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If emissions are not reduced drastically, middle-income and low-income countries will suffer the most, the report said.

Billions at risk of heat and humidity exposure beyond human levels: Report

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

If global temperatures increase by 1°C or more than current levels, each year billions of people will be exposed to heat and humidity so extreme they will be unable to naturally cool themselves, according to research published in Proceedings of the National Academy of Sciences. 

The article indicated that warming of the planet beyond 1.5°C above pre-industrial levels will be increasingly devastating for human health across the planet. 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 researcher team modeled global temperature increases ranging between 1.5°C and 4°C — considered the worst-case scenario where warming would begin to accelerate — to identify areas of the planet where warming would lead to heat and humidity levels that exceed human limits.

This composite map shows land areas that could face extreme heat if the planet continues to warm (indicated in yellow and orange). The darker the color, the longer the projected exposure to extreme heat. Source.

The study found that 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 heatwaves. Heatwaves with higher humidity can be more dangerous because the air cannot absorb excess moisture, which limits sweat evaporates from human bodies and moisture from some infrastructure, like evaporative coolers. Troublingly, researchers said, these regions are also in lower-to-middle income nations, so many of the affected people may not have access to air conditioning or any effective way to mitigate the negative health effects of the heat.

Wet bulb temperatures

The report mentioned that the ambient wet-bulb temperature limit for young, healthy people is about 31°C at 100% humidity. Having said that, in addition to temperature and 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. Historically, 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, according to the researchers.

If warming of the planet continues to 3°C above pre-industrial levels, the researchers concluded, heat and humidity levels that surpass human tolerance would begin to affect the Eastern Seaboard and the middle of the United States. South America and Australia would also experience extreme heat at that level of warming, the report added.

At current levels of heating, the researchers said, the United States will experience more heatwaves, but these heatwaves 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 are good at predicting trends, but they do not predict specific events like the 2021 heatwave in Oregon that killed more than 700 people or London reaching 40°C last summer,” said lead author Daniel Vecellio. “And remember, heat levels then were all below the limits of human tolerance that we identified. So, even though the United States will escape some of the worst direct effects of this warming, we will see deadly and unbearable heat more often. And, if temperatures continue to rise, we will live in a world where crops are failing and millions or billions of people are trying to migrate because their native regions are uninhabitable.”

Future warming

A 2022 report demonstrated that the limits of heat and humidity people can withstand are lower than were previously theorized. 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. At certain levels of heat and humidity, these adjustments are no longer sufficient, and body core temperature begins to rise. While this is not an immediate threat, 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.

“Around the world, official strategies for adapting to the weather focus on temperature only,” one of the researchers said. “But this research shows that humid heat is going to be a much bigger threat than dry heat. Governments and policymakers need to re-evaluate the effectiveness of heat-mitigation strategies to invest in programs that will address the greatest dangers people will face.”

Regardless of how much the planet warms, the researchers said that people should always be concerned about extreme heat and humidity, even when they remain below the identified human limits. In preliminary studies of older populations, they’ve found that older adults experience heat stress and the associated health consequences at lower heat and humidity levels than young people.

If emissions are not reduced drastically, middle-income and low-income countries will suffer the most, the report said. As one example, the researchers pointed to Al Hudaydah, Yemen, a port city of more than 7,00,000 people on the Red Sea. Results of the study indicated that if the planet warms by 4°C, the 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,” Matthew Huber, one of the researchers, said. “This is true despite the fact that these nations generate far fewer greenhouse gas emissions than wealthy nations. As a result, billions of poor people will suffer, and many could die. But wealthy nations will suffer from this heat as well, and in this interconnected world, everyone can expect to be negatively affected in some way.”

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