If you have ever languished in a sauna, you’ll know how enervating the combination of extreme heat and high humidity can be. Now, imagine being in a sauna wherein the temperature and humidity just keep climbing and climbing. Very quickly the experience will stop being relaxing and become deeply unpleasant.
You’ll manage to lose some heat to begin with by sweating, but ultimately a threshold will be reached beyond which sweating is no longer physically possible. You try the door and it’s locked. There’s no escape – and you have just hours to live.
It’s a nightmare vision, but such a conspiracy of heat and humidity is increasingly being encountered in the hottest parts of our world. It’s slated to become ever more common as global heating continues to
build in the decades ahead, threatening the lives of hundreds of millions of people.
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To measure both heat and humidity, we use a wet-bulb thermometer. This is different to the thermometers that take our temperature when we’re sick or measure how hot it is outside, which are dry-bulb. In simple terms, a wet-bulb thermometer encases the bulb at the base in a soaked cloth, across which air is passed.
Sometimes, the device has a handle to allow the user to ensure air flow by whirling it around, like the rattles beloved by football fans in years gone by.
Given that heat is lost by evaporation from the soaked cloth – mimicking how the human body loses heat through sweating – the temperature recorded will always be lower than that seen on a dry-bulb thermometer. That is, until humidity reaches 100 per cent.
At that point, heat loss by evaporation is no longer possible, so the measured temperature becomes the same on wet- and dry- bulb thermometers.
The wet-bulb temperature (WBT) is closer to what we actually feel, which is why it always seems hotter and more uncomfortable when the air is both hot and humid. Those are the circumstances, say, in advance of a big thunderstorm.
The human body loses heat in various ways to keep our core temperature within safe limits. As heat and humidity build, however, they become less efficient or stop working altogether. Heat loss by radiation and conduction works as long as the skin temperature is higher than the surroundings; at very high temperatures, this no longer happens.
Then, the only way heat can be lost from the body is through evaporation from sweat. So if the humidity is high enough to make sweating impossible, our body temperature can climb to dangerous levels.
Normally, our core temperature is around 37°C (98.6°F). If it rises above 40°C (104°F) then unconsciousness, seizures and organ failure can follow. A core temperature of 44°C (111°F) is a virtual death sentence. Once sweating is prevented, the body’s core temperature will just keep rising, typically leading to death within as little as six hours.
Humidity readings
While the temperatures read on a dry-bulb thermometer may remain constant on a hot day, the WBT varies as humidity fluctuates. In essence, it provides a measure of how effectively we can cool ourselves by sweating. In this way, the WBT can tell us when humid heat is approaching a level that could mean a danger to health, or even a threat to life.
The critical WBT has long been thought to be 35°C (95°F), or the equivalent of a dry-bulb temperature reading of 35°C at 100-per-cent humidity. For comparison, on 19 July 2022, when the UK temperature reached an all-time high of 40.3°C (104.5°F), the humidity was 25 per cent, meaning that the WBT was around 25°C (77°F).
A WBT of 35°C (95°F) equates to a combination of a higher dry-bulb temperature with a humidity lower than 100 per cent: for example, 39°C (102°F) at 75-per-cent humidity or 45°C (113°F) at 50-per-cent humidity. But whatever the combination the endgame for a human will be the same.
Even for a young, fit person sitting in the shade with plenty of water, death will likely come within six hours. A fan won’t help either; only access to air conditioning to prevent the terminal decline of the body’s heat-regulating mechanisms.
This is already pretty scary stuff, and it gets worse.
A study was recently undertaken at Penn State University, USA, in which young and healthy subjects were exposed in the lab to increasing heat and humidity. It showed that the lethal threshold is actually quite a bit lower than a WBT of 35°C (95°F) and closer to 31°C (87.8°F).
This marks what’s known as the ‘critical environmental limit’, above which the human body can’t adapt and so a previously stable core temperature begins to rise steadily. What’s more, the subjects were sitting or moving slowly, as one might be if undertaking simple everyday tasks like cooking or taking a shower.
It goes without saying that, in the real world, people engaged in hard and physical work – such as tending crops or working in construction – would be far more at risk. The same is true for the elderly and infirm, as well as young children.
Today, while a WBT of 35°C (95°F) is very rare, it is becoming more common to have a WBT hovering around the 31°C (87.8°F) mark. The former has only been recorded on around a dozen occasions in a handful of countries, including the usual suspects of Saudi Arabia, India and Pakistan, and only then for a few hours at a time.
The lower threshold, meanwhile, has been breached at least a thousand times. Records show that incidences of a WBT of 30°C (86°F) have more than doubled between 1979 and 2017.
Looking ahead, the tropics will see the maximum WBT climb by 1°C (1.8°F) for every 1°C of global average temperature rise, which, for the 12 months to June 2024, was 1.63°C (2.93°F).
A recent survey of climate scientists by the Guardian newspaper revealed that a significant majority expect the planet to heat up by at least 2.5°C (4.5°F). This translates to a big hike in tropical WBT temperatures, meaning many hundreds of millions of people will be exposed to increasingly dangerous, if not deadly, conditions.
As I write in mid-June 2024, much of the world’s population is suffering from the effects of blistering early summer heatwaves, including extreme temperatures across much of the USA and Mexico, which have been made more than 35 times more likely by global heating.
Meanwhile, dry-bulb temperatures just short of 52°C (125°F) claimed more than 1,300 lives during the annual Hajj pilgrimage in Mecca, Saudi Arabia. Such circumstances provide a frightening foretaste of what is coming in the decades ahead.
A study published last year by researchers at Penn State and Purdue universities in the US gave a grim bottom line: as the planet continues to heat up, billions of people will face combinations of heat and humidity that “lie outside the bounds of past human experience, and beyond current heat mitigation strategies."
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Urban dangers
For a 2°C (3.6°F) global average temperature rise, which – given inaction on emissions reductions – is almost certain, dangerous humid heatwaves will become prevalent in many parts of the world.
Most at risk will be those living in northern India, Pakistan, Bangladesh, the Arabian Gulf, the Sahel region of Africa and East Asia. Because of the so-called ‘heat island effect’, whereby urban centres dominated by concrete and tarmac are hotter than their surroundings, those in cities will be most at risk.
WBTs approaching 31°C (87.8°F), at which working outdoors would be all but impossible, are forecast to occur in excess of 200 hours a year in Aden (Yemen), more than 150 hours in Lahore (Pakistan), and 30–40 hours in the Indian cities of Delhi and Kolkata.
A global temperature rise of 3°C (5.4°F) would see an extension of these conditions, which would begin to be encountered in Tokyo (Japan), Lagos (Nigeria), Shenzhen (China), Chicago and New York (USA).
Cities are invariably hotter than the countryside, but this is not to say that rural regions won’t suffer. A 2018 study highlighted a particular threat to the North China Plain: the country’s breadbasket, criss-crossed by a dense network of irrigation courses.
Not only does all this water push up humidity levels, it's predicted to raise the temperature across the region by 0.5°C (0.9°F). While I suspect there are other candidates, the authors of the study warn that the region faces “the greatest risk to human life from rising temperatures of any location on Earth.”
Bearing in mind that this is one of the most densely populated parts of the planet – home to more than 400 million people, mostly farmers – this is not a surprise.
The increased risk arising from extreme heat and humidity is compounded here due to the majority of people being involved in physical outdoor work, and a lower availability of air conditioning than in cities. The study predicts that heatwaves in which the WBT exceeds 35°C (95°F) will occur repeatedly from about 2070, if we do nothing to curb emissions.
Individual events will, of course, happen much sooner, along with an increasing frequency of WBTs of 31°C (87.8°F), which, at the very least, will bring working outside to a grinding halt. It’s even questionable whether places like the North China Plain will continue to be habitable.
Certainly, the heat-related disaster that hit the 2024 Hajj could pale into insignificance compared to future death tolls from humid heatwaves in this part of China.
Looking to the future
This brings me to the broader outlook for our world as global temperatures continue to surge. Agriculture across the planet is already under pressure from super-charged weather, notably drought, floods, hail and wildfires, leading to reduced harvests of coffee, cocoa, olives and staples like rice, maize and wheat in recent years.
Even in temperate regions like the UK, yields of wheat, oats, barley and oilseed rape are forecast to be down collectively by almost 18 per cent this year.
In tropical regions, on top of the drops in yields, farmers will have to contend with extreme conditions that will make it difficult, if not impossible, to tend crops.
Productivity will inevitably take a massive hit. In the decades to come, working outside during the summer in places as far apart as southern Europe, North America, South and South East Asia, and large parts of Africa, is going to be at best excessively unpleasant and, at worst, fatal.
This won’t just apply to agriculture either.
Extreme heat and humidity will severely impact increasingly other sectors that depend upon outdoor work and leisure, such as construction, tourism, sport, fisheries and the mining industries. Economies around the world will suffer. A number of recent studies predict significant and continuing falls in GDP in the decades to come.
In the absence of air conditioning, working indoors won’t be all that fun either. Productivity will shrink as people struggle to maintain focus. While air conditioning might appear to be a cure- all, it’s worth noting that a major ramping up will require a massive hike in power generation.
That would be all well and good if this is provided by renewables, but self-defeating if it’s generated by burning more fossil fuels.
In any case, it’s hard to see how this can help the 400 million Chinese citizens at risk of future humid heatwaves, nor the many hundreds of millions – even billions – more across the planet, who have no choice but to work outdoors.
The harsh reality is that the only way of coping with the new conditions on our hothouse planet will be to abandon those parts of it that no longer support human life year-round without artificial aid.
Perhaps for the first time in our history, humankind will begin to pull in its horns, retreating from those regions, undoubtedly in the tropics, that we have made unliveable by our own carbon-polluting activities.
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