Sixty degrees no longer seemed unthinkable on planet Earth. Extreme heat as it is currently in India and Pakistan puts a strain on the body – but extreme humidity is even worse. How does the body react to this?
The people of India and Pakistan are going through tough weeks. A temporary peak of the brutal heat wave was reached in mid-May: Pakistan’s Jacobabad weather station recorded 51 degrees, a new record that has just been lost. For Central Europe, these are unimaginable values, but even heat-tolerant people in the densely populated region are pushed to the limit by the intense dry heat.
One temperature record has been chasing the next day since March
A heat wave is essentially unprecedented because of its duration. He’s been chasing one record for the next since March. And climate researchers worry about what will happen – in the short term in early summer, but above all in the future. Sixty degrees and more is no longer unimaginable on planet Earth, climate change could turn South Asia into an inescapable sauna.
Even more frightening is the extreme scenario that has received little research: extreme humidity. Moist heat puts more stress on an organism than dry heat, and everyone feels that during steamy thunderstorms like this week: the higher the humidity in summer, the bigger the complaint. The only thing that helps is drinking and sweating.
However, if the outside area is turned into a real steam sauna, and the humidity is 100 percent, that won’t help either. Fully saturated hot air no longer absorbs water, and drops of sweat on the surface of the skin can no longer evaporate – a failure of the natural cooling system of the human body. And so the sweltering heat is a real killer: without sweating, the body temperature rises, and the core body temperature rises by more than one degree per hour. It can be fatal.
This horror scenario does not come from a Hollywood script, but it can become a reality in a warm climate. It occurs when the air is very hot and the humidity is very high at the same time – meteorologists talk about a very high temperature of the moist bulb. This scale indicates the maximum temperature to which a wet surface, such as a person’s sweaty skin, can cool down by evaporation. At a wet bulb temperature of 35 degrees, it becomes critical: then a person is no longer physically able to dissipate heat into the environment. Physiologists talk about the maximum load for humans.
“The maximum human endurance is the point at which the temperature rises, even if you are getting wet and naked in the shade in front of a large fan” – said climate researcher Stephen Sherwood from the University of New South Wales in Sydney, describing the phenomenon. Twelve years ago he published a study on the topic in the journal PNAS and it caused a great deal of scientific debate.
Especially dangerous for the elderly, infants and the already sick
However, intense stress on the body begins not at a wet temperature of 35 degrees, but much earlier. Even at humidity levels of around 30 degrees, as is currently measured in India and Pakistan, it becomes dangerous for the elderly, infants and those with pre-existing illnesses. The wet bulb temperature reached 33.1 degrees in Jacobabad. A current US study conducted two months ago on young, healthy people concluded that humid temperatures of 31 degrees are indeed critical. The test subjects’ basal body temperature actually increased this value, according to the study, which was published in the Journal of Applied Physiology.
Unlike the familiar scale of air temperature, wet-bulb temperature is not a simple quantity. It is measured with a hygrometer, which consists of two thermometers: one measures the air temperature, and the other is wrapped in a wet cotton sock to determine the temperature of the wet bulb. If the humidity is one hundred percent, then both scales show the same temperature. Otherwise, the temperature of the wet bulb is always lower than the air temperature: the drier the air, the lower it is, the more effective the evaporation can cool the skin.
Most areas are still far from life-threatening humid temperatures. Globally, a wet bulb temperature rarely rises above the 30 mark. But it shouldn’t stay that way. Southern Asia, the Arabian Peninsula, coastal parts of Africa, Australia and Central America are considered hot humid areas. Frankfurt climate researcher Joachim Curtius considers steam sauna conditions around the Arabian Sea and Persian Gulf to be a realistic catastrophic scenario with many deaths likely in the near future. “In my opinion, this risk was underestimated and little was known,” he says.
The limits of human resilience have occurred many times
Erich Fischer investigates such extreme scenarios at ETH Zurich. The lead author of the latest Global Climate Report would like to better understand how such conditions arise and whether entire regions will become uninhabitable as a result. The proximity to the warm ocean, large rivers and extensive irrigation areas make the area between the Indus and Ganges rivers vulnerable to high humidity.
Climate researchers have found evidence that the limits of human resilience have already occurred many times in today’s climate. Two years ago, Colin Raymond of the California Institute of Technology in Advanced Sciences presented an analysis of data from 1979 to 2017 that showed such extreme conditions. Accordingly, wet temperatures of more than 33 degrees 80 times have already been reached on the Arabian Peninsula, and the 35 degrees threshold has been briefly exceeded twice. Examining the data showed something else: Extreme, humid heat was twice as common at the end of the study period as it was at the beginning.
One event in particular has electrified climate researchers: the last week of July 2015 in the Persian Gulf. At the time, the area was sweltering hot for a week, with the temperature rising to more than 40 degrees daily. The heat wave reached its climax on the last day of July. The Gulf water temperature rose to more than 34 degrees. In the late afternoon, the wind turned and moved the laundry room air over the bay toward the land. In the Iranian port city of Mahshahr, the temperature soared to 46 degrees, and the humidity reached 49 percent – the equivalent of about 35 degrees humid. So far, the event is considered the worst wet heat wave ever recorded.
After the humid heat, the epidemiologists got to work. They feared a high death rate, especially in populations that could hardly tolerate extreme stress. So they assessed the number of deaths and looked for indications of high humidity levels. However, there was no evidence of increased mortality in the data, and other studies have not yet shown that humidity associated with heat waves leads to higher mortality. How could that happen? “In fact, the results of epidemiological investigations do not confirm the hypotheses of laboratory studies,” says Antonio Gasparini, an epidemiologist from the London School of Hygiene. “They also do not confirm the physiological knowledge that moist heat is necessarily worse than dry heat.”
The extent of moist heat stress depends on the behavior
However, there is no doubt about the maximum load which is 35 degrees wet temperature. Gasparini says, “Several laboratory studies in animals and humans have confirmed that burglary becomes more dangerous to the body as it becomes more severe. However, real studies show significant differences between regions and populations,” he says.
One reason for this may be that the data examined is not the best: High-quality mortality data is only available from industrialized countries, says Gasparini. There is also the human factor. Because the extent of moist heat stress also depends on the behavior of each individual. However, heat stress indicators cannot reflect this. In this regard, they should be “used with caution,” says Jacob Zschichler, aquatic system researcher at the Helmholtz Center for Environmental Research in Leipzig. So there’s a big difference in the clothes you’re wearing, whether you’re working or resting, whether there’s a breeze or whether you’re exposed to direct sunlight. It’s also important how long the unlikely conditions last – and whether you have access to cold storage.
To solve the mystery of the excess mortality shortage, epidemiologists and climate researchers are now uniting under the umbrella of the European Geoscientists Union (EGU). One explanation could be that humidity varies little over time at the local level, but the temperature does. Because epidemiological models are often generated on a small scale, the potential impact of humidity may not be detectable locally, at most between regions or countries.
Only one thing seems clear: the clock is ticking. With an average global temperature increase of three degrees, wet temperatures of about 35 degrees can occur each year in vulnerable areas – and raise questions about their habitability.