threshold “well before 3C of warming,” scientists say, though further global warming would bring unprecedented levels of humidity even to typically dry regions such as North America and the Middle East.
“If people do not find a way to cool down within hours, it can lead to heat exhaustion, heat stroke and strain on the cardiovascular system that can lead to heart attacks in vulnerable people,” explained W. Larry Kenney, co-author of the study and professor of physiology and kinesiology and the Marie Underhill Noll Chair in Human Performance at Penn State.
Under 2C of warming by the end of the century, cities in India, the United Arab Emirates, and Saudi Arabia could experience up to 223.6 hours – nearly 10 days – of unbearable heat each year, levels that have thus far only been breached for a few hours in parts of the Middle East and Southeast Asia.
The situation is much different if the 2C threshold is breached. According to the study, this much warming would put up to 4 billion people across Pakistan, India, China, and several African countries a risk of lethal outdoor conditions, whereas 3C of warming would result in extreme humidity in generally dry areas, including major US cities like New York and Chicago. The coast of the Gulf of Mexico as well as parts of the Missouri and Mississippi River Valleys will also turn into heat and humidity stress spots, with huge repercussions not only on local populations but also on the economy, with tourism, agriculture as well as outdoor recreation and other industry activities irreversibly affected.
“Humans will struggle to adapt to these conditions in a warmer world as they will present widespread challenges across many aspects of food–energy–water security, human health, and economic development including in the world’s most populous and most vulnerable regions,” the study reads.
The rapidly intensifying climate crisis is already increasing the frequency, intensity, and duration of heatwaves and other extreme weather events.
A study published in October 2022 said the world is already on track to warm above 2C, despite “staggering” growth in green energy development praised in the International Energy Agency’s (IEA) latest update of the Net Zero Roadmap. Indeed, while the Agency said the unprecedented progress in clean energy development in recent years is promising, substantial additional efforts are still required by 2030 to reach net zero emissions by mid-century, including a tripling of global renewable capacity, a doubling of energy-efficient infrastructure, a significant boost in heat pump sales, and further widespread adoption of electric vehicles.
“These results are another indication of the importance of restricting warming to 2C as suggested in the ratification of the Paris Agreement in 2015,” researchers said, adding that physiological adaptation and other adaptive measures – particularly in urban design – will be crucial in determining the habitability of regions in the future.
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