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By Karina Shah

The tropics could become uninhabitable if we don’t limit global warming to less than 1.5°C, the target set in the Paris Agreement on climate change. Above this, the equatorial region, which is home to around 43 per cent of the world’s population, could see air temperatures increase beyond the limit that the human body can withstand.

Yi Zhang at Princeton University in New Jersey and her colleagues used data from 22 climate models to determine how rising air temperatures in the tropics will affect wet bulb temperatures – a measure of both heat and humidity made with a thermometer covered by a wet cloth.

“One can think of this wetted thermometer bulb as mimicking the process of human skin cooling off by evaporating sweat – this is why it is relevant for the heat stress of our bodies,” says Zhang. Humans’ ability to regulate an optimum body temperature of 37°C is dependent on this. If the wet bulb temperature exceeds 35°C, the human body is unable to sufficiently cool itself down.

“The drier the environment is, the more effective the evaporation is and the lower the wet bulb temperature,” says Zhang. “Most of us can tell from life experience that a hot and humid day feels hotter than an equally hot but dry day.”

Although wet bulb temperature is often lower than air temperature, the team’s models suggested that an increase in air temperature in the tropics would correspond to an equal increase in wet bulb temperature.

Temperatures in the tropics are already high on average, but as the humidity is also generally high, this places the region at higher risk of becoming uninhabitable than other areas of Earth.

The team’s analysis suggests that limiting global warming to 1.5°C will prevent the region from reaching the 35°C threshold of wet bulb temperature.

Robert E. Kopp at Rutgers University in New Jersey, who wasn’t involved with the research, says the work helps illustrate that sticking to the Paris climate goals is key to avoiding some of the most extreme heat-related health outcomes of global warming.

Journal reference: Nature Geoscience, DOI: 10.1038/s41561-021-00695-3

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