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A double whammy for heat islands: fewer trees in cities and more paved surfaces

A green roof can help reduce the heat island effect.

The amount of tree cover in our cities is dropping, and we have more paved surfaces. Those are the main findings from a national studyby the U.S. Forest Service.

Dave Nowak is a senior scientist with the Forest Service. He and his colleague Eric Greenfield studied aerial images from 2009 to 2014, and found we’re losing an average of 28.5 million trees a year in urban areas. 

“A lot of the loss has to do with development. We’re finding that often, areas where the trees are, they’re converting to impervious surfaces, which would be roads or buildings. So the trees are swapping out, and being replaced by these hard surfaces," says Nowak.

But he says that’s only part of the issue.

"It’s not all due to that. In some cases, when we did the assessment, the trees were just gone. We looked five years ago, the trees were there. Then, in the current images, the trees were gone. But it looked exactly the same, the grass was there," he explains.

In those cases, he says, it could be due to people taking trees down just because they want to. Or trees could've died of old age, or been killed by pests such as the emerald ash borer and people removed them.

Nowak says most of the changes were in the eastern part of the U.S.

"Around the Great Lakes, a lot of the states are near the highest classes of losing canopy cover, so they’re losing 0.1 to 0.2% per year,” he says. “So it’s one of the areas that may be hot spots.”

He says in areas that have replaced trees with buildings, roads or parking lots, there can be a double whammy for people, by increasing the heat island effect.

“Trees cool the environment, because they evaporate water and create shade. But when you replace that with black surfaces or hard surfaces that absorb heat and re-radiate the heat, we tend to warm our cities up," he says.

You can listen to the interview with Dave Nowak above.

Rebecca Williams is senior editor in the newsroom, where she edits stories and helps guide news coverage.
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