But the extreme heat waves of the Western US are a mere example of what could happen: A new study finds that in the future the heat risk for the country’s largest cities could be much greater than previously thought.
Without cuts in greenhouse gas emissions, large U.S. cities could see roughly between 13 and 30 times more population-adjusted exposure to extreme heat by 2100 compared to the beginning of this century, the study found.
Along with the role of greenhouse gas emissions, the scientists also considered how population growth and urban development can affect a city’s heat risk.
The study focused on 47 of the country’s largest cities.
Each hour that a person is exposed to extreme temperatures equals one “person hour,” which is one of the key metrics the researchers used to quantify the projected growth in heat risk for each city.
They found that the New York region could see an increase of 24.6 billion people in hours of exposure to heat, the largest of any subway region surveyed and more than twice the increase projected in every other city. New York is followed by other major metro areas such as Washington, Atlanta and Los Angeles.
Compared to the level of threat faced in the early 2000s, fast-growing Sun Belt cities such as Atlanta, Austin, Miami and Orlando are projected to see the largest relative growth in heat hazard at the end of this century, due to projected locally climate change, population growth and urban development. Of the 29 cities, the research project could see the ‘worst impact’ relative to its risk of heat at the turn of the century, 23 located in the Sun Belt.
Unlike some previous studies that have predicted future extreme heat risk, the researchers here say they used locally defined temperature thresholds to determine what qualifies as a hot day. Specifically, the researchers define “hot days” as those where local temperatures exceed the 99th percentile in the region in the decades between 2000 and 2009.
“This is important because people are acclimatizing to their environment,” said Matei Georgescu, an associate professor at the Arizona State University School of Geographical Sciences and Urban Planning and one of the study’s authors. “If you have a 35-degree Celsius (95 degrees Fahrenheit) in Phoenix during the summer, that’s actually a very nice, pleasant day. But a day with a similar temperature reading in New York can be quite excessive.”
Although they found that warming caused by burning fossil fuels is the main driver of projected increases in heat load, urban development and the effects of asphalt-covered heat islands also play an important role.
“The growth of the built environment – our roads, our buildings, the very concrete jungle – all the warming caused by this is not negative,” said Georgescu. “In fact, the effect of built-up upliftment of built-up environment can be as great as on increase in glass fiber-induced arousal on a city scale.”
“Many cities are considering implementing various policies and programs on climate change,” Ebi said. “Most of those have focused on mitigation, but adaptation is just as important, and we need to think about what needs to be done to make cities comfortable places to live.”
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