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Combination of Heat and Humidity Becoming Too Much for Humans

(CN) — It’s no secret that humid heat is much more difficult to handle than dry, desert-like heat, and recent studies have found that by the end of the century the tropics and subtropics could see heat and humidity reaching levels rarely, if ever, experienced by humans before.

According to scientists, the combined heat and humidity will be detrimental to economies and may even pass the physiological limits of human survival.

But in a study published Friday in the journal Science Advances, authors Colin Raymond, a postdoctoral researcher at NASA's Jet Propulsion Laboratory and Tom Matthews, a lecturer in climate science at Loughborough University in the United Kingdom, say the previous predictions are incorrect — because they’re already occurring.

The authors identify thousands of previously rare or unprecedented occurrences of extreme heat and humidity in Asia, Africa, Australia, South America and North America, including in the U.S. Gulf Coast region. For example, along the Persian Gulf researchers recorded more than a dozen events in which conditions surpassed the theoretical limit of human survivability. So far, these outbreaks have been localized and only lasted for a couple hours at a time, but the authors say they are increasing in frequency and intensity.

"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 doctoral student at Columbia University's Lamont-Doherty Earth Observatory. "The times these events last will increase, and the areas they affect will grow in direct correlation with global warming."

To get a complete picture of the issue, the authors analyzed data from weather stations from 1979 to 2017 and found events of combined extreme heat and humidity doubled over the study period. Several of these repeated incidents took place across India, Bangladesh and Pakistan, northwestern Australia, and along the coasts of the Red Sea and Mexico's Gulf of California.

The highest and most potentially fatal readings were spotted 14 times in the cities of Dhahran/Damman, Saudi Arabia; Doha, Qatar; and Ras Al Khaimah, United Arab Emirates, with combined populations of over 3 million. Other locations also hit with these extreme conditions include parts of southeast Asia, southern China, subtropical Africa and the Caribbean.

The southeastern region of the United States experienced extreme conditions dozens of times, particularly near the Gulf Coast in east Texas, Louisiana, Mississippi, Alabama and the Florida Panhandle. These conditions also reached inland into Arkansas and along the southeastern coastal plain, with the worst cases hitting New Orleans and Biloxi, Mississippi.

As expected, such incidents tended to cluster on coastlines along confined seas, gulfs and straits, where evaporating seawater provides abundant moisture that gets sucked into the hot air. Areas more inland that experience moisture-laden monsoon winds or wide areas of crop irrigation seem to follow the same trend.

The authors note previous climate studies failed to recognize these incidents in the past because climate researchers normally look at averages of heat and humidity measured over large areas and over several hours at a time. On the other hand, Raymond and his colleagues drilled directly into hourly data from 7,877 individual weather stations, which allowed them to identify shorter-lived bouts affecting smaller areas.

Humid heat is much more dangerous than dry heat because humans cool their bodies by sweating, which carries away excess body heat through evaporation. This process is effective in desert regions, but in humid areas the air is already too heavy with moisture to take on much more and the evaporation of sweat is slowed or stopped altogether. 

In the most extreme instances, unless a person can escape to air conditioning, their body’s core can heat beyond its narrow survivability range and their organs will begin to fail. Under these conditions, even the strongest, healthiest person with no clothes, abundant shade and drinking water would perish within hours.


Meteorologists measure the heat/humidity effect using what is called the “wet bulb” centigrade scale, more well known in the U.S. as the "heat index" or "real-feel" Fahrenheit readings. Earlier studies show that even the most well-adapted, able-bodied humans cannot carry out normal outdoor activities when the wet bulb hits 32 degrees Celsius — equivalent to a heat index of 132 degrees Fahrenheit — and most anyone else would succumb far before that. 

The Persian Gulf cities reached a peak reading of 35 degrees Celsius, said to be the theoretical survivability limit, and roughly translates to a heat index reading of 160 degrees Fahrenheit. Considering the heat index stops at 127 degrees Fahrenheit, these readings are literally off the charts.

"It's hard to exaggerate the effects of anything that gets into the 30s," said Raymond.

According to the study, worldwide wet-bulb readings approaching or exceeding 30 degrees Celsius have doubled since 1979. Readings of 31 degrees Celsiuis — once thought of as rare occurrences — happened about 1,000 times while and readings of 33, previously thought to be nonexistent, occurred around 80 times.

In July 2019, a heat wave hit that scorched much of the U.S. maxed out at about 30 degrees Celsius on the wet bulb, translating into heat indexes approaching 115 degrees Fahrenheit in select locations and reaching 122 degrees Fahrenheit in Baltimore, Maryland. The city also experienced a similar heat wave event later in August.

The heat waves hit communities hard and led to at least a half-dozen deaths, including an air-conditioning technician in Phoenix, Arizona, and former National Football League lineman Mitch Petrus, who died while working outdoors in Arkansas.

Heat-related illnesses already kill more U.S. residents than any other weather-related hazard including cold, hurricanes or floods. Last year, the website InsideClimate News revealed that cases of heat stroke or heat exhaustion among U.S. troops on domestic bases grew 60% from 2008 to 2018 and killed a total of 17 soldiers, almost all in the muggy U.S. Southeast.

High-humidity heat waves in Russia and Europe have killed tens of thousands because air conditioning is far less prevalent than in the United States.

"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-wrote a 2017 paper projecting that such conditions would not take hold until later in the century.

Although air conditioning can assuage some of the effects in the United States and some other wealthy countries, it comes at a price. Prior to this study, one of the highest heat/humidity events ever reported occurred in the city of Bandar Mahshahr, Iran, with a wet-bulb reading of 35 degrees Celsius in July 2015. Residents avoided tragedy by staying inside air-conditioned vehicles and buildings and showering after going outside and there were no reported deaths.

But Horton notes that if people are increasingly forced indoors for longer periods, farming, commerce and other activities could potentially grind to a halt — similar to what we’re seeing globally due to the novel coronavirus.

And in poorer countries with no electricity or air conditioning, the risks are much greater especially since many in these countries rely on farming and daily outdoor heavy labor. Many of these areas could be rendered basically uninhabitable, Horton said.

Kristina Dahl, a climatologist with the Union of Concerned Scientists who led a study last year warning of increasing future heat and humidity in the United States, said the new paper shows "how close communities around the world are to the limits."

She added that some places may already be seeing conditions worse than the study suggests, since weather stations do not necessarily pick up hot spots in dense city neighborhoods filled with heat-trapping concrete and pavement.

Steven Sherwood, a climatologist at the Australia's 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."

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