Climate Change May Make Sections of Earth Uninhabitable

Results from a new article published today (Oct. 9) in Proceedings of the National Academy of Sciences indicated that warming of the planet beyond 1.5 C above preindustrial levels will be increasingly detrimental for human health across the planet.

If global temperatures increase by 1 degree Celsius (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 interdisciplinary research from the Penn State College of Health and Human Development, Purdue University College of Sciences and Purdue Institute for a Sustainable Future.

Before the onset of heat-related health problems, such as heat stroke or heart attack, the human body can only withstand a limited number of combinations of heat and humidity. As global temperatures rise due to climate change, billions of people may be driven beyond these limits. Since the beginning of the Industrial Revolution, when humans began to consume fossil fuels in machines and factories, global temperatures have increased by approximately 1 degree Celsius (C) or 1.8 degrees Fahrenheit (F). The Paris Agreement, signed by 196 nations in 2015, seeks to limit global temperature increases to 1.5 degrees Celsius above pre-industrial levels. The research team modeled global temperature increases between 1.5 C and 4 C -- the worst-case scenario in which warming would begin to accelerate -- in order to identify regions of the planet where warming would result in heat and humidity levels that exceed human limits.

"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," said co-author W. Larry Kenney, professor of physiology and kinesiology, the Marie Underhill Noll Chair in Human Performance at Penn State and co-author of the new study. "I am not a climate scientist, and my collaborators are not physiologists. Collaboration is the only way to understand the complex ways that the environment will affect people's lives and begin to develop solutions to the problems that we all must face together."

The ambient wet-bulb temperature limit for young, healthy individuals is approximately 31 degrees Celsius, or 87.8 degrees Fahrenheit at 100 percent humidity, according to work published by Penn State researchers last year. In addition to temperature and humidity, a person's specific threshold at a given time also depends on their level of exertion and other environmental factors, such as wind speed and solar radiation. According to the researchers, temperatures, and humidity exceeding human limits have been recorded in the Middle East and Southeast Asia only a handful of times in human history, and only for a few hours at a time.

If global temperatures increase by 2 degrees Celsius above pre-industrial levels, the 2.2 billion residents of Pakistan and India's Indus River Valley, the one billion residents of eastern China, and the 800 million residents of sub-Saharan Africa will experience many hours of intolerable heat each year, according to the study's findings. These regions would predominantly experience heat waves with significant humidity. Heatwaves with higher humidity can be more hazardous because the air cannot absorb excess moisture, limiting the amount of perspiration that can evaporate from human bodies and from certain infrastructure, such as evaporative coolers. Concerningly, these regions are located in low- to middle-income countries, so many of the affected individuals may not have access to air conditioning or any other effective means of mitigating the adverse health impacts of the heat. The researchers concluded that if the planet continues to warm by 3 degrees Celsius above pre-industrial levels, heat and humidity levels that exceed human tolerance will begin to affect the Eastern Seaboard and the center of the United States, from Florida to New York and Houston to Chicago. At that level of warming, South America and Australia would also experience extreme temperatures.

"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 with Kenney. "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."

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 globe. However, the researchers cautioned that these types of models frequently fail to account for the most extreme and atypical weather events.

Understanding future climate and human limits Kenney and his colleagues have conducted 462 separate investigations over the past several years to document the levels of heat, humidity, and physical exertion that humans can withstand before their bodies can no longer maintain a stable core temperature. In 2022, Kenney, Vecellio, and their co-workers demonstrated that the human tolerance for heat and humidity is lower than previously believed.

"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," Kenney said. "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."

When this work was published, Huber, who had already begun documenting the effects of climate change, reached out to Vecellio about the possibility of a collaboration. Prior to this, Huber had published a widely cited paper proposing a theoretical upper limit for human heat and humidity tolerance. The researchers, along with Huber's graduate student Qinqin Kong, decided to investigate how people in various regions of the globe would be affected if the planet warmed by 1.5 to 4 degrees Celsius. The researchers estimated that the planet will warm by 3 degrees Celsius by 2100 if no action is taken.

"The data collected by Kenney's team at Penn State provided much-needed empirical evidence about the human body's ability to tolerate heat. Those studies were the foundation of these new predictions about where climate change will create conditions that humans cannot tolerate for long," said co-author Matthew Huber, professor of earth, atmospheric and planetary sciences at Purdue University.

Keeping cool in the heat

 No matter how much the planet warms, extreme temperatures and humidity should always be a concern, according to the researchers, even if they remain below the established human limits. Kenney found in preliminary investigations of older populations that older adults experience heat stress and its associated health effects at lower heat and humidity levels than younger individuals. 
Around the world, official strategies for adapting to the weather focus on temperature only," Kong 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."

"Heat is already the weather phenomenon that kills the most people in the United States," Vecellio, now a postdoctoral researcher at George Mason University's Virginia Climate Center, said. "People should care for themselves and their neighbors -- especially the elderly and sick -- when heatwaves hit."

This study's data examined the body's core temperature, but the researchers noted that during heatwaves, individuals also experience health issues due to other factors. Kenney stated that the majority of the 739 individuals who died during Chicago's 1995 heatwave were over the age of 65 and suffered from a combination of high body temperature and cardiovascular issues, resulting in heart attacks and other cardiovascular causes of death. Considering the future The researchers cite decades of research indicating that humans must reduce the emission of greenhouse gases, particularly carbon dioxide emitted by the combustion of fossil fuels, to prevent an increase in global temperatures. Vecellio stated that if no adjustments are made, middle-income and low-income countries will suffer the most. As an illustration, the researchers cited Al Hudaydah, Yemen, a port city on the Red Sea with more than 700,000 inhabitants. According to the study's findings, if the planet warms by 4 degrees Celsius, this city will experience more than 300 days per year with temperatures exceeding the limits of human tolerance, rendering it nearly 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," Huber 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."

This investigation was funded by grants from the National Institute of Health, the National Aeronautics and Space Administration, and the National Science Foundation. 

WNCTimes

Citation:

Penn State. "Climate-driven extreme heat may make parts of Earth too hot for humans." ScienceDaily. ScienceDaily, 9 October 2023. <www.sciencedaily.com/releases/2023/10/231009191623.htm>.