Who will be most at risk from increasing fatal humidity?

Deadly levels of humid heat are not a common phenomenon, yet.

But on current climate pathways, it’s likely parts of the world will get so hot and humid that humans will be literally unable to survive the weather in coming decades.

And according to an Australasian research team, populations under the most threat are also often the least well-equipped to handle it.

They are sounding the alarm on humid heat stress in a commentary published in One Earth.

The “theoretical limit of human survival”, as the researchers call it, is a wet-bulb temperature of 35°C.

“The wet bulb temperature is a measure that incorporates both air temperature and humidity,” explains lead author Emma Ramsay, a postdoctoral research fellow at Nanyang Technological University in Singapore.

“If you’re measuring it on the ground, it’s a thermometer with a wet cloth wrapped around it. Essentially, it’s measuring the temperature if the water evaporated.”

Evaporating water – or rather, evaporating sweat – is how humans cool down. The more moisture in the air, the warmer something needs to be for water to evaporate and the harder it is for the body to cool itself.

“Eventually, our core body temperature would rise, and we wouldn’t be able to survive for much longer than a few hours,” Ramsay tells Cosmos.

The wet-bulb 35°C limit, according to Ramsay, is “purely based on physics”.

“It really was an ideal limit, and any real human would almost certainly struggle in conditions below that.”

There have been a few isolated recordings of the lethal 35°C limit in recent years – although only briefly. People haven’t seen longer and more deadly periods of deadly humid heat.

“In human history, it’s never passed these limits,” says Ramsay.

“But what’s kind of scary is that the best available climate projections now show that parts of the world are on track to reach these upper limits. In some parts, only 1 or 2°C warming could be [enough].”

Unfortunately, the places that are at most risk are also often the least prepared.

“There’s one billion people living in informal settlements – so, what we think of as slums – primarily located in developing countries across the tropics,” says Ramsay.

“When you look at a map of where the most informal settlements are, in sub-Saharan Africa and Asia, they line up almost exactly with these really high-risk locations for humid heat stress.”

Beyond taking action to limit greenhouse gas emissions, how should the world cope with humid heat stress? Evacuating these areas isn’t a feasible option, says Ramsay.

“I think any kind of widespread migration due to climate change would be hugely catastrophic for the world. So helping communities adapt is a priority. And there are a number of ways that we can do that.”

A significant one, according to the researchers’ paper, is urban planning. Cities are particularly hot places, but tree planting and other green spaces have a significant cooling effect.

“Green and blue spaces in the city help to keep it cool,” says Ramsay.

Buildings to shelter from the heat can help too – but not necessarily private homes.

“Air conditioning obviously seems like an almost inevitable solution, but it’s not going to be equitable, and it’s not going to be sustainable,” says Ramsay.

“We already see during heatwaves that everyone turning on the air conditioning can put a huge load on the power grid, and a blackout can be deadly in that situation. So we know that air conditioning isn’t a viable solution for the future.

“What we can do is have places like humid heat shelters where, during the very worst heat waves, people can go and seek shelter in an air-conditioned space.”

Cities also need better meteorological science to manage this. In sub-Saharan Africa, according to the paper, just 6% of urban settlements have a meteorological station within 5km.

“For this to be effective, we need to be able to send out warnings and alerts through what are known as early warning systems,” says Ramsay.

“In Australia, if there’s a heatwave or a flood, you’ll get a text or an alert from the BOM. But in many developing countries, these just don’t exist, and they don’t have the climate data to really be feeding into these systems either.”

Ramsay and colleagues write in their paper that “such efforts must be prioritised to protect the most vulnerable from the existential threat of humid heat”.

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