The ‘Heat Island’ Effect: Urban Infrastructure Causes City Temperatures to Rise

Tens of millions of people who live or work in the urban areas of America’s largest cities are exposed to significantly higher temperatures than those who live in suburban or rural areas, according to a newly released analysis by the environmental justice nonprofit Climate Central.

The cause of the higher temperatures is the human-built environment in cities — miles of concrete and blacktop, a lack of vegetation and shade trees, tall boxy buildings that block breezes and structures that absorb and then radiate heat. The analysis focuses on 65 major U.S. cities, and the 50 million people who live in those cities, and divides them into subsets of census tract maps, often no more than a few blocks. The analysis offers a granular look at what are known as “urban heat islands,” essentially hot spots where temperatures soar and millions suffer because of the surrounding hardscape. The temperature gap between suburban neighborhoods and urban areas continues to widen. 

The results are varied. The city with the largest percentage (97%) of residents living in a heat island was determined to be Newark, N.J., where few can escape the soaring temperatures. It was followed closely by Ft. Myers, Fla. (96%). The city with the largest number of people living or working in an urban heat island was New York City, with more than 7 million people enduring temperatures at least eight degrees higher than in a park-like setting outside the city. Los Angeles, Houston, Chicago and San Antonio rounded out the top five.

 “Urban heat islands already expose residents to disproportionate heat risks and cooling costs, which will only climb as long as carbon pollution drives up global temperatures,” said Jennifer Brady, Climate Central’s senior data analyst.

The analysis also found that people of color and those living beneath the poverty line are disproportionately exposed to intense urban heat. The analysis concluded, as have other studies, that the disparity is a result of racially biased residential planning in the early development of U.S. cities, an issue that persists.

Together, climate change and the urban islands form something of a two-stage heat rocket. While climate change has warmed the planet and caused more intense and more frequent heat waves, the heat island effect has been an accelerant, pushing the temperatures even higher.

The health consequences of higher and higher temperatures are significant. The National Weather Service rates extreme heat as the deadliest weather-related hazard in the country, more so than hurricanes, tornados or firestorms. Last year in Maricopa County, Arizona, which includes Phoenix, 645 people died as a result of heat, up from 61 in 2014.

Reducing the impact of heat islands would be arduous and expensive, but not impossible. Potential solutions, the Climate Central analysis concluded, include planting more leafy trees, particularly along paved streets; planting gardens or lawns on the roofs of buildings; and constructing solar reflective sidewalks that can lower temperatures.

In 2019, then-Mayor Eric Garcetti launched an ambitious effort to plant 90,000 shade trees in Los Angeles but the push fell far short of its goal, and the city’s chief forest officer later said there was an abundance of neighborhoods where there was simply not enough space to plant trees and increase vegetation.

“Until cities take action to cool these areas,” Climate Central’s Brady said, “people who live there will face the worst impacts of climate change.”

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