Global climate change could unleash environmental catastrophes ranging from melting glaciers, rising sea levels, and climatic aberrations that lead to floods, droughts, and forest fires. A new study finds that the combination of high heat and humidity as a result of climate change could result in an increase in heat-related fatalities and make many regions in the world quite uninhabitable.

To understand the interactions between real-world problems like climate change and their effect on human health, the new study included interdisciplinary expertise from climatologists and physiologists at Purdue University and other research facilities in the United States.

The combination of heat and humidity has a devastating effect on the human body, as it can withstand only certain combinations of heat and humidity before beginning to experience heat-related health problems, such as heat stroke or heart attack. 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.

The study shows that an increase of 1.5 degree Celsius (℃) or more than current levels, could result in billions of people being exposed to heat and humidity so extreme that their body will be unable to cool itself naturally. To identify areas of the planet where warming would lead to heat and humidity levels that exceed human limits, the researchers modeled global temperature increases ranging between 1.5℃ and the worst case scenario of 4℃.

Over a period of several years, the research team conducted more than 460 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.

The scientists explained that 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.
The rise in core temperature is not an immediate threat, provided the body has immediate access to 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.

The study found that the limits of heat and humidity people can withstand are lower than were previously theorized. They found that the ambient wet-bulb temperature limit for young, healthy people is about 31℃, at 100 percent humidity. The research noted that babies, older adults, and people with cardio-pulmonary vulnerabilities could experience heat stress and the associated health risks at lower heat and humidity levels than young people.

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. Since the start of the industrial revolution, when humans began to burn fossil fuels in machines and factories, temperatures around the world have increased by nearly 1℃. The Paris Climate Agreement signed in 2015 by 196 countries aims to limit worldwide temperature increases to 1.5℃ above pre-industrial levels.

Results of the study indicate that if global temperatures increase by 2℃ 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. Worryingly, 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 warming of the planet continues to 3℃ above pre-industrial levels, the researchers concluded, heat and humidity levels that surpass human tolerance would begin to affect wider areas. This would increase health risks and fatalities among billions of people in areas of the United States, in South America, in Australia, and elsewhere.

It is important to keep in mind that while climate models, such as those used in the above study are good at predicting trends, they often do not account for most unusual weathers, or of specific events such as the 2021 heatwave in Oregon in the US that claimed the lives of more than 700 people, despite heat levels being below limits of human tolerance.

Around the world, official strategies for adapting to the weather usually focus only on temperature only. However, the new study 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.

Unless mitigatory measures are introduced to curb use of fossil fuels, which is the main source of greenhouse gas (GHG) emissions that contribute to recent higher temperatures and changing weather patterns, we will soon be confronted with a world where food insecurity is rising, and billions of people migrating because their native regions are uninhabitable.

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