Why large parts of Earth might become uninhabitable due to climate change

Rob Waugh
·Contributor
·4 min read
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Wet and dry bulb thermometer, an instrument used to measure the relative humidity of the atmosphere. Beige colour equipment is sticked to soil in greenhouse in botanic gardens. Behind are cactuses.
Wet and dry bulb thermometer, an instrument used to measure the relative humidity of the atmosphere. (Getty)

If global temperatures increase by one degree more than current levels, billions of people will be exposed to heat and humidity so extreme they will be unable to cool themselves.

Humans can only withstand certain combinations of heat and humidity before their bodies begin to experience heat-related health problems, such as heat stroke or heart attack.

As climate change pushes temperatures higher around the world, billions of people could be pushed beyond these limits.

New research from the Penn State College of Health and Human Development, Purdue University College of Sciences and Purdue Institute for a Sustainable Future suggests that huge areas of the world including Pakistan, India and China could see ‘unsurvivable’ heatwaves.

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What are 'wet bulb' temperatures?

‘Wet bulb’ temperatures refer to a sort of thermometer measurement built to measure heat AND humidity.

Human beings can survive very high temperatures (well over 50C) when humidity is low, but in high humidity, humans cannot survive temperatures of even 35C for long periods, because there is no way to cool down by sweating.

Wet-bulb thermometers are a specialised device to allow researchers to work out whether humans can sweat: if the water evaporates, the thermometer cools down, so that ‘wet bulb’ temperatures are lower than ‘dry bulb’ temperatures.

In high humidity, the water will not evaporate, and the wet-bulb temperature will be the same as the dry-bulb temperature.

The ambient wet-bulb temperature limit for young, healthy people is about 31C at 100% humidity, according to work published last year by Penn State researchers.

Coast in Al Hudaydah in Yemen.
Al Hudaydah in Yemen could become 'almost uninhabitable' (Getty)

Where could temperatures exceed human limits?

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.

Results of the study indicate that if global temperatures increase by 2C 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.

Troublingly, researchers said, 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.

What could happen at higher temperatures?

If warming of the planet continues to 3C 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 US — from Florida to New York and from Houston to Chicago.

South America and Australia would also experience extreme heat at that level of warming.

Lead author Daniel Vecellio, a bioclimatologist who completed a postdoctoral fellowship at Penn State, said, "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 40C last summer,.

“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."

Where could be worst affected?

As one example, the researchers pointed to Al Hudaydah, Yemen, a port city of more than 700,000 people on the Red Sea.

Results of the study indicated that if the planet warms by 4C, this city can expect more than 300 days when temperatures exceed the limits of human tolerance every year, making it almost uninhabitable.