“A young tree has the cooling effect of five room-sized air conditioners working for 20 hours a day.”
~ Former London Councilor Jon Burke
“… the net cooling effect of a young healthy tree is equivalent to ten room-size air conditioners operating 20 hours a day.“
~ U.S. Forest Service
The small town in northern Arizona where I lived for a couple of years in the 1970s is in the high, dry desert. Coming from north-central Texas and Oklahoma, the lack of trees was a real shock to me. On the Navaho Reservation where I worked, the few streams often had cottonwoods growing on the banks, but the size was dependent on groundwater. Places with groundwater, usually canyons or ravines, could be lined with the trees that offered a shady refuge. In the towns, people tried with limited success to grow and maintain trees, but big trees were scarce. Any shade was welcome, but the fierce winds meant it gave less comfort. I’ve since lived in tree-filled places, Georgia, Alaska, California, and Colorado, where I can relish the friendly shade.
But trees do more than just make us comfortable, they make the whole planet more comfortable. According to reporter Lloyd Alter, “Theoretically, trees can help provide cooling in two ways: by providing shade, and through a process known as evapotranspiration. Locally, trees provide most of their cooling effect by shading … Evapotranspiration occurs when the sun’s rays hit the trees’ canopy, causing water to evaporate from the leaves. This cools them down — just as sweating cools our skin — thereby reducing the amount of energy left to warm the air.”
Alter quotes a study by Carly Ziter of Concordia University, “We found that to get the most cooling, you have to have about 40 percent canopy cover, and this was strongest around the scale of a city block … So if your neighborhood has less than 40 percent canopy cover, you’ll get a little bit of cooling, but not very much.”
She continues, “Effects of canopy were limited at night; thus, reduction of impervious surfaces remains critical for reducing nighttime urban heat … the study suggests we should rip up those impervious surfaces that cause the heat island effect, mostly roads and parking, and replace them with trees with a coverage of at least 40%.”
The emphasis for trees is on neighborhoods and neighbors. Even though a single spot of shade in the desert was always a welcome respite, its cooling effects were more emotional than physical. Writer Matt Hickman notes, “We’ve long sung the praises of urban canopies and their unmatched ability to scrub the air, mitigate flooding, elevate moods and cool overheated cities … Based on what we know about the myriad benefits of urban trees, it’s safe to assume that people living on blocks with at least 40 percent coverage are a little less testy and enjoy summertime electric bills lower than the residents of neighboring blocks who, in the absence of an ample number of temp-lowering trees, are forced to crank the AC to full blast.”
“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,” says Ziter. “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.”
The implication is something I noted living in Arizona, a neighborhood under siege by the sun is not a pleasant place — not just due to the heat, but because there are limited opportunities to spend time with neighbors. In the summer where I live now, neighbors stop and chat in the shade, or share a drink together on their patios and porches. People are not only cooler; they are happier and nicer when shade is nearby.
So, go plant a tree or, even better, many trees. Your mood will improve and so will your neighbors’.
Lloyd Alter, The Best Way to Cool Our Cities Is to Plant More Trees, July 25, 2022, TreeHugger
Matt Hickman, Trees Are the Not-So-Secret Weapon in Keeping Cities Cool, April 1, 2019, TreeHugger
I appreciate your writings on these topics.