Global temperatures rising by one degree Celsius or more each year would expose billions of people to extreme heat and humidity, making natural cooling insufficient for many. This finding comes from research conducted at the Purdue University School of Science and the Purdue Sustainable Future Institute in the United States.
The study’s outcomes, published in the Proceedings of the National Academy of Sciences, indicate that warming to 1.5°C above pre-industrial levels will increasingly harm human health across the globe.
Humans can only tolerate certain combinations of heat and humidity before health begins to suffer, including risks like heat stroke and heart attack. As climate change pushes temperatures higher worldwide, billions could exceed their physiological limits.
Extreme heat, the researchers note, could render some regions uninhabitable. The project mapped temperature and humidity increases from 1.5°C to 4°C, aiming to locate areas most at risk when heat and humidity surpass human tolerance.
To explore how climate change may affect health, W. Larry Kenney, a physiology and kinesiology professor at Pennsylvania State University and a co-author of the study, emphasizes the need for knowledge of both planetary conditions and human biology.
Temperatures and humidity beyond human limits
The study notes that the ambient wet-bulb temperature limit for young, healthy individuals sits around 31°C. But the actual threshold at any moment depends on effort levels and other environmental factors such as wind speed and solar radiation.
Historically, temperatures and humidity above human limits have occurred only on a handful of occasions for short periods, notably in parts of the Middle East and Southeast Asia.
India, Pakistan, Eastern China and Sub-Saharan Africa
The accompanying map shows expected annual hours of heat beyond human limits based on different warming scenarios.
The research suggests that if global temperatures rise by 2°C above pre-industrial levels, roughly 2.2 billion people in the Indus River Valley region (spanning Pakistan and India), about 1 billion people in eastern China, and around 800 million people in sub-Saharan Africa would experience hours of heat beyond what human bodies can safely endure each year.
These regions are largely in lower-middle-income countries where access to air conditioning or effective heat mitigation methods may be limited, amplifying health risks.
Will heat reach the United States, South America and Australia at 3°C
If warming persists to 3°C above pre-industrial levels, higher temperatures and humidity could affect the eastern and central United States, as well as parts of South America and Australia.
While the models are strong at projecting broad trends, they cannot predict specific events such as the 2021 Oregon heatwave or a single heat event in London. The lead author notes that these models show probable patterns rather than precise incidents.
The projection that extreme heat could render certain areas uninhabitable has prompted calls for action—reducing greenhouse gas emissions, especially carbon dioxide from fossil fuels, remains critical to curb rising temperatures.
Millions may migrate as livelihoods and living conditions become untenable, a consequence explored by the team after conducting hundreds of controlled experiments to determine safe heat exposure levels for varying physical exertion and humidity scenarios.
As temperatures climb, the body’s cooling mechanisms intensify: sweating increases, blood flow to the skin rises, and heat exchange with the environment accelerates. When these processes can no longer maintain core temperature, health risks escalate. Earlier work by the Purdue team demonstrated that human heat tolerance may be lower than some theoretical estimates suggested, reinforcing the relevance of empirical data to predicting future conditions.
These findings underscore the need for robust climate adaptation strategies that do more than address temperature alone. Humid heat, in particular, poses a greater threat than dry heat, according to the researchers, who advocate evaluating heat-reduction investments with a focus on the most dangerous hazards people face.
People of all ages should remain vigilant about heat extremes, even when overall warming seems moderate. Preliminary studies indicate older adults may experience heat stress at lower heat and humidity levels than younger people, highlighting the importance of protecting vulnerable populations through targeted public health measures.
Heating events affect health in multiple ways beyond core temperature—dehydration, cardiovascular strain, and heat-related illnesses can arise during heat waves. The researchers stress the broader imperative to cut greenhouse gas emissions and accelerate energy transitions to stabilize global temperatures.
In conclusion, the work emphasizes that the worst heat stress is likely to hit less affluent regions first, with rapid population growth compounding the impacts. Yet even wealthier nations will feel the ripple effects in an interconnected world, underscoring a shared global risk.
Note: The reference material for this analysis comes from a study published by the Proceedings of the National Academy of Sciences. Further details are provided within the accompanying research article and supporting materials.