The study, conducted by researchers from the Penn State College of Health and Human Development, Purdue University College of Sciences, and Purdue Institute for a Sustainable Future in the United States, reveals that if global temperatures rise by at least 1 degree Celsius (C) compared to current levels, vast populations will be exposed to heat and humidity levels so severe that their bodies will struggle to naturally regulate temperature.

The implications of this research highlight that any global temperature increase beyond 1.5 C above preindustrial levels will have profoundly negative consequences for human health worldwide. The human body can only tolerate specific combinations of heat and humidity before experiencing severe heat-related health issues like heatstroke or heart attacks. As climate change continues to drive temperatures upward globally, billions of people may find themselves pushed beyond these critical limits.

This study also underscores the historical context of climate change. Since the onset of the industrial revolution, when fossil fuels began to be burned in machinery and factories, global temperatures have risen by approximately 1 C. In response to this crisis, in 2015, 196 nations came together to sign the Paris Agreement, which aims to curtail global temperature increases and keep them within 1.5 C above pre-industrial levels.

To assess the potential consequences of climate change, the research team employed models projecting temperature increases spanning from 1.5 C to 4 C. The latter represents a worst-case scenario where warming accelerates, enabling them to identify regions of the world where rising temperatures would result in heat and humidity levels surpassing human tolerance thresholds.

?W. Larry Kenney, professor at Penn State and co-author of the new study said,?"To understand how complex, real-world problems like climate change will affect human health, you need expertise both about the planet and the human body."

The ambient wet-bulb temperature limit for young, healthy people is about 31 C, according to work published last year by Penn State researchers.

However, 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.

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, according to the researchers.

If the global temperature rises by 2 C

Results of the study indicate 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.

The 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.

If the global temperature rises by 3 C

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 US -- 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 US 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, a bioclimatologist who completed a postdoctoral fellowship at Penn State.

"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," Vecellio added.