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Urbanization Can Heat a Place as Much as Climate Change

The urban heat island effect is indeed very strong.
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Sunset on San Joaquin River, Central Valley, California. (Photo: Terrance Emerson/Shutterstock)

Sunset on San Joaquin River, Central Valley, California. (Photo: Terrance Emerson/Shutterstock)

In 50 years, the population of California's Central Valley is expected to grow by five million, and by 2100, it'll have grown by millions more. The upshot here is that people will have built enough new roads and buildings to raise the valley's average temperatures by one to two degrees Celsius through urbanization alone, according to a new analysis.

"One or two degrees might not seem like a lot, until you see your energy bill in the summer," says Matei Georgescu, a climate scientist at Arizona State University who ran the analysis, published last week in the Journal of Climate. Plus, by that time, the Central Valley will be seeing higher temperatures due to global warming. Georgescu's one-to-two degrees estimate might account for "50 percent" of the overall warming the Central Valley can expect to experience by the end of the century, he says.

The heating comes from the so-called urban heat island effect, which describes how the dark, metal-and-asphalt materials of a city absorb and trap warmth from the sun. The urban heat island effect makes cities hotter than they would be otherwise, if they weren't developed. The Central Valley now is highly agricultural, but the United States Environmental Protection Agency predicts that by 2100, a lot of that farmland will have been converted to buildings. Georgescu used the EPA's model as a basis for his own calculations for what urban heat island effect Central California will experience in 2100.

By 2100, people will have built so many new roads and buildings that it will raise California's Central Valley's average temperatures by one to two degrees Celsius through urbanization alone.

He also modeled a potential solution: Either paint the valley's roofs white, or plant vegetation on them, so they won't heat up under the sun as much as traditional, black roofs do. Georgescu's math found that so-called cool and green roofs, which have been proven to lower urban temperatures, could bring down the Central Valley's average daily temperatures by one or two degrees Celsius. Unfortunately, the effect works only during the day. At night, cities release a lot of the heat they stored during the sunlight hours. And the hotter nighttime temperatures could still affect residents, making them turn up their air conditioning to help them get to sleep.

Georgescu thinks a better solution might be for cities to build in specific orientations that help buildings release their heat into space, instead of keeping it trapped. Although we've never heard of a city mandating such arrangements, there is some research on the issue. Georgescu also notes cities could reduce their electricity use, which always generates waste heat—a lesson that could apply to any city in the world.

Georgescu was inspired to run his model in Central California because it's a rapidly growing region, albeit one many outside the state don't think about. Many climate scientists work on global models, but Georgescu focused on a smaller area because his model can actually tell residents what temperature differences they'll experience. "We don't live in a global model," he says. "We live in specific locations."

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