According to interdisciplinary research from Penn State's College of Health and Human Development, Purdue University's College of Science and Purdue University's Institute for Sustainable Futures, if global temperatures rise by 1 degree Celsius or more above current levels, billions of people will be at risk of extreme heat and humidity that prevent the body from cooling naturally.
Research results from a new article recently published in the Proceedings of the National Academy of Sciences indicate that warming the earth by more than 1.5 degrees Celsius above pre-industrial levels will cause increasing damage to human health across the planet.
Humans can only tolerate a certain combination of heat and humidity before the body begins to experience heat-related health problems, such as heat stroke or heart attack. As climate change raises global temperatures, billions of people could be pushed beyond these limits.
Since the start of the Industrial Revolution, when humans began burning fossil fuels in machines and factories, global temperatures have risen by about 1 degree Celsius, or 1.8 degrees Fahrenheit. In 2015, 196 countries signed the Paris Agreement, which aims to limit global temperature rise to 1.5 degrees Celsius above pre-industrial levels.
This comprehensive map shows land areas (indicated in yellow and orange) that may face extreme heat if the Earth continues to warm. The darker the color, the longer you can expect to be exposed to extreme heat. Image credit: Daniel Vecellio, Qinqin Kong, W. Larry Kenney, and Matthew Huber; composite image courtesy of Dennis Maney, Pennsylvania State University
The team simulated global temperature increases between 1.5 degrees Celsius and 4 degrees Celsius - considered the worst-case scenario in which climate warming begins to accelerate - to identify areas of the planet where warming would push heat and humidity beyond human limits.
"To understand complex events like climate change," said W. Larry Kenney, co-author of the new study and professor of physiology and kinesiology and the Marie Underhill Noll Chair in Human Function at Penn State, You need expert knowledge about how the environment affects human health. I am not a climate scientist, and my collaborators are not physiologists. Collaboration is the only way to understand the complex ways in which the environment affects people's lives and begin to develop solutions to the problems we must face together."
According to research published last year by Penn State researchers, the environmental wet-bulb temperature limit for young, healthy people is about 31 degrees Celsius, which is equivalent to 87.8 degrees Fahrenheit at 100 percent humidity. However, in addition to temperature and humidity, any individual's specific threshold at any given moment depends on their level of physical exertion and other environmental factors, including wind speed and solar radiation. Researchers say temperatures and humidity exceeding human limits have only occurred a limited number of times in the Middle East and Southeast Asia in human history, and only for a few hours each time.
The findings show that if global temperatures rise 2 degrees Celsius above pre-industrial levels, 2.2 billion residents of Pakistan and the Indus Valley, 1 billion people in eastern China, and 800 million residents in sub-Saharan Africa will experience many hours of heat that exceeds human tolerance each year.
These areas will primarily experience high humidity heat waves. Heat waves with higher humidity are more dangerous because the air cannot absorb excess moisture, which limits the evaporation of human sweat and the humidity of some infrastructure, such as evaporative coolers. Worryingly, these areas are also located in low- and middle-income countries, so many affected may not have access to air conditioning or any effective means of mitigating the negative health effects of high temperatures, the researchers said.
The researchers concluded that if Earth's warming continues to 3 degrees Celsius above pre-industrial levels, heat and humidity beyond human tolerance will begin to affect the eastern seaboard and central regions of the United States - from Florida to New York, and from Houston to Chicago. At this level of warming, extreme heat will also occur in South America and Australia.
Researchers say the United States will experience more heat waves at current levels of warming, but they are not expected to exceed human limits as frequently as in other parts of the world. However, the researchers caution that these types of models often fail to account for the worst and most unusual weather events.
First author Daniel Vecellio is a bioclimatologist who completed postdoctoral research with Kenny at Pennsylvania State University. Keep in mind that heat levels at that time were all below our established limits of human tolerance. So while the United States will escape some of the worst direct impacts of a warming climate, we will see deadly and intolerable heat more frequently. And if temperatures continue to rise, we will live in a world where crops fail and millions or billions of people try to migrate because their original places are uninhabitable. "
Over the past few years, Kenny and his collaborators have conducted 462 independent experiments, documenting the combined levels of heat, humidity and physical exertion that humans can endure before their bodies can no longer maintain a stable core temperature.
"As temperatures rise, people sweat and more blood is pumped to the skin, thereby maintaining core body temperature by losing heat to the environment. At a certain level of heat and humidity, these regulatory measures are no longer sufficient and the body's core temperature begins to rise. This does not pose an immediate threat, but it does require some form of mitigation. If people cannot find a way to cool down within a few hours, it can lead to heat exhaustion, heat stroke, and a load on the cardiovascular system that can lead to heart attacks in vulnerable people," Kenny said.
In 2022, Kenny, Vecellio and their collaborators showed that the limits of heat and humidity that people can tolerate are lower than previously theorized.
"The data collected by Kenny's team at Penn State provide much-needed empirical evidence of the human body's ability to tolerate heat," said Matthew Huber, professor of earth, atmospheric and planetary sciences at Purdue University. "These studies are the basis for these new predictions of where climate change will create conditions that humans cannot tolerate in the long term."
After the study was published, Huber, who had already begun mapping the impacts of climate change, contacted Vecellio to discuss a potential collaboration. Huber previously published a widely cited study proposing theoretical limits for heat and humidity in humans.
The researchers and Huber's graduate student Kong Qinqin decided to explore how people in different parts of the world would be affected if the Earth warmed by 1.5 degrees Celsius to 4 degrees Celsius. Researchers say 3 degrees Celsius of warming by 2100 is the best estimate if no action is taken.
"Official weather adaptation strategies around the world focus solely on temperature," Kong said. "But this research shows that wet heat will pose a greater threat than dry heat. Governments and policymakers need to reassess the effectiveness of heatstroke cooling strategies and invest in programs that address the greatest dangers people will face.
Researchers say that no matter how much the planet warms, people should always be concerned about extreme heat and humidity - even if they remain below established human limits. In preliminary studies of older adults, Kenny found that older adults develop heat stress and related health consequences at lower levels of heat and humidity than younger adults.
"Heat is already the deadliest weather phenomenon in the United States," said Veselio, now a postdoctoral fellow at George Mason University's Virginia Climate Center. "When heat waves hit, people should care about themselves and their neighbors — especially the elderly and sick."
The study used data that examined the body's core temperature, but researchers say people can also experience health problems for other reasons when heat waves hit. Kenney gave the example of the 739 people who died during the 1995 Chicago heat wave, most of whom were over 65 and experienced both hyperthermia and cardiovascular problems, leading to heart attacks and other deaths from cardiovascular disease.
To stop rising temperatures, the researchers cited decades of research arguing that humans must reduce greenhouse gas emissions, especially carbon dioxide from burning fossil fuels. Veselio said that if changes were not made, middle- and low-income countries would suffer the most.
The researchers used the example of Hodeidah in Yemen, a port city on the Red Sea with a population of more than 700,000. Research results show that if the earth warms up by 4 degrees Celsius, the city will have more than 300 days every year where the temperature exceeds the human tolerance limit and will be almost uninhabitable.
Huber said: "The most severe heat stress will occur in less affluent regions, whose populations are expected to grow rapidly in the coming decades. This is despite the fact that greenhouse gas emissions in these countries are much lower than in rich countries. As a result, billions of poor people will suffer and many may die. But rich countries will also be affected by this high temperature, and in this interconnected world, everyone will be affected in some way."