Humidity could be the killer with climate change

In January 2015, thermometers in Marble Bar, Western Australia, touched 50 °C – a single degree shy of the national record. But it’s extreme humidity records we should be taking more notice of, a wave of new research suggests.

As the climate changes, deadly heatwaves that combine high temperatures with humidity so severe that the human body can no longer cool itself, could start to affect regions of the world currently home to hundreds of millions of people. That’s the conclusion reached by Columbia University’s Ethan Coffel, reported at an American Geophysical Union meeting in San Francisco in December.

In 2010, climate scientist Steven Sherwood from the University of New South Wales first highlighted the humidity problem. He modelled the widespread occurrence of extreme humidity events we might see by the end of the century, should worst-case CO₂ emissions raise average global temperatures by 7 °C.

Coffel’s study, however, used the latest IPCC climate projections for 2060 and found regional, relatively near-future effects from much more modest heating.

This extreme humidity is less likely to occur in arid spots like Marble Bar. Coffel’s climate models suggest that there is more risk in India, West Africa, Iran, Saudi Arabia and other countries along the Persian Gulf – environments where hot air meets very warm coastal waters.

 ‘A 35 °C wet-bulb temperature is the point at which your sweat will not evaporate’

To model these events, Coffel looked at a number called the wet-bulb temperature, which combines heat and humidity into a single metric.

Wet-bulb temperature is taken by placing a damp cloth over the thermometer’s bulb. Evaporation cools the bulb, the same way perspiring cools the body. As humidity increases, the cooling effect slows. For many mammals, including humans, 35 °C wet-bulb temperature is critical.

“In theory, a 35 °C wet-bulb temperature is the point at which your sweat will not evaporate,” Coffel says.

At that point, even the fittest young adult is unlikely to survive more than a few hours before fatally overheating. But lower wet-bulb temperatures can still claim the lives of the elderly or infirm. Deadly 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, he says.

Coffel found that by 2060, an estimated 600 million people will live in regions at risk of heat waves producing wet bulb temperatures hitting 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 shy of the 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.”

There will also be major demands on developing countries’ infrastructures. “There will be conditions where power and air-conditioning systems can’t fail,” he says.

Noah Diffenbaugh, a climate scientist at Stanford University, agrees. Part of the problem last year in India, he says, was that its 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 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 climate-change danger. A recent paper in Nature Climate Change, focused specifically on the Persian Gulf, found very 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.”