Record heat was also set elsewhere this year, including the UK, which smashed its previous record by 1.6 degrees Celsius, reaching over 40 degrees Celsius. Portugal reached 47C on the 21st of this month, the hottest July day on record, while many parts of France recorded new highs. These heatwaves have reignited the debate about how we can protect people from rising temperatures – and how high we can bear them. But the headlines don’t tell the whole story when it comes to the impact of high temperatures on humans, because humidity, which isn’t factored into these numbers, plays a huge role in how we actually experience heat. Recent research has found that we may indeed already be approaching the threshold values for human survival of temperature and humidity for short periods of time in some parts of the world – a measure known as the “wet bulb” temperature – and that this threshold may actually they are much lower than we thought.
What does liquid bulb temperature mean?
Wet bulb temperature (WBT) combines dry air temperature (as you would see 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 wet cloth over the bulb of a thermometer, the water that evaporates from the cloth will cool the thermometer. This lowest temperature is the WBT, which cannot exceed the dry-top temperature. If, however, the humidity in the surrounding air is high – meaning the air is already more saturated with water – there will be less evaporation, so the WBT will be closer to the dry-land temperature. The Yamuna riverbed 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, a climatologist at the Union of Concerned Scientists. “That’s the real purpose, to measure how well we’ll be able to cool ourselves by sweating.” 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 in heat waves, says W Larry Kenney, professor of physiology at Penn State University.
When do liquid 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. It is usually considered to be 35 C, roughly equivalent to an air temperature of 40 C with a relative humidity of 75%. (At the UK maximum 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 ourselves this way, leading 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 core body can exceed the survivability range and organs begin to fail. The oft-quoted figure of 35C comes from a theoretical study in 2010. However, research authored by Kenney this year found that the actual limit our bodies can tolerate could be much lower. “Our data is real human data and shows that the critical liquid bulb temperature is closer to 31.5C,” 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 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 liquid bulb limit be crossed?
In a global context, the UK is a relatively low-risk area for extreme wet-bulb conditions – 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 it may be 31C later in the century,” says McGuire. “Again, the Met Office certainly didn’t expect 40C [dry temperature] heat in 2022″. However, the risk of exceeding the WBT limit is greater elsewhere. A 2015 study concluded that extremes are likely to approach and exceed 35 degrees Celsius in the region around the Persian Gulf 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 coastal subtropical locations have already experienced WBT 35C, albeit only for a few hours. An Iraqi cools off in Baghdad. Temperatures in the country reached 53 degrees Celsius in 2020. Photo: Ahmad Al-Rubaye/AFP/Getty Images “Previous studies predicted this would happen several 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 increase in direct correlation with global warming.” The study also found that globally, the number of times a WBT of 30 C – 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 31 C and about a dozen above 35 C, in Pakistan, India, Saudi Arabia, Mexico and Australia. An important question is how warming due to the climate crisis correlates with increases in WBT extremes. A study last year found that maximum WBT in the tropics would increase by 1C for every 1C of average warming. This means that limiting global warming to 1.5C above pre-industrial times would prevent the majority of the tropics – home to 40% of the world’s population – from reaching the 35C survival limit, the paper said. Heat waves are worsening many times faster than any other type of extreme weather due to the climate crisis. Scientists estimate it made heatwaves in India and Pakistan 30 times more likely. As another paper put it, asking whether today’s most dangerous heat waves could have occurred in a pre-industrial climate is “quickly becoming an outdated question.” Instead, as heat waves begin to affect more people’s lives more often, the question of what we can do about them becomes increasingly important. As the world sees more and more of the deadly mix of high humidity and high temperature, this could eventually mean that some places become too hot to live in, opening up the need for migration routes that will allow millions of people to escape domestic spaces.
