Global warming is causing atmospheric heat and humidity to combine in several parts of the world, including in India, into a deadly extreme weather event, according to a study which says such conditions may also ravage economies.
Analysing data from weather stations from 1979 to 2017, the researchers, including those from Columbia University in the US, found that extreme heat and humidity combinations doubled over the study period.
According to the study, published in the journal Science Advances, repeated incidents appeared in much of India, Bangladesh, Pakistan, northwestern Australia, 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 in Saudi Arabia, Doha in Qatar, and Ras Al Khaimah, UAE, which have combined populations of over three million, the scientists said.
They spotted more than a dozen recent brief outbreaks surpassing the theoretical human survivability limit.
These extreme weather events, according to the researchers, have so far been confined to localised areas and lasted just hours, but they said the outbreaks 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 study lead author Colin Raymond, who did the research as a PhD. candidate at Columbia University.
"The times these events last will increase, and the areas they affect will grow in direct correlation with global warming," Raymond said.
These 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, the study noted.
In some areas further inland, moisture-laden monsoon winds, or wide areas of crop irrigation appear to play the same role, it said.
Explaining why these freak events were not spotted earlier, the scientists said prior studies usually looked at averages of heat and humidity measured over large areas and over several hours at a time.
In the current study, Raymond and his colleagues instead drilled directly into hourly data from 7,877 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, according to the study.
Water expelled through the skin removes excess body heat, and when it evaporates, it carries that heat away, the scientists explained.
While the process works nicely in deserts, they said it works less efficiently in humid regions, where the air is already too laden with moisture to take on more.
As a result, evaporation of sweat slows and in the most extreme instances, it could stop, the researchers warned.
In such cases, unless one can retreat to an air-conditioned room, the body's core heats beyond its narrow survivable range, and organs begin to fail, they explained.
Even a strong, physically fit person resting in the shade with no clothes and unlimited access to drinking water would die within hours, the study cautioned.
The researchers said meteorologists measure the heat/humidity effect on the "wet bulb" Centigrade scale.
Prior studies had suggested that even the strongest, best-adapted people cannot carry out normal outdoor activities when the wet bulb hits 32 degree Celsius.
A reading of 35, the peak briefly reached in the Persian Gulf cities, is considered the theoretical survivability limit, the researchers explained.
Worldwide, wet-bulb readings approaching or exceeding 30 degree Celsius on the wet bulb have doubled since 1979, the current study noted.
During this period, the number of readings of 31, previously believed to occur only rarely, totaled around 1,000, the scientists said.
Readings of 33, which were previously thought to be almost nonexistent, totaled around 80, they added.
"We may be closer to a real tipping point on this than we think," said Radley Horton, a co-author of the study from Columbia University.
One of the previously highest heat/humidity events ever reported was in the Iranian city of Bandar Mahshahr, which almost reached a 35 degree Celsius wet-bulb reading on July 31, 2015, the scientists said.
There were no known deaths from this incident, however, residents reported staying inside air-conditioned vehicles and buildings, and showering after brief visits outside.
But Horton said if people are forced indoors for longer periods, commerce and other activities could 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.
Many people in poor countries are most at risk since they do not even have electricity.
According to Horton, these facts could make some of the most affected areas basically uninhabitable.
Some localities may already be seeing conditions worse than the study suggests, the researchers warned.
This is because weather stations do not necessarily pick up hot spots in dense city neighbourhoods built with heat-trapping concrete and pavement, they said.
"These measurements imply that some areas of the Earth are much closer than expected to attaining sustained intolerable heat," said study co-author Steven Sherwood, a climatologist at the Australia's University of New South Wales.