According to interdisciplinary research from the Penn State College of Health and Human Development, Purdue University College of Sciences, and the Purdue Institute for a Sustainable Future, if global temperatures rise by 1 degree Celsius (C) or more above current levels, billions of people will be exposed to heat and humidity so extreme that they will be unable to naturally cool themselves.
According to the findings of a recent study published in the Proceedings of the National Academy of Sciences, global warming of more than 1.5 degrees Celsius beyond preindustrial levels will be increasingly harmful to human health around the world.
Humans can only take a particular amount of heat and humidity before developing heat-related health issues like heat stroke or heart attack. As climate change raises global temperatures, billions of people may be pushed beyond these boundaries.
To understand how complex, real-world problems like climate change will affect human health, you need expertise in both the planet and the human body.
W. Larry Kenney
Temperatures around the world have risen by around 1 degree Celsius (1.8 degrees Fahrenheit) since the dawn of the Industrial Revolution when humans began to consume fossil fuels in machinery and factories. The Paris Agreement, adopted by 196 countries in 2015, aims to restrict global temperature increases to 1.5 degrees Celsius over pre-industrial levels.
The research 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.
“To understand how complex, real-world problems like climate change will affect human health, you need expertise in both the planet and the human body,” said co-author W. Larry Kenney, professor of physiology and kinesiology at Penn State and the Marie Underhill Noll Chair in Human Performance. “I am not a climate scientist, and neither are my colleagues. Collaboration is the only way to comprehend the intricate ways in which the environment will affect people’s lives and to begin developing answers to the difficulties that we must all tackle together.”
A threat to billions
According to Penn State researchers, the ambient wet-bulb temperature limit for young, healthy people is around 31 C, which is equal to 87.8 F with 100% humidity. However, in addition to temperature and humidity, the specific threshold for any individual at any given time is determined by their level of exertion as well as other environmental elements such as wind speed and solar radiation. Temperatures and humidity that surpass human limits have only been recorded a few times in human history, and only for a few hours at a time, in the Middle East and Southeast Asia, according to the researchers.
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.
High-humidity heatwaves would predominantly affect these areas. High humidity heatwaves can be more dangerous since the air cannot absorb excess moisture, limiting sweat evaporation from human bodies and moisture from some equipment, such as evaporative coolers. Worryingly, many of the affected people may not have access to air conditioning or any other effective strategy to offset the severe health impacts of the heat, according to the study.
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 — 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 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.
Understanding human limits and future warming
Over the last several years, Kenney and his collaborators have 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,” he said. “At certain temperatures and humidity levels, these adjustments are no longer adequate, and body core temperature begins to rise.” Although this is not an immediate threat, it does necessitate some type of alleviation. Heat exhaustion, heat stroke, and strain on the cardiovascular system, which can lead to heart attacks in sensitive persons, can occur if people do not find a way to cool down within hours.”
In 2022, Kenney, Vecellio, and their collaborators demonstrated that the limits of heat and humidity people can withstand are lower than were previously theorized.
“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.
When this paper was released, Huber, who had already started working on mapping the effects of climate change, approached Vecellio about a possible collaboration. Huber’s prior study, which was highly recognized, proposed a theoretical limit of humans’ heat and humidity limitations.
The researchers, along with Huber’s PhD student Qinqin Kong, decided to investigate how people in various parts of the world might be affected if the earth warmed by 1.5 to 4 degrees Celsius. According to the researchers, 3 degrees Celsius is the best estimate of how much the world will warm by 2100 if no action is taken.
Staying safe in the heat
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, Kenney found that older adults experience heat stress and the associated health consequences at lower heat and humidity levels than young people.
“Heat is already the weather phenomenon that kills the most people in the United States,” said Vecellio, who is now a postdoctoral researcher at George Mason University’s Virginia Climate Center. “People should care for themselves and their neighbors – especially the elderly and sick – when heatwaves hit.”
The data utilized in this study looked at the core temperatures of the body, but the researchers stated that during heatwaves, people have health concerns for other reasons as well. Kenney, for example, stated that the majority of the 739 people who died during Chicago’s 1995 heatwave were over 65 and suffered from a combination of high body temperature and cardiovascular difficulties, resulting in heart attacks and other cardiovascular causes of death.
Looking to the future
To prevent temperature rises, the researchers reference decades of studies demonstrating that humans must cut greenhouse gas emissions, particularly carbon dioxide released by burning fossil fuels. Middle- and low-income countries will suffer the most if no adjustments are made, according to Vecellio.
As an example, the researchers cited Al Hudaydah, Yemen, a Red Sea port city of about 700,000 people. According to the study’s findings, if the earth heats by 4 degrees Celsius, this metropolis will experience more than 300 days per year when temperatures exceed the boundaries of human endurance, rendering it nearly uninhabitable.
