Heat risk a more pressing problem for urban than rural

It’s pretty indisputable that the planet is warming up, but the resulting exposure to extreme heat is likely to hit city dwellers harder than their rural cousins, a new University of Guelph study warns.

The study, published last week in PNAS (Proceedings of the National Academy of Sciences), is billed as “the first to project how much extreme heat city dwellers may experience in future.”

The study takes into account not only the greenhouse effect, but also warming that’s being caused by increasing urbanization and urban population growth at the same time.

That’s “critical” in assessing the numbers of people who would be exposed to extreme heat under different scenarios, according to Scott Krayenhoff of Guelph’s School of Environmental Sciences, who worked on the study with researchers from Arizona State University.

Extreme heat has impacts in urban areas that it doesn’t have in rural areas, he said — not just because of the hotter urban environment, but also because cities house more people.

“A major reason that cities are often warmer is the built infrastructure — the paved roads and the building and residential roofs that absorb and emit heat, the tall buildings that retain that heat — those are major factors that make cities hotter than rural areas,” Krayenhoff said in a U of G release.

“Previous population heat and cold exposure estimates have not accounted for urban development-induced climate impacts, have neglected interactions between urban development-induced warming and (greenhouse gas)-induced climate change, and have used fixed temperature thresholds that may be inappropriate for some cities,” the authors wrote in their study.

Put another way, “we used a regionally adapted definition of extreme heat exposure that essentially accounts for the fact that a summertime temperature of 35 C is extreme in some cities, such as Detroit or Toronto, but not in others, such as Phoenix,” Krayenhoff said in the U of G’s release.

With that added focus on “locally defined” extreme heat conditions, the authors wrote, their study offers up “a more detailed and nuanced definition of extreme heat and cold exposure through key innovations, and our predicted exposure is substantially greater than previous assessments.”

The study models indicate that by the end of the 21st century, population-weighted exposure to extreme heat will increase by at least 12.7 times compared to the start of the century — but that number could reach up to 29.5 times under a “worst-case scenario of high greenhouse gas emissions, high urbanization and continued population growth.”

The study also looked not just at the number of hot days, but at the number of hours of extreme heat exposure, since each day can include a different number of hours of extreme heat.

In other words, they tallied all the hours that cities are expected to exceed the local threshold for a hot day. That threshold is in the 40 C range in Phoenix, for instance, while up in the Niagara area, around Buffalo, N.Y., it’s in the low 30s.

They then multiplied the number of projected hot hours by population to create a metric that measures “person hours” of extreme heat exposure.

Who would get the worst of it? Areas of the so-called “Sunbelt” of the U.S. South would be most affected, but big cities such as New York, Washington, D.C., Atlanta and Chicago would also see spikes in “person hours” of extreme heat, particularly as more people migrate to cities from rural areas.

The study focused on 47 urban centres in the U.S., but given that two-thirds of Canada’s urban population lives within 100 km of the U.S. border, Krayenhoff said, its findings would likely apply to most large cities in Canada also.

The study authors say their findings may be helpful to city and government planners in prioritizing “adaptation measures” for climate change and policy-making such as street tree-planting, use of more reflective materials on roofs or pavements, or setting up more cooling centres for urban residents.