The shade of a single tree can provide welcome relief from the hot summer sun. But when that single tree is part of a small forest, it creates a considerable cooling effect. According to a study published today in the Proceedings of the National Academy of Sciences, trees play a big role in keeping our cities cool.
According to the study, the right amount of tree cover can lower summer daytime temperatures by as much as 10 degrees Fahrenheit. And the effect is quite noticeable from neighborhood to neighborhood, even down to the scale of a single city block. "We knew that cities are warmer than the surrounding countryside, but we found that temperatures vary just as much within cities," says Monica Turner, a professor in the department of Integrative Biology, Wisconsin-Madison University and a co-author of the study.
With climate change making extreme heat events more common each summer, city planners are working on how to prepare. Heat waves drive up energy demands and costs and can have big human health impacts. One potentially powerful tool, the study's authors say, are organisms that have been around long before human civilizations could appreciate their leafy benefits. And those trees may be the secret to keeping the places we live livable.
Essentially, says Turner, roads, sidewalks and buildings absorb heat from the sun during the day and slowly release that heat at night. Trees, on the other hand, not only shade those surfaces from the sun's rays, they also release water into the air through their leaves, a process that cools things down.
To get the maximum benefit of this cooling service, the study found that tree cover must be more than 40 percent. In other words, an aerial picture of a single city block would need to be nearly half-way covered by a leafy green network of branches and leaves.
