Above: A volunteer showers a woman with water during a heat wave in Karachi, Pakistan, on May 29, 2018. (Rizwan Tabbasum/AFP via Getty Images)
Bolstering a decade of research on the risks of unprecedented heat and humidity from human-caused climate change, a new study finds evidence for more than a dozen cases of heat-humidity combinations that could be deadly if experienced for more than a few hours. Led by scientist Colin Raymond, the paper, “Potentially Fatal Combinations of Humidity and Heat Are Emerging across the Globe,” was published Friday in the open-access journal Science Advances.
The study focuses on observations of wet-bulb temperature, which serves as an indicator of how much a person would be able to cool off by sweating. Since human skin temperature averages close to 35°C (95°F), wet-bulb temperatures above that value would in theory prevent people from dispelling internal heat and potentially lead to fatal consequences within a few hours. (Wet-bulb temperatures are calculated in a different manner than heat indexes and are much lower for the same physiological impact; a wet-bulb temperature of 35°C would literally be off the chart on a U.S. heat index table.)
Previous high-profile studies had warned of the risks posed by increasingly high wet-bulb temperatures in future climates, including the chance that wet-bulb readings (TWs) of 35°C might emerge later this century. The new paper goes a step further by identifying 14 examples of 35°C wet-bulb readings that have already occurred since 1979 in Pakistan, Saudi Arabia, and the United Arab Emirates (UAE).
Prior to this study, only one of these readings was widely known and acknowledged to be an informal global record: a value of 35°C at Dhahran, Saudi Arabia, on July 8, 2003.
More broadly, the paper finds that the frequency of TW values reaching 27°C, 29°C, 31°C, and 33°C across the world all showed doubling trends over the period from 1979 to 2017. Extrapolating from the relationship between global warming and increasing wet-bulb temperatures over the past four decades, the authors find that dangerous web-bulb readings will continue to spread across vulnerable parts of the world, affecting millions more people, as human-caused climate change unfolds.
“The most important takeaway is the steepness of the trends,” Raymond told weather.com in an interview. “It didn’t matter what level of extremeness we looked at or what part of the world—the trends were upward and very steep across all of those levels.”
Where short-term heat has already reached potentially fatal values
The locations and dates of the 35°C-or-greater wet-bulb temperatures since 1979, as confirmed by Raymond and colleagues, include:
Jacobabad, Pakistan: July 25, 1987; June 5, 2005; June 7, 2005; June 27, 2010; June 30, 2010; Jul 13, 2012
Ras Al-Khaimah, UAE: August 2, 1995; August 12, 1995; August 11, 2009; July 8, 2010
Jeddah, Saudi Arabia: August 15, 1999; June 24, 2010
Dera Ismail Khan, Pakistan: August 17, 2017
Dhahran, Saudi Arabia: July 8, 2003
Several dozen other such cases were filtered out in quality control checks, Raymond said in an email. The authors did not calculate the length of each event, but extreme TW values to date are generally concentrated into periods less than three hours, they said.
Using ocean data, the authors found that summer sea surface temperatures exceeding 35°C over the Persian Gulf are linked to several of these events. The worst heat and humidity occurred when this air moved onshore—like a diabolical sea breeze, but driven by different weather mechanisms. Every station on the Persian Gulf coast with adequate data reported wet-bulb readings of 31°C (88°F) or higher at least once every three years during the study period.
“One of the things that was most surprising to us as researchers was that in these hot-spot regions…the intensity of the humid heat is confined very closely to the coastline, say 10 or 20 kilometers [6-12 miles],” Raymond said.
In the case of Pakistan, extremely hot, humid air can surge up the populous Indus Valley to affect cities like Jacobabad, which sits about 500 kilometers (300 miles) inland. Widespread irrigation is likely another factor in boosting the moisture values in this region and elsewhere across western South Asia, noted the authors.
Other parts of the world that are already yielding wet-bulb readings of 31°C or higher at least every three years include eastern coastal India, Pakistan and northwestern India, and the shores of the Red Sea, Gulf of California, and southern Gulf of Mexico. All of these feature subtropical coastlines with access to both intense continental heat and marine air flowing off high sea surface temperatures.
Drilling into the data
Previous studies on this topic have relied largely on gridded analyses such as ERA-Interim (European Center for Medium-Range Weather Forecasts). Such datasets pool temperatures across grid points that each represent a small area—comparable to having one to three data points in a state like Rhode Island. Such averaging obscures the localized short-term peaks in wet-bulb temperature that were brought to light in this study. The authors found that the discrepancy between ERA-Interim gridded values and observed values of dew point grew larger as the wet-bulb temperature rose.
“According to our weather station analysis, emphasizing land grid points underplays the true risks of extreme TW along coastlines, which tends to occur when marine air masses are advected even slightly onshore,” note the authors.
Based on the study’s findings, many thousands of people have already experienced 95°F wet-bulb temperatures, though these episodes were almost certainly too brief to raise the question of survivability. Jeddah has a metro-area population of close to 4 million, and Jacobabad, where 6 of the 14 identified cases occurred, is home to about 200,000 people.
Jacobabad is accustomed to extreme heat in general: its average daily high and low in June are 112°F (44°C) and 85°F (29°C), which is hotter than any average month in Jeddah, or Phoenix for that matter.
The deadliest heat waves in recent decades have struck parts of the globe where wet-bulb readings surge to levels that are less extreme than 35°C but still hugely dangerous when sustained over multiple days. “Severe mortality and morbidity impacts typically occur at much lower values [than 35°C TW],” noted Raymond and colleagues. Heat waves felled an estimated 70,000-plus people across Europe in 2003 and more than 55,000 people in Russia in 2010, according to the database EM-DAT. Regional wet-bulb values in these heat waves never exceeded 28°C (82°F), the paper points out.
In contrast, relatively few heat deaths are reported near the Persian Gulf, where air conditioning is widespread and societies have long dealt with extreme heat. Raymond and colleagues point to “the need to ascertain how acclimation to high-heat-stress conditions is diminished as the physiological survivability limit is approached. Such efforts may also help resolve the reasons for the paucity of reported mortality and morbidity impacts associated with observed near 35°C [wet-bulb] conditions.”
A future of increased extreme heat and humidity
Looking ahead, the authors used a technique called generalized extreme value analysis to estimate the extent to which 35°C wet-bulb readings will increase with global warming. They found that a temperature increase over preindustrial values of around 2.3°C—which could occur over the next few decades—would be enough for 35°C wet-bulb values to show up regularly in gridded analyses, which suggests that multiple locations within a grid box could encounter such heat and humidity on a recurring basis.
Going forward, one of the prime areas of concern is northern India and Pakistan, where many thousands of laborers work outdoors in premonsoon heat that can reach dangerous levels. A 2015 heat wave killed more than 2500 people in northern India. As pointed out in a 2019 post by Jeff Masters, four of the ten deadliest heat waves on record in the EM-DAT database affected India and/or Pakistan.
Adaptive measures such as cooling centers have successfully reduced the toll taken by heat waves in many northern midlatitude areas over the last decade, but air conditioning is not an option for many impoverished South Indians.
“The southern Persian Gulf shoreline and northern South Asia are home to millions of people, situating them on the front lines of exposure to TW extremes at the edge of and outside the range of natural variability in which our physiology evolved,” Raymond and colleagues state in the paper. “The deadly heat events already experienced in recent decades are indicative of the continuing trend toward increasingly extreme humid heat, and our findings underline that their diverse, consequential, and growing impacts represent a major societal challenge for the coming decades.”
Jan Wesner Childs contributed to this post.
