The study is the first to find that wet-bulb temperatures of 95 degrees Fahrenheit (35 Celsius) — which render ineffective the human heat response of sweating to shed heat through evaporation, leading to hyperthermia — are already occurring for short periods of time at a few weather stations.
These tend to occur in parts of the Persian Gulf shoreline and coastal southwest North America, where sizzling lands border sultry seas, as well as in northern South Asia, where extreme heat and humidity combinations overlap just before the annual monsoon season begins.
With computer-model projections showing the world will continue to warm rapidly in response to increasing amounts of greenhouse gases in the atmosphere, the study, published Friday in the journal Science Advances, warns that highly populated regions of the world will be rendered uninhabitable sooner than previously thought for parts of each year.
This will come to pass unless people take wide-ranging and costly steps to adapt to the heat during the next few decades, while nations undertake measures to slash emissions of greenhouse gases.
The study depicts a world steadily marching toward a future in which many other locations approach or reach that survivability threshold, a trend that could throw a spotlight on the divide between rich nations that are able to adapt to this new reality and poor countries that suffer productivity losses and deaths.
The heat in the subtropics constitutes a threat to global stability, some researchers say, since it could prompt millions to become climate migrants, searching for more temperate conditions elsewhere.
For the research, scientists at NASA’s Jet Propulsion Laboratory, Loughborough University in the U.K. and Columbia University examined surface temperature data around the world on land and sea. They detected, for the first time, the arrival of what some scientists have called “Steambath Earth,” with heat waves that humans cannot survive after prolonged exposure.
The threshold for survivability occurs when the wet-bulb temperature hits 95 degrees.
Meteorologists measure wet-bulb temperatures by wrapping a wet wick around the bulb of a thermometer. While it may seem to be an esoteric figure, it’s highly significant in heat-wave situations, since the higher the wet-bulb temperature gets, the more difficult it becomes for the human body to shed metabolic heat into the air through the evaporation of sweat.
In fact, mortality increases during heat waves when wet-bulb temperatures reach temperatures of 88 to 91 degrees (31 to 33 Celsius), the study states.
‘Underappreciated’ climate hazard
According to study co-author Radley Horton of Columbia University, the combination of high temperature with high humidity is “maybe the most underappreciated of the direct climate hazards.” In an interview, he said that for too long attention has mainly focused on increasing global temperatures and heat extremes overall.
“People are late to the game in thinking about these multiple variate risks, such as heat plus humidity,” Horton said.
Horton said the researchers were surprised to find such sharp increases in moist heat waves, in part because computer-model projections don’t show these occurrences becoming common for a few more decades.
However, models often miss local-level interactions between coastal regions and hot seas, for example, or between hot inland regions of South Asia and moist air flowing northward from the Indian Ocean just as the monsoon season begins. The results add to emerging evidence that, at the local and regional scale, climate changes can be more extreme than global averages suggest, and have more severe impacts.
Matthew Huber, a heat-extremes researcher at Purdue University who was not involved in the study, said excessive combinations of heat and humidity are not a widespread health concern yet but probably will be as global warming progresses.
“We expect these extreme wet-bulb values to be rare but to become more common as the world warms,” he said in an email. “That this is observed to be happening is therefore exactly as we would predict from standard models of global warming, but it is disturbing to see it happening in real time.
“It will only get worse and more widespread from here.”
In the Washington region, which is notorious for its hot and humid summers, the results show moderate levels of heat and humidity have increased significantly since 1979, although extremely high wet-bulb temperatures are not yet commonplace, said study co-author Colin Raymond of NASA.
The region’s all-time maximum wet-bulb temperature is 87.2 degrees (30.7 Celsius), set at Newington, Va., on July 18, 2013.
Middle East, South Asia among first regions to see such deadly heat emerge
Parts of the Middle East have endured suffocating levels of heat and humidity in recent years, producing off-the-chart heat-index values — another metric showing the combination of heat and humidity.
In July 2015, Bandar Mahshahr, a city in southwest Iran, registered an air temperature of 115 degrees (46 Celsius) and a dew point of 90 degrees (32 Celsius). The computed heat index for this brutal combination was 165 degrees (74 Celsius), among the highest recorded. However, the U.S. National Weather Service cautions such conditions are so extreme that they are beyond the limit the heat index was designed to calculate.
The highest known combination of heat and humidity occurred in Dhahran, Saudi Arabia, according to Christopher Burt, author of “Extreme Weather: A Guide & Record Book.” On July 8, 2003, the temperature hit 108 (42.2 Celsius) with a dew point of 95 (35 Celsius).
Both the Dhahran and Bandar Mahshahr temperature and humidity pairings equate to wet-bulb temperatures readings around 95 (35 Celsius), at the theoretical survivability limit. The two cities lie adjacent to the Persian Gulf, where water temperatures can soar into the 90s.
In another recent example of extremely humid heat at a Middle East location adjacent to water, on June 26, 2018, the coastal city of Quriyat, Oman, never dropped below 108.7 degrees (42.6 Celsius) over the course of a calendar day. This was most likely the highest minimum temperature observed on Earth.
With the intensity of humid heat escalating, wealthy countries in the Middle East are pursuing innovative but expensive strategies to bring their people indoors, and even to air-condition the outdoors. Qatar, for example, is air-conditioning its outdoor markets, sidewalks and outdoor malls, for example.
However, these are not steps that rapidly warming but comparatively poor nations in the subtropics can take in the face of global warming.
“Of all of the climate risks coming this century, the thing I am most concerned about is excessive heat in the tropics,” Ken Caldeira, senior scientist emeritus with the Carnegie Institution for Science at Stanford University, said in an email.
“Rich countries have brought much of their economy indoors where air conditioning is a possibility, but many developing economies rely on labor-intensive outdoor agriculture,” he wrote. “The combination of poverty and extreme temperatures can be lethal.”
For Jonathan Buzan, a climate scientist at the University of Bern in Switzerland, the new study contains extreme-heat warnings not just for the Middle East and South Asia but for the United States, too. His research has found that wet-bulb temperature extremes in the Midwest increase by about 1.8 degrees (1 Celsius) for every 1.8 degrees of global warming.
This means that warming of 3.6 degrees (2 Celsius) would bring periods of wet-bulb temperatures of 95 degrees (35 Celsius) for a limited amount of time. With global warming on course to exceed 3.6 degrees, that could bring “a few days per year” with unsurvivable heat “sustained for a few days per year.” This would make agriculture that relies on outdoor picking impossible.