But the most dangerous heat waves are often not just from high temperatures alone but also from humidity, according to climate scientists from Columbia University who presented their study at the American Geophysical Union fall meeting this week (14 December) in San Francisco.
When humidity starts to rise, sweat fails to evaporate, so people cannot cool themselves off effectively. In muggy, dense cities that see many power outages, or in rural areas with limited power, the heat stress from combinations of heat and humidity can be lethal.
The study used the high-emissions, business-as-usual climate scenario called RCP 8.5 to project population growth and “wet-bulb” temperatures, a combination of high temperatures with humidity and commonly used as a measure of heat stress. RCP 8.5 is one of the representative concentration pathways adopted by the Intergovernmental Panel on Climate Change for climate modelling and research.
By 2060, the study found that some 700 million people around the world could suffer oppressive wet-bulb temperatures of 32 degrees Celsius at least once a year.
Ethan Coffel, who led the research team, says that while 32 or 33 degrees Celsius does not seem extreme, consider this: In August 2015, the city of Bandar Mahshahr in Iran registered a stifling combination of humidity and 46 degrees Celsius heat that soared to near-record levels. The wet-bulb temperature there would have been roughly 31 degrees Celsius.
Radley Horton, a member of the research team, points out that the very young, very old, and people doing strenuous labour outdoors would be most vulnerable even before heat stress rises that high. There would be direct impacts on labour and agricultural productivity, with knock-on effects on the economy.
Heat stress would sharply increase the demand for air-conditioning, he adds, but at the same time, high humidity might impair its cooling performance.
Heat and humidity could also interact with poor air quality and other climate phenomena, and those interactions need to be studied further, Horton notes. “A bigger wild card is large mammals, which share some similarities with humans in terms of vulnerability to heat stress.”
However, there are uncertainties in models predicting heat stress, says Steven Sherwood of the University of New South Wales in Australia, who did not participate in the research but whose 2010 study outlined the theoretical limits of climate-change heat and humidity that humans can bear.
“Still, the [Columbia University] research is a worthwhile study because it gives you a rough idea of what you would expect,” Sherwood posits. “People don’t realise what a threat it is.”
This piece was produced by SciDev.Net’s South-East Asia & Pacific desk.