Why You Need to Worry About the “Wet Bulb Temperature”

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In March, April and May this year, India and its neighbors endured repeated heat waves that exposed more than a billion people to dangerously hot conditions. India broke several temperature records. The hottest March in more than a century was recorded across the country and a new high of more than 49C was hit in Delhi in May.

Record heat has also been recorded elsewhere this year, including the UK, which smashed its previous record by an incredible 1.6C, reaching more than 40C. Portugal reached 47C on the 21st of this month, the hottest July day on record, while several places in France recorded new highs.

These heat waves have sparked the debate about how we can protect people from rising temperatures – and how high we can tolerate them going. But the headline numbers don’t tell the whole story when it comes to the impact of high temperatures on humans, because humidity, which is not included in these numbers, plays a huge role in how we actually experience heat.

Recent research has found that we are actually already approaching thresholds for human survival of temperature and humidity for short periods in some parts of the world – a measure known as the “wet-bulb” temperature – and that this threshold may actually be far lower than previously thought .

What does wet bulb temperature mean?

Wet-bulb temperature (WBT) combines dry air temperature (as you see on a thermometer) with humidity – essentially a measure of heat stress conditions in humans.

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

A man and a boy walk across a dry, cracked river bed

The bed of the river Yamuna in Delhi in May. Photo: Manish Swarup/AP

“The [wet-bulb] the temperature reading you get will actually change depending on how humid it is, says Kristina Dahl, climate scientist at the Union of Concerned Scientists. “That’s the real purpose, to measure how well we can cool ourselves by sweating.”

Humidity and temperature are not 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 in heat waves, says W Larry Kenney, a physiology professor at Penn State University.

When do wet bulb temperatures become dangerous?

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

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

The oft-cited 35C value comes from a theoretical study from 2010. However, research co-authored by Kenney this year found that the real threshold our bodies can tolerate may be far lower. “Our data is actual human data and shows that the critical wet bulb temperature is closer to 31.5 C,” he says.

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

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

Where can the threshold for wet bulb be passed?

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

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

In 2020, research found that some subtropical coastal locations have already experienced WBTs of 35C, albeit only for a few hours.

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

An Iraqi man cools off in Baghdad. Temperatures in the country reached 53C in 2020. Photo: Ahmad Al-Rubaye/AFP/Getty Images

“Previous studies suggested this would happen decades from now, but this shows it’s happening right now,” said lead author Colin Raymond, a climate scientist at NASA’s Jet Propulsion Laboratory. “The times these events last will increase and the areas they affect will grow in direct correlation with global warming.”

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

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An important question is how temperature increases due to the climate crisis correlate with increases in WBT extremities. A study last year found that maximum WBT in the tropics will rise by 1C for every 1C of average warming. This means limiting global warming to 1.5C above the pre-industrial era would prevent the majority of the tropics – home to 40% of the global population – from reaching the survival limit of 35C, the paper said.

Heat waves are worsening many times faster than any other type of extreme weather due to the climate crisis. Scientists estimate that it made the heat wave in India and Pakistan 30 times more likely. As another paper put it, asking whether today’s most impactful heat waves could have occurred in a pre-industrial climate is “quickly becoming an obsolete question”.

Instead, as heat waves begin to affect more people’s lives more frequently, the question of what we can do about them becomes increasingly important. As the world sees the deadly combination of high humidity and high temperature more and more often, this could eventually mean that some places simply become too hot to live in, opening up the need for migration routes to enable millions of people to get away from their home areas.

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