Risk of deadly heat surges for city dwellers, new study shows

Exposure to fatal levels of heat and humidity has tripled for city residents in recent decades, a new study found.

The dangerous combination of heat and humidity, which impacts almost a quarter of the world’s population, is the result of rising temperatures and urban population growth, according to the study “Global urban population exposure to extreme heat” published Monday in the Proceedings of the National Academy of Sciences.

Hundreds of millions of people worldwide have migrated in recent decades from rural areas to cities, which now contain more than half of the global population. In those settings, researchers observed, sparse vegetation and abundant concrete tend to trap and concentrate heat, generating an “urban heat island effect.”

“It increases morbidity and mortality,” lead author Cascade Tuholske, a postdoctoral researcher at Columbia University’s Earth Institute, said in a statement. “It impacts people’s ability to work, and results in lower economic output. It exacerbates pre-existing health conditions.” 

The researchers collected maximum daily heat and humidity measurements for 13,115 cities from 1983 to 2016. They defined extreme heat as 86 degrees Fahrenheit using a gauge that accounts for the physiological effects of high humidity. Most healthy people have trouble functioning outside and unhealthy individuals risk death at high levels of heat and humidity.

The analysis found that the number of “person-days” — days per year that exceeded a heat exposure threshold, multiplied by the total population exposed — rose from 40 billion in 1983 to 119 billion in 2016.

The authors noted that in the years following that timeframe, numerous heat waves have claimed lives in places like the U.S. Northwest and southern Canada. 

While results varied among cities, the study concluded that population growth accounted for two-thirds of overall exposure increase, while warming was responsible for one-third.

The most affected cities tended to be those in lower altitudes, with the worst-hit being Dhaka, Bangladesh, which saw an increase of 575 million person-days of extreme heat due to its “ballooning population,” according to the authors.

Other big cities with similar population-heavy trends included Shanghai and Guangzhou in China; Yangon, Myanmar; Bangkok, Thailand; Dubai, United Arab Emirates; Hanoi, Vietnam; and Khartoum, Sudan.

Cities where exposure increases were due mostly to warming included Baghdad, Iraq; Cairo, Egypt; Kuwait City, Kuwait; Lagos, Nigeria; and Kolkata and Mumbai in India. Warming was also almost exclusively responsible for heat exposure surges in Europe, as population growth there has been relatively static, the study found.

In the U.S., the authors identified about 40 major cities with rapidly growing exposure, mostly clustered along the Gulf Coast. Houston, Dallas-Fort Worth, San Antonio and Austin, Texas, as well as Pensacola, Fla., experienced surges due to combined increases in population and heat.

The American cities in which population growth was the main driver included Las Vegas; Savannah, Ga.; and Charleston, S.C. Those in which fast-rising heat was the main driver included Baton Rouge, Houma and Lake Charles, La.; and Gulfport, Miss.

One major outliner was the northeastern city of Providence, where warmer, more humid weather was responsible for 93 percent of rising exposure.

“A lot of these cities show the pattern of how human civilization has evolved over the past 15,000 years,” said Tuholske, noting that many of the cities are in warm climates near big river systems. “Now, those areas may become uninhabitable. Are people really going to want to live there?”