Temperatures are continuing to rise to fresh all-time highs as global warming accelerates. People have started to die because of the extreme heat and it’s only a matter of time before the “wet-bulb” effect will kill large numbers of people.

Humans’ internal temperature is ideally supposed to be 36.6C, but they can only tolerate a relatively tight band of hotter temperatures. The wet-bulb effect kicks in when temperatures rise to 35C or more on the wet-bulb scale, coupled with very high humidity that prevents sweat from evaporating. The evaporation of sweat accounts for 80% of the cooling of the human body. If the body cannot cool down it will eventually overheat, triggering respiratory and cardiovascular issues and even death. If the victim cannot quickly find another way to cool off then the wet-bulb effect kills within six hours, according to a landmark study  in 2010, reports Reuters.

Note that confusingly, the wet-bulb temperature includes an assumption of near 100% humidity and is thus not directly comparable with simple temperature measures, which can easily be far in excess of 35C and non-lethal.

The wet-bulb effect gets its name from measuring the temperature by covering a thermometer with a water-soaked cloth. The process of the water evaporating from the cloth, thus lowering the temperature, mirrors how the human body cools down with sweat. At 100% relative humidity, wet-bulb temperature will be the same as the dry air temperature, as there is no evaporation because the air is already saturated with water.

Wet-bulb conditions have been rare, but the death toll from overheating is starting to rise, mainly amongst the old and medically vulnerable. And this spring has seen fresh extreme temperatures across the world breaking all-time highs that were only set last year – the hottest on record.

India heatwave

Indian TV presenter Lopamudra Sinha collapsed while reading the news on April 21 on the Doordarshan news channel and had to be revived by her colleagues during the broadcast.

Sinha said she was feeling unwell before the broadcast but attempted to carry on after having a glass of water. The Doordarshan anchor slumped on her seat, fainting on air while ironically reporting live on the heatwave updates sweeping West Bengal. Sinha said she had fainted "due to intense heat and because her blood pressure plummeted suddenly". The anchor also said that, due to some snag in the cooling system, there was extreme heat inside the studio.

India is being seared by a heatwave that has seen temperatures range from 40C to 46C, which are starting to approach the upper band of what humans can tolerate.

African deaths

The situation in Mali in Africa is even worse, where temperatures have rocketed to over 48C and where one hospital has reported hundreds of deaths as a result of the extreme heat.

A number of countries in the Sahel region and across West Africa were hit by a strong heatwave that began in March and continued into April, with Mali and Burkina Faso being the worst affected.

The Gabriel Toure Hospital in Bamako, Mali’s capital, recorded 102 deaths in the first days of April during the worst of the heatwave, the BBC reported. About half of those that died were over 60 years of age. These deaths were more probably not the direct result of wet-bulb conditions, but stress caused by extreme heat and heatstroke; the hospital said that “heat played a role” in many of these fatalities.

Climate change has raised temperatures 1.5C higher than normal in Mali and Burkina Faso, and made the night even hotter at 2C above the average.

Gulf states wet-bulb

The Gulf states came close to wet-bulb conditions last August when a wave of extreme heat and very high humidity swept the region, hitting cities from Dubai to Doha. This year Dubai has been hit again by extreme weather, after a whole year’s worth of rain fell on the United Arab Emirates’ (UAE) capital in a single day, turning the runways at the international airport into a lake and flooding the metro.

Rain by itself is not life-threatening, but last year temperatures soared to 43C in August coupled with 35-40% humidity that comes close to being life-threatening.

And humidity is expected to rise rapidly. As bne IntelliNews reported in a deep dive into changing rainfall levels, with each one degree rise in temperature water vaporisation increases by 7%. As water vapour is itself a greenhouse gas (GHG), the more humid the air, the faster temperatures will rise.

The almost wet-bulb conditions in the Gulf didn’t last very long; they only become life-threatening if they persist. But a 2020 study in the Journal of Scientific Advances found that dangerous wet-bulb temperatures could increase from approximately one to two hours that has been occasionally recorded in Asia and the Middle East, to six or more hours by 2060, killing anyone who can't take refuge from the heat and humidity.

Tropical regions with a lot of humidity, especially those along the monsoon belt, are generally at greatest risk of experiencing lethal wet-bulb temperatures, including China, India, Bangladesh, Pakistan and Africa's Sahel region, which are all classed as key risk zones. 

“The hottest city on earth”, Jacobabad in Pakistan, has passed the wet-bulb threshold four times and La Paz, Mexico, Port Hedland, Australia, and Abu Dhabi, UAE have also breached the limit, according to the 2020 study that detailed which countries are at risk. 

Global warming is accelerating

The problem is only set to get worse. As bne IntelliNews reported, according to some estimates around 3bn people will find themselves living in countries that have become uninhabitable by 2070, say scientists due to rising temperatures.

Officially the world is only 1.2C warmer than the long-term pre-industrial benchmark established in Paris in 2015. With average global temperatures 1.2C warmer, scientists say the extreme heatwave in Mali should happen once every 200 years, but if global temperatures breach 2C, powerful heat waves would happen every 20 years, according to climate models.

Recently some scientists have been asking if climate change is accelerating following the unprecedented increases in temperature in 2023. Temperatures were 1.5C higher than the pre-industrial benchmark every day of last year, more than the Paris accords’ maximum recommended level, and are on course to repeat that this year. That doesn’t mean the Paris target has already been missed, as the 1.5C target is an average over three years and so still less than 1.5C, but clearly that threshold will be surpassed by the end of 2025 if nothing changes.