Why you have to worry about “wet bulb temperature”

Why you have to worry about “wet bulb temperature”

Why you have to worry about “wet bulb temperature”

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In March, April and May of this year, India and its neighbors experienced repeated heatwaves that exposed more than a billion people to dangerously hot conditions. India broke several temperature records. The warmest March in over a century was recorded across the country and in May a new high of over 49 ° C was reached in Delhi.

Record heat was also recorded elsewhere this year, including the UK, which broke its previous record by a staggering 1.6 ° C, hitting more than 40 ° C. Portugal hit 47 ° C on 21st of this month, the hottest July day on record, while several places in France recorded new highs.

These heatwaves have reignited the debate about how to protect people from rising temperatures and how high we can stand them. But the main figures don’t give the whole story when it comes to the impact of high temperatures on humans, because humidity, which isn’t factored into these figures, plays a huge role in how we actually experience heat. .

Recent research has found that in some places around the world we may already be close to the threshold values ​​for human survival of temperature and humidity for short periods – a measure known as “wet bulb” temperature – and that this threshold could actually be a lot. lower than previously thought.

What does wet bulb temperature mean?

Wet bulb temperature (WBT) combines dry air temperature (as you would see on a thermometer) with humidity – essentially, it’s a measure of the heat stress conditions on humans.

The term comes from how it is measured. If you run a damp cloth over the bulb of a thermometer, the water evaporating from the cloth will cool the thermometer. This lowest temperature is the WBT, which cannot exceed the dry temperature. If the humidity in the surrounding air is high, however, which means the air is already more saturated with water, less evaporation will occur, so the WBT will be closer to dry temperature.

A man and a boy walking on a dry and cracked river bed

The bed of the Yamuna River in Delhi in May. Photography: Manish Swarup / AP

“The [wet-bulb] The temperature reading you get will actually change depending on how humid it is, ”says Kristina Dahl, a climate scientist at the Union of Concerned Scientists.

Humidity and temperature aren’t the only things that affect a person’s body temperature – solar radiation and wind speed are other factors. But WBT is especially important as a measure of indoor environments, where deaths often occur during heatwaves, says W Larry Kenney, a professor of physiology at Penn State University.

When do wet bulb temperatures become dangerous?

Concern often centers on the “threshold” or “critical” WBT for humans, the point at which a healthy person could only survive for six hours. This is generally considered 35 ° C, roughly equivalent to an air temperature of 40 ° C with a relative humidity of 75%. (At the UK temperature peak on 19 July, the relative humidity was around 25% and the wet bulb temperature around 25 ° C.)

Humans usually regulate their core body temperature by sweating, but above the wet bulb temperature, we can no longer cool down in this way, causing our body temperature to rise steadily. This essentially marks a limit to human adaptability to extreme heat: if we can’t escape the conditions, our body’s core can rise beyond survival range and organs can begin to fail.

The often-quoted 35C value comes from a 2010 theoretical study. However, Kenney’s co-authored research this year found that the actual threshold our bodies can tolerate could be much lower. “Our data is real data from human subjects and shows that the critical wet bulb temperature is closer to 31.5 ° C,” she says.

Bill McGuire, director of the Benfield UCL Hazard Research Center in the UK, says if the new finding is true, we’re in “a whole new ball game” when it comes to extreme heat. “The number of people exposed to potentially deadly combinations of heat and humidity worldwide would be much higher than previously thought.”

It is important to note that heat becomes dangerous for many people far below the WBT threshold.

Where could the wet bulb threshold be exceeded?

In a global context, the UK is a relatively low risk area for extreme wet bulbs – so far it has rarely exceeded 28 ° C. “My personal feeling is that a wet bulb temperature of 35 ° C would not be possible in the UK, even though 31 ° C may be later in the century,” says McGuire.. “Then again, the Met Office certainly wasn’t expecting 40C [dry temperature] hot in 2022 “.

However, the risk of exceeding the WBT threshold is greater elsewhere. A 2015 study concluded that extremes are likely to approach and exceed 35 ° C in the region around the Persian Gulf towards the turn of the century if greenhouse gas emissions are not curbed, asking questions about human habitability there.

In 2020, research found that some coastal subtropical locations have already experienced WBT of 35 ° C, even if only for a few hours.

An Iraqi man wipes his face in front of two large, fogged fans

An Iraqi cools down in Baghdad. Temperatures in the country reached 53 ° C in 2020. Photograph: Ahmad Al-Rubaye / AFP / Getty Images

“Previous studies predicted this would happen in several decades, but this shows it is happening right now,” said lead author Colin Raymond, a climate scientist at NASA’s Jet Propulsion Laboratory. “The timing of these events will increase and the areas they hit will increase in direct correlation with global warming.”

The study also found that globally, the number of times a WBT of 30 ° C was reached – still considered an event of extreme humidity and heat – more than doubled between 1979 and 2017. about 1,000 occurrences of a WBT of 31 ° C and about a dozen above 35 ° C, in Pakistan, India, Saudi Arabia, Mexico and Australia.

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An important question is how the temperature rises due to the climate crisis correlates with increasing WBT extremes. A study last year found that the maximum WBT in the tropics will increase by 1 ° C for every 1 ° C of average warming. This means that limiting global warming to 1.5 ° C above the pre-industrial era would prevent much of the tropical area – where 40% of the world’s population lives – from reaching the 35 ° C survival limit, he says. the document.

Heatwaves are getting worse many times faster than any other type of extreme climate due to the climate crisis. Scientists estimate it made the heatwave in India and Pakistan 30 times more likely. As another paper put it, asking whether today’s most impacting heatwaves might have occurred in a pre-industrial climate is “rapidly becoming an obsolete question.”

Instead, as heat waves begin to affect the lives of more and more people with greater frequency, the question of what we can do about it is becoming increasingly important. As the world increasingly sees the deadly mix of high humidity and high temperature, this could ultimately mean that some places simply become too hot to live, opening up the need for migration routes to allow millions of people to move away from their home areas. .

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