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Above 1.5°C, heat and humidity can be deadly for billions

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New research shows human tolerance for global heating is lower than previously thought

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Research published this week in Proceedings of the National Academy of Sciences shows that warming beyond 1.5°C above pre-industrial levels will be devastating for human health across the planet.

The research team modeled global temperature increases ranging between 1.5°C and 4°C  to identify areas of the planet where warming would lead to heat and humidity levels that exceed human limits. They found that if global temperatures increase by 1°C or more above current levels, billions of people will be exposed to heat and humidity so extreme they will be unable to naturally cool themselves.

Humans can only withstand certain combinations of heat and humidity before their bodies begin to experience heat-related health problems, such as heat stroke or heart attack. As climate change pushes temperatures higher around the world, billions of people could be pushed beyond these limits.

The ambient wet-bulb temperature limit for young, healthy people is about 31°C, which is equal to 87.8°F at 100% humidity. The specific threshold for any individual at a specific moment also depends on their exertion level and other environmental factors, including wind speed and solar radiation.

In human history, temperatures and humidity that exceed human limits have been recorded only a limited number of times — and only for a few hours at a time — in the Middle East and Southeast Asia.

If global temperatures increase by 2°C above pre-industrial levels, the 2.2 billion residents of Pakistan and India’s Indus River Valley, the one billion people living in eastern China and the 800 million residents of sub-Saharan Africa will annually experience many hours of heat that surpass human tolerance.

These regions would primarily experience high-humidity heat waves, which are more dangerous because the air cannot absorb excess moisture, which limits sweat evaporation. These regions are in lower-to-middle income nations, so many of the affected people will not have access to air conditioning or any effective way to mitigate the heat.

If warming of the planet continues to 3°C above pre-industrial levels, heat and humidity levels that surpass human tolerance would begin to affect the Eastern Seaboard and the middle of the United States— from Florida to New York and from Houston to Chicago. South America and Australia would also experience extreme heat at that level of warming.

At current levels of heating, the researchers said, the United States will experience more heat waves, but these heat waves are not predicted to surpass human limits as often as in other regions of the world. Still, the researchers cautioned that these types of models often do not account for the worst, most unusual weather events.

Models like these predict trends, but they do not predict specific events like the 2021 heat wave in Oregon that killed more than 700 people or London reaching 40°C last summer. Even though the United States will escape some of the worst direct effects of this warming, it will see deadly and unbearable heat more often. If temperatures continue to rise, crops will fail and millions or even billions of people will seek to escape their native regions.

The researchers conducted 462 separate experiments to document the combined levels of heat, humidity and physical exertion that humans can tolerate before their bodies can no longer maintain a stable core temperature.

“As people get warmer, they sweat, and more blood is pumped to their skin so that they can maintain their core temperatures by losing heat to the environment,” physiologist Larry Kenney says. “At certain levels of heat and humidity, these adjustments are no longer sufficient, and body core temperature begins to rise. This is not an immediate threat, but it does require some form of relief. 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.”

The research concluded that the limits of heat and humidity people can withstand are lower than were thought. Most official strategies for adapting to the weather focus on temperature only, but humid heat is going to be a much bigger threat than dry heat.

Older adults experience heat stress and the associated health consequences at lower heat and humidity levels than young people. Most of the 739 people who died during Chicago’s 1995 heat wave were over 65: they experienced a combination of high body temperature and cardiovascular problems, leading to heart attacks and other cardiovascular causes of death.

To stop temperatures from increasing, the researchers cite decades of research indicating that humans must reduce the emission of greenhouse gases, especially the carbon dioxide emitted by burning fossil fuels. If changes are not made, middle-income and low-income countries will suffer the most.

As one example, the researchers pointed to Al Hudaydah, Yemen, a port city of more than 700,000 people on the Red Sea. Results of the study indicated that if the planet warms by 4°C, this city can expect more than 300 days when temperatures exceed the limits of human tolerance every year, making it almost uninhabitable.

The worst heat stress will occur in regions that are not wealthy and that are expected to experience rapid population growth in the coming decades. Billions of poor people will suffer, and many could die.

(This article includes material provided by the University of Pennsylvania.)

Extract from
“Greatly enhanced risk to humans as a consequence of empirically determined lower moist heat stress tolerance.”
Proceedings of the National Academy of Sciences, October 9, 2023

Significance: Increased heat and humidity potentially threaten people and societies. Here, we incorporate our laboratory-measured, physiologically based wet-bulb temperature thresholds across a range of air temperatures and relative humidities, to project future heat stress risk from bias-corrected climate model output. These vulnerability thresholds substantially increase the calculated risk of widespread potentially dangerous, uncompensable humid heat stress. Some of the most populated regions, typically lower-middle income countries in the moist tropics and subtropics, violate this threshold well before 3 °C of warming. Further global warming increases the extent of threshold crossing into drier regions, e.g., in North America and the Middle East. These differentiated patterns imply vastly different heat adaption strategies. Limiting warming to under 2 °C nearly eliminates this risk.

Abstract: As heatwaves become more frequent, intense, and longer-lasting due to climate change, the question of breaching thermal limits becomes pressing. A wet-bulb temperature (Tw) of 35 °C has been proposed as a theoretical upper limit on human abilities to biologically thermoregulate. But, recent — empirical — research using human subjects found a significantly lower maximum Tw at which thermoregulation is possible even with minimal metabolic activity. Projecting future exposure to this empirical critical environmental limit has not been done. Here, using this more accurate threshold and the latest coupled climate model results, we quantify exposure to dangerous, potentially lethal heat for future climates at various global warming levels. We find that humanity is more vulnerable to moist heat stress than previously proposed because of these lower thermal limits. Still, limiting warming to under 2 °C nearly eliminates exposure and risk of widespread uncompensable moist heatwaves as a sharp rise in exposure occurs at 3 °C of warming. Parts of the Middle East and the Indus River Valley experience brief exceedances with only 1.5 °C warming. More widespread, but brief, dangerous heat stress occurs in a +2 °C climate, including in eastern China and sub-Saharan Africa, while the US Midwest emerges as a moist heat stress hotspot in a +3 °C climate. In the future, moist heat extremes will lie outside the bounds of past human experience and beyond current heat mitigation strategies for billions of people. While some physiological adaptation from the thresholds described here is possible, additional behavioral, cultural, and technical adaptation will be required to maintain healthy lifestyles.