Lots of trees can help keep cities cooler in summer

On hot summer days hanging out in a city park can help you cool down. The trees there provide pools of shade while gentle breezes blow even as on streets without trees an unrelenting heat reigns with nary a breeze.

We’ve long known this, but a study now confirms that trees play a large part in keeping urban landscapes cool. According to a study published in the Proceedings of the National Academy of Sciences, ample enough tree cover can lower summer daytime temperatures considerably, thereby mitigating the heat island effect.

Manmade surfaces such as roads, sidewalks and buildings are slow to release the heat they absorb from sunrays by day. Not so trees. They help keep things cooler in two ways: they shade those surfaces from sunrays, and they also release water into the air through their leaves, which too has a cooling effect.

“We knew that cities are warmer than the surrounding countryside, but we found that temperatures vary just as much within cities,” explains says Monica Turner, a professor of biology at the University of Wisconsin-Madison who was a co-author of the study. “Keeping temperatures more comfortable on hot summer days can make a big difference for those of us who live and work there.”

The scientists demonstrated that even within cities temperatures could vary widely based on their relative amounts of tree cover. Blocks with more trees are usually cooler than those with fewer or none. In other words, the heat island effect isn’t uniform within a city and can be alleviated simply by planting more trees in sidewalks and other available spaces.

“Tree canopy cover can actually do more than offset the effects of impervious surfaces,” says Carly Ziter, the paper’s lead author. During the day, “an equivalent amount of canopy cover can cool the air down more than pavement will warm it up.”

To maximize the natural cooling effects of trees in cities, the authors say that canopy cover should be over 40%, which means that a single city block should be nearly half covered by branches and leaves above street level. “It’s not really enough to just kind of go out and plant trees, we really need to think about how many we’re planting and where we’re planting them,” Ziter stresses

“We’re not saying planting one tree does nothing, but you’re going to have a bigger effect if you plant a tree and your neighbor plants a tree and their neighbor plants a tree,” she adds.