CLIMATE WATCH
— Making sense of the latest data
In January 2015, thermometers in Marble Bar, Western Australia, touched 50°C – one degree shy of the national record. But it’s extreme humidity we should be taking more notice of, research suggests.
In some regions of the world, climate change could bring deadly heat waves that combine high temperatures with humidity so severe the human body can’t cool itself. Ethan Coffel, a doctoral student in atmospheric science at Columbia University, reported that conclusion at an American Geophysical Union meeting in San Francisco in December.
Some studies suggest average global temperatures could rise by as much as 7°C by the end of this century. In 2010, climate scientists Steven Sherwood from the University of New South Wales and Matthew Huber of Purdue University calculated that if so, some regions would experience heat and humidity so extreme that humans could not survive being outdoors for prolonged periods.
Coffel’s study made even more dire predictions. Using the latest climate change projections from the International Panel on Climate Change, he and his colleagues found conditions that cause extreme stress could arise within five decades, at significantly lower temperatures. The greatest risk is in places where hot air meets warm coastal waters, such as in India, West Africa, Iran, Saudi Arabia and other Persian Gulf countries.
To model future heat and humidity in such places, Coffel, Sherwood and Huber used a metric called the wet-bulb temperature which combines heat and humidity into a single figure.
A wet-bulb temperature is taken by placing a damp cloth over a thermometer’s bulb. Evaporation cools the bulb, the same way perspiration cools the body. As humidity increases, the cooling effect slows.
For many mammals, including humans, a wet-bulb temperature of 35°C is critical because it’s the point at which sweat will not evaporate. Even the fittest young adult is unlikely to survive outdoors in these conditions for more than a few hours. Lower wet-bulb temperatures can claim the lives of the elderly or infirm. Heat waves in India and Pakistan that killed 5,000 people in 2015 only produced wet-bulb temperatures in the range of 29-31°C.
Coffel found that by 2060, an estimated 600 million people will live in regions where wet-bulb temperatures may hit 32°C. Of these, 250 million could see heat waves with wet-bulb temperatures of 33°C, and 50 million could see 34°C – one degree off the critical limit.
Even if these areas are never truly rendered uninhabitable, people living there will have to make major lifestyle changes, says Radley Horton, another Columbia climate scientist involved in the project. Outdoor labour will become increasingly difficult, for example: “We will see more [rest] breaks, more people working overnight, changes in clothing, less strenuous activity.”
Extreme heat and humidity will also place major demands on infrastructure. “There will be conditions where power and air-conditioning systems can’t fail,” Horton says. Noah Diffenbaugh, a climate scientist at Stanford University, agrees, adding that part of the reason for the fatalities in India was that the country’s electrical grid wasn’t up to the demands of a severe heat wave.
Haiyan Teng, a climate scientist at the National Centre for Atmospheric Research in Boulder, Colorado, notes that Coffel’s study is just the most recent in a growing body of work showing that heat alone is not the best predictor of the climatechange danger. An article published in Nature Climate Change, focused on the Persian Gulf, found similar results under business-as-usual carbon emissions.
“Climate change is often measured by the mean surface air temperature,” Teng says, “but the impacts on human health depend on multiple factors.”
“The question,” adds Diffenbaugh, “is how do we prepare ourselves? We’re going to face these hazards more frequently in the future.”
A WET- BULB TEMPERATURE OF 35° C IS CRITICAL BECAUSE IT’S THE POINT AT WHICH SWEAT WILL NOT EVAPORATE.
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