Most people know that humid heat is harder to handle than the “dry” kind.
Recently, scientists have projected that later in the century, in parts of the tropics and subtropics, warming climate could cause combined heat and humidity to reach levels rarely, if ever, experienced before by humans. Such conditions would ravage economies, and possibly even surpass the physiological limits of human survival.
According to a new study, which appeared in the journal Science Advances on May 8, such conditions are already appearing. The study identifies thousands of previously rare or unprecedented bouts of extreme heat and humidity in Asia, Africa, Australia, South America and North America, including in the US Gulf Coast region.
Along the Persian Gulf, researchers spotted more than a dozen recent brief outbreaks surpassing the theoretical human survivability limit. The outbreaks have so far been confined to localised areas and lasted just hours, but they are increasing in frequency and intensity, say the authors.
“Previous studies projected that this would happen several decades from now, but this shows it’s happening right now,” said lead author Colin Raymond, who did the research as a postgraduate student at Columbia University’s Lamont-Doherty Earth Observatory. “The time these events last will increase, and the areas they affect will grow in direct correlation with global warming.”
Analysing data from weather stations from 1979 to 2017, the authors found that extreme heat/humidity combinations doubled over the study period.
Repeated incidents appeared in much of India, Bangladesh, Pakistan, northwestern Australia and along the coasts of the Red Sea and Mexico’s Gulf of California. The highest, potentially fatal, readings were spotted 14 times in the cities of Dhahran/Damman in Saudi Arabia, Doha in Qatar and Ras Al Khaimah in the United Arab Emirates, which have combined populations of more than 3 million. Parts of south-east Asia, southern China, subtropical Africa and the Caribbean were also hit.
Not surprisingly, incidents tended to cluster on coastlines along confined seas, gulfs and straits, where evaporating seawater provides abundant moisture to be sucked up by hot air. In some areas further inland, moisture-laden monsoon winds or wide areas of crop irrigation appear to play the same role.
Prior climate studies failed to recognise most past incidents because climate researchers usually look at averages of heat and humidity measured over large areas and over several hours at a time. Raymond and his colleagues instead drilled directly into hourly data from 7877 individual weather stations, allowing them to pinpoint shorter-lived bouts affecting smaller areas.
Humidity worsens the effects of heat because humans cool their bodies by sweating; water expelled through the skin removes excess body heat, and when it evaporates it carries that heat away. The process works nicely in deserts, but less well in humid regions, where the air is already too laden with moisture to take on much more. Evaporation of sweat slows. In the most extreme instances, it could stop.
In that case, unless one can retreat to an air-conditioned room, the body’s core heats beyond its narrow survivable range and organs begin to fail. Even a strong, physically fit person resting in the shade with no clothes and unlimited access to drinking water would die within hours.
Meteorologists measure the heat/humidity effect on the “wet bulb” Celsius scale.
Prior studies suggest that even the strongest, best-adapted people cannot carry out normal outdoor activities when the wet bulb hits 32°C. Most others would crumble well before that. A reading of 35°C — the peak briefly reached in the Persian Gulf cities — is considered the theoretical survivability limit. “It’s hard to exaggerate the effects of anything that gets into the 30s,” said Raymond.
The study found that worldwide, wet-bulb readings approaching or exceeding 30°C have doubled since 1979. The number of readings of 31°C — previously believed to occur only rarely — totalled around 1000. Readings of 33°C — previously thought to be almost nonexistent — totalled around 80.
A heat wave that struck much of the US last July maxed out at about 30°C on the wet bulb and a similar wave hit in August. The waves paralysed communities and led to at least six deaths, including those of an air-conditioning technician in Phoenix, Arizona, and former NFL player Mitch Petrus, who died in Arkansas while working outside.
It was a modest toll; heat-related illnesses already kill more US residents than any other weather-related hazard including cold, hurricanes or floods. An investigation last year by the website Inside Climate News revealed that cases of heat stroke or heat exhaustion among US troops on domestic bases grew 60% from 2008–18. Seventeen soldiers died, almost all in the muggy US Southeast.
High-humidity heat waves in Russia and Europe, where far fewer people have air conditioning, have killed tens of thousands.
“We may be closer to a real tipping point on this than we think,” said Radley Horton, a Lamont-Doherty research scientist and co-author of the paper. Horton co-authored a 2017 paper projecting that such conditions would not take hold until later in the century.
While air conditioning may blunt the effects in the US and other wealthy countries, there are limits.
Before the new study, one of the previously highest heat/humidity events ever reported was in the Iranian city of Bandar Mahshahr, which almost reached a 35°C wet-bulb reading on July 31, 2015. There were no known deaths; residents reported staying inside air-conditioned vehicles and buildings, and showering after brief sojourns outside.
But Horton points out that if people are increasingly forced indoors for longer periods, farming, commerce and other activities could potentially grind to a halt, even in rich nations — a lesson already brought home by the collapse of economies in the face of the novel coronavirus.
In any case, many people in the poorer countries most at risk do not have electricity, never mind air conditioning. There, many rely on subsistence farming, requiring daily outdoor heavy labour. These facts could make some of the most affected areas basically uninhabitable, says Horton.
Kristina Dahl, a climatologist at the Union of Concerned Scientists who led a study last year warning of increasing future heat and humidity in the US, said the new paper shows “how close communities around the world are to the limits”. She added that some localities may already be seeing conditions worse than the study suggests, because weather stations do not necessarily pick up hot spots in dense city neighbourhoods built with heat-trapping concrete and pavement.
A separate study led by Stanford University and published last month shows that from 1979–2013 the average US agricultural worker has already experienced dangerously high heat-index conditions on 21 days of the May-August growing season. It projects that, without emissions reductions or adaptive measures, exposure will double; in the Southeast, those conditions will span the entire growing season.
Steven Sherwood, a climatologist at the University of New South Wales, said: “These measurements imply that some areas of Earth are much closer than expected to attaining sustained intolerable heat. It was previously believed we had a much larger margin of safety.”