TOO HOT TO LIVE

There's a new kind of extreme weather scorching the planet

If you have ever languished in a sauna, you will 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. Very quickly the experience will stop being relaxing and become increasingly unpleasant. To begin with, you will lose some heat by sweating, but ultimately a threshold will be reached beyond which sweating is no longer possible. You try the door, but it's locked. There is no escape – and you have just hours to live.

It is a grim scenario, but such a conspiracy of heat and humidity is now increasingly encountered in the hottest parts of our world, and is slated to become ever more common as global heating continues to build in the decades ahead, threatening the lives of hundreds of millions. The combination of heat and humidity is measured on a so-called wet-bulb thermometer. The thermometers we use to take our temperature when we are sick, or to measure how hot or cold it is outside before we decide whether to wear shorts or a warm coat are – by contrast - dry-bulb. In simple terms, a wet-bulb thermometer is one in which the bulb at the base is encased in a soaked cloth across which air is passed. Sometimes the thermometer has a handle so that this can be accomplished by whirling it around, a little like the rattles beloved of football fans in years gone by.

Because heat is lost by evaporation from the wet cloth – mimicking how the human body loses heat by sweating – the temperature recorded by a wet-bulb thermometer is always lower than that measured on a dry bulb instrument, until humidity reaches 100 percent. Then heat loss by evaporation is no longer possible, so the measured temperature is the same on both. The wet-bulb temperature 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, for example in advance of a big thunderstorm.

The human body loses heat in various ways which, together, act to keep its core temperature within safe limits, but as heat and humidity build, they become less efficient, before stopping working altogether. Heat loss by radiation and conduction, for example, works as long as the skin temperature is higher than the surroundings, but at very high temperatures this no longer happens. Then, the only way heat can be lost from the body is by evaporation from sweat. But if the humidity is high enough, sweating becomes impossible, which leads to body temperature climbing to dangerous levels. Normally, the core temperature is around 37°C, but if it rises to above 40°C, then unconsciousness, seizures and organ failure can follow. A core temperature of 44C° is a virtual death sentence. Once the external temperature and humidity prevent sweating, the core temperature will just keep rising, typically leading to death within six hours.

While the temperature read on a dry-bulb thermometer may be constant on a hot day, the wet-bulb temperature (WBT) varies as humidity goes up and down, providing what is – in effect – a measure of how effectively we can cool ourselves by sweating. In this way, the WBT can tell us when humid heat is reaching a level that could mean a danger to health or even a threat to life – a level at which we can no longer sweat. The critical WBT has long been thought to be 35°C, which is equivalent to a dry-bulb temperature of 35°C at 100 percent humidity. For comparison, on July 19^th 2022, when the UK temperature reached an all-time high of 40.3°C, the humidity was 25 percent, and the WTB around 25°C.

A WBT of 35°C also equates to higher temperatures at lower humidities, for example, 39°C at 75 percent humidity or 45°C at 50 percent humidity. Whatever the combination, the end-game for a human exposed to it are 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 preventing terminal decline of the body's heat regulating mechanisms.

This is pretty scary stuff, but it gets worse. A recent study, undertaken at Penn State University, in which young, healthy, subjects, were exposed in the lab to increasing heat and humidity, revealed that the lethal threshold is actually quit a bit lower than a WBT of 35°C, and closer to 31°C. This marks what's known as the critical environmental limit above which the human body can't adapt, so that a previously stable core temperature begins to rise steadily, leading – with continued exposure – to an increasing threat to life. The experiment subjects were sitting or moving slowly, as one might undertaking simple everyday tasks like cooking or taking a shower. It goes without saying that, in the real world, people engaged in hard, physical, work – for example tending crops in fields or building – would be far more at risk. As would the elderly and infirm, and young children.

Today, while a WBT of 35°C is very rare, it is becoming increasingly common for a WBT of 31°C, or just below this, to be achieved or even exceeded. The 35°C mark has only been recorded on around a dozen occasions in a handful of countries, including the usual suspects, Saudi Arabia, India and Pakistan, and only then for a few hours at a time. But the 31°C threshold has been breached at least a thousand times, while records show that incidences of a WBT of 30°C more than doubled between 1979 and 2017. Looking ahead, the tropics will see the maximum WBT climb by 1°C for every 1°C of global average temperature rise which, for the 12 months to June this year, was 1.63°C. A recent survey of climate scientists by the Guardian newspaper revealed that a significant majority expect the planet ultimately to heat up by at least 2.5°C. This translates to a big hike in tropical WBT temperatures that will mean many hundreds of millions of people will be exposed to increasingly deadly conditions.

As I write this in mid-June, much of the world's population is suffering as a consequence of blistering early summer heatwaves, including extreme temperatures prevailing across much of the US and Mexico, which have been made more than 35 times more likely by global heating. Meanwhile, dry-bulb temperatures just short of 52°C have taken more than 1300 lives during the annual Hajj pilgrimage in Mecca, Saudi Arabia. Such circumstances provide a frightening foretaste of what is coming in the decades ahead. The results of a study published last year by researchers at Penn State and Purdue universities in the US, makes grim reading; the bottom line being that, as the planet continue to heat up, billions of people will face combinations of extreme heat and humidity that 'lie outside the bounds of past human experience, and beyond current heat mitigation strategies'.

For a 2°C global average temperature rise, which – given inaction on emissions reductions – is almost certain, dangerous heat and humidity will become increasingly 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 living in cities will be most at risk. WBTs approaching 31°C – at which young, healthy, people would struggle to undertake simple, basic, activities, and 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 would see an extension of these potentially lethal conditions, which would also begin to be encountered in Tokyo (Japan), Lagos (Nigeria), Shenzhen (China) and Chicago and New york (US).

Cities are invariably hotter than the surrounding countryside, but this is not to say that rural regions will not also suffer under future conditions of extreme heat and humidity. A 2018 study highlighted a particular threat to the North China Plain; the country's breadbasket, which is criss-crossed by a dense network of irrigation courses. Not only does all this water push up humidity levels, it is also predicted to raise the temperature across the region by 0.5°C as global heating proceeds. 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 exactly a surprise. The increased risk arising from combinations of extreme heat and humidity is compounded here due to the majority of the population being involved in hard, physical, work outside and because the availability of air conditioning is much lower than in cities. The study predicts that heatwaves in which the WBT exceeds 35°C, 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 WBT 31°C conditions which, at the very least, will bring working outside to a grinding halt. It is even questionable whether – without widely accessible air conditioning – places like the North China Plain will continue to be habitable. Certainly, the more than a thousand heat-related deaths at the aforementioned 2024 Hajj could pale into insignificance compared to future death tolls from extreme humid heatwaves in this part of China

And this brings me to the broader outlook for our world as global temperatures continue to surge upwards. Agriculture across the planet is already under escalating pressure from super-charged weather, notably drought, floods, hail and wildfires, leading to reduced harvests in recent years of coffee, cocoa, olives and staples like rice, maize and wheat. Even in temperate regions like the UK, yields of wheat, oats, barley and oilseed rape are forecast to be down – collectively – almost 18 percent this year. In tropical regions, on top of crop damage due to weather hazards, farmers will have to contend with future extreme heat and humidity that will make it increasingly difficult, if not impossible, for them to tend their crops, resulting in productivity taking a massive hit. In the decades to come, working outside during much of 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 extremely unpleasant, and at worst lethal.

This won't just apply to agriculture either, extreme heat and humidity will impact increasingly severely upon other sectors that depend upon outdoor work and leisure, including construction, tourism, sport, fisheries and the extractive industries. Together, this will mean that economies the world over will suffer, and 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 much fun either, as temperatures and humidity climb, reducing productivity as people struggle to maintain focus. While air conditioning might appear to be a cure-all, it is worth noting that a major ramping up will also require a massive hike in power generation. All well and good if this is provided by renewables, but self-defeating if it is generated by burning more fossil fuels. In any case, it is hard to see how this can help the 400 million Chinese citizens at risk of future deadly heatwaves, nor the many hundreds of millions – even billions – more across the planet, who have no choice but to work out of doors. The grim reality is that the only way of coping with the new conditions on our hothouse planet is to abandon those parts of it that will 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.

An illustrated version of this article was published in BBC Science Focus magazine on July 29th 2024