IIn March, April and May this year, India and neighboring countries endured repeated heat waves that exposed more than a billion people to dangerously hot conditions. India has broken many temperature records. The warmest March in more than a century was recorded across the country and a new high of more than 49 degrees Celsius was reached in Delhi in May.

Record temperatures have also been recorded elsewhere this year, including the UK, breaking the previous record of a staggering 1.6 degrees Celsius, reaching over 40 degrees Celsius. Portugal hit 47C on the 21st of this month, the hottest July day on record, while parts of France recorded new highs.

These heatwaves have sparked debate about how we can protect people from rising temperatures – and at what altitudes we can tolerate them. But the headline numbers don’t tell the whole story when it comes to the effects of high temperatures on humans, because humidity, which isn’t factored into these numbers, plays a huge role in how they work. We actually experience heat.

Recent research has found that we may indeed be close to reaching a threshold value for human survivability of short-term heat and humidity in some parts of the world – a measure of measurement is known as the “wet bulb” temperature – and 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 see on a thermometer) with humidity – it is, in essence, a measure of heat-stressed conditions for humans.

The term comes from the way it is measured. If you slide a wet cloth over the thermometer bulb, the water that evaporates from the cloth will cool the thermometer. This lower temperature is the WBT, which cannot be higher than the dry temperature. However, if the humidity in the ambient air is high – meaning the air is more saturated with water – less will evaporate, so the WBT will be closer to the drier temperature.

A man and a boy walking across the dry cracked riverbed
Yamuna riverbed in Delhi in May. Photo: Manish Swarup / AP

“The [wet-bulb] Kristina Dahl, a climate scientist at the Union of Concerned Scientists, says the temperature reading you get will actually vary depending on how humid it is. “That’s the real purpose, to measure how well we can 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 others. But WBT is particularly important as a measure of the indoor environment, where deaths often occur during heat waves, said W Larry Kenney, a professor of physiology at Penn State University.

When do wet bulb temperatures become dangerous?

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

Humans normally regulate their internal body temperature by sweating, but above wet bulb temperature we cannot cool down this way anymore, resulting in a steady rise in body temperature. This essentially marks a limit to humans’ ability to adapt to extreme temperatures – if we can’t escape the conditions, our body’s core could be out of range. and organs may begin to fail.

The quoted 35C value comes from a 2010 theoretical study, however, research Kenney’s co-author this year found that the actual threshold our bodies can tolerate can be much lower. “Our data is actual human subject data and shows that the critical wet bulb temperature is close to 31.5C,” he said.

Bill McGuire, Director of the UCL Benfield Hazard Research Center in the UK, said if the new findings are true, we are in “a whole new ball game” when it comes to extreme heat. “The number of people exposed to a potentially lethal combination of heat and humidity worldwide will be much higher than previously thought.”

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

Where can the wet bulb threshold be crossed?

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

However, the risk of crossing the WBT threshold is greater elsewhere. A 2015 study concluded that extremes are likely to reach and exceed 35 degrees Celsius in the area around the Arabian Gulf by the end of the century if greenhouse gas emissions are not curbed, raises questions about the ability of people to live there.

In 2020, the study found that several coastal subtropical sites experienced 35C WBTs, albeit for only a few hours.

An Iraqi man is wiping his face in front of two large misting fans
An Iraqi man cools down in Baghdad. The temperature of the whole country will reach 53 degrees Celsius in 2020. Photo: Ahmad Al-Rubaye / AFP / Getty Images

Lead author Colin Raymond, a climate scientist at Nasa’s Jet Propulsion Laboratory, said: “Previous studies predicted that this would happen a few decades from now, but this suggests it does. is happening right now.. “The duration of these events will increase, and the areas they affect will develop a direct correlation with global warming.”

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

An important question is how temperature increases due to the climate crisis correlate with increases in the WBT poles. A study last year showed that maximum WBT in the tropics would increase by 1C for every 1 degree of average warmth. This means that limiting global warming to 1.5 degrees Celsius above pre-industrial levels would prevent much of the tropics – where 40% of the global population lives – from reaching the living limit. missed is 35 degrees Celsius, the newspaper said.

Heat waves are getting worse many times faster than any other type of extreme weather because of the climate crisis. Scientists estimate that it is 30 times more likely to cause heatwaves in India and Pakistan. As another article questioned, whether today’s most impactful heatwaves could occur in a pre-industrial environment is “a question that is quickly becoming an old-fashioned question.”

Instead, as heat waves begin to affect more people’s lives more frequently, the question of what we can do about them is becoming ever more important. As the world sees a deadly combination of high humidity and high temperatures increasingly frequent, this could ultimately mean some places simply becoming too hot to liveopens up the need for migration pathways to allow millions of people to leave their home areas.