Why you should care about ‘wet bulb temperature’

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

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

These heat waves have reignited the debate about how we can protect people from rising temperatures and how high we can take them to get. But the headline numbers don’t tell the whole story when it comes to the impact of high temperatures on humans, because humidity, which isn’t accounted for in these numbers, plays a huge role in how we actually experience heat. heat.

Recent research has found that we may already be approaching threshold values ​​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 much lower than previously thought.

What does wet bulb temperature mean?

Wet-bulb temperature (WBT) combines dry air temperature (as you’d see it on a thermometer) with humidity; in essence, it is a measure of heat stress conditions in humans.

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

The Yamuna river bed in Delhi in May. Photograph: 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. “That’s the real purpose, to measure how well we can cool off 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 professor of physiology at Pennsylvania 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 might survive only six hours. It is generally considered to be 35°C, roughly equivalent to 40°C air temperature at 75% relative humidity. (At the maximum temperature of 19 July in the UK, the relative humidity was about 25% and the wet bulb temperature was about 25°C).

We humans generally regulate our internal 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 oft-cited 35C value comes from a 2010 theoretical study. However, research Kenney co-authored 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 that if the new finding is true, we are in “a whole new ballgame” when it comes to extreme heat. “The number of people exposed to life-threatening combinations of heat and humidity around the world would be much higher than previously thought.”

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

Where could the threshold of the wet bulb be crossed?

In a global context, the UK is a relatively low-risk area for wet-bulb extremes: so far it has rarely gone above 28°C. “My personal feeling is that a wet-bulb temperature of 35°C would not be possible in the UK, although 31°C could well be later in the century,” says McGuire.. “On the other hand, the Met Office certainly wasn’t expecting 40C [dry temperature] heat in 2022”.

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

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

An Iraqi man wiping his face in front of two large misting fans

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

“Previous studies projected 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 duration times of these events 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, which is still considered an extreme heat and humidity event, was reached more than doubled between 1979 and 2017. There were about 1,000 occurrences of a WBT of 31C, 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 extremes. A study from last year found that the maximum WBT in the tropics will increase by 1C for every 1C of average warming. This means that limiting global warming to 1.5°C above pre-industrial times would prevent most of the tropics, where 40% of the world’s population lives, from reaching the 35°C survival limit, according to The document.

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 article put it, asking whether today’s most shocking heat waves could have occurred in a pre-industrial climate “is fast becoming an outdated question.”

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

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