The result is the urban heat island effect, a phenomenon that is expected to intensify as greenhouse gas emissions raise global temperatures.
In a study recently published in the journal Computers, Environment and Urban Systems, researchers at the University of Georgia looked at the 50 largest metropolitan areas in the United States to see which cities were experiencing the most intense temperature changes owing to the UHI effect.
Salt Lake City; Louisville, Kentucky; Miami; and Cleveland topped the list with jumps in annual temperatures between 0.92 degrees Celsius and 1.49 degrees Celsius in 2010 thanks to asphalt, density, and urban sprawl.
Urban areas with increases between a half to nearly 1 degree Celsius included Los Angeles; San Francisco; Dallas–Fort Worth; Minneapolis–St. Paul; Detroit; Washington, D.C.–Baltimore; New York City; Philadelphia; Norfolk, Virginia; Birmingham, Alabama; and Jacksonville and Tampa, Florida.
During a 1995 heat wave, 739 Chicago residents died in just five days. Record heat across Europe during the summer of 2003 is blamed for the deaths of more than 70,000 people. And just this year, multiple days of temperatures above 120 degrees Fahrenheit in Pakistan and India killed 1,000 and 2,300 people, respectively.
Rising temperatures also worsen air quality as pollution from car exhaust and factories increases during hotter days. That leads to heightened health risks for people who suffer from asthma, allergies, and other respiratory diseases.
The point of the study—besides creating a list of cities to avoid during heat waves—was to decipher the factors behind the UHI effect. Was it New York and other high-density cities or those sprawling urban metropolises like Los Angeles?
The answer appears to be both.
“We found that more contiguous sprawling and dense urban development both enhanced UHI intensities,” study lead author Neil Debbage, a doctoral student at the University of Georgia, said in a statement. “In other words, it does not appear to be a simplistic ‘either-or’ situation regarding sprawl or density.”
The best way to fight the UHI effect is to break up the blacktop with green, the study found. Marshall Shepherd, a University of Georgia professor of atmospheric sciences and the study’s coauthor, said well-placed parks, greenbelts, and trees has a direct impact on heat island intensity.
“It’s not just whether cities have high-density development but how the built infrastructure is connected and disconnected by green spaces,” Shepherd said in a statement.
The study’s findings could help city planners determine where trees, parks, and green space are needed to avoid urban hot spots.
But the problem is only growing. For the first time in human history, a majority of the world’s population now lives in cities. At the same time, climate change is expected to trigger heat waves that will kill 12,000 people annually in 49 U.S. cities by 2100.
“It's crucial to work toward a better understanding of the complex processes at the intersection of urbanization, climate, and human health,” Shepherd said. “Current and future cities will be modified or designed with weather and climate in mind, and research at UGA will play a key role.”
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