The Best Way to Cool Our Cities Is to Plant More Trees

What's more powerful—an air conditioner or a tree? Former London Councilor Jon Burke recently took to Twitter to share an interesting statistic: A young tree has the cooling effect of five room-sized air conditioners working for 20 hours a day. There are many different conflicting versions of this stat. The U.S. Forest Service, for example, states: "Did you know the net cooling effect of a young healthy tree is equivalent to ten room-size air conditioners operating 20 hours a day? In addition, Healthy, mature trees add an average of 10 percent to a property’s value. Check out Forest Service’s Sustaining America’s Urban Trees and Forests report for more details." Interestingly, that is the same link Burke quotes, but with twice the number of air conditioners. I have searched the document and cannot find any reference to air conditioners, but have many questions, including why 20 hours a day and not all day? And where did this really come from?

The oldest version of it that I can find is from the United States Senate Committee on Appropriations hearings in 1970, in a statement by Arnold D. Rhodes of the University of Massachusetts, looking for money to "study and enhance the role played by trees, parks, and open space in creating a quality environment for people living and working in urban complexes." But at least we know how the math worked: It's based on the transpiration—water evaporating from leaves—of a tree having a cooling effect of a million BTUs per day. Divided by the weird 20 hours and you get 50,000 BTUs and depending on the size of the room (the standard rule of thumb of 20 BTUs per square foot), Burke and the U.S. Forest Service could both be right.

It is a lousy analogy but an important issue. In a recent post, I calculated how much heat cars add to our urban environment and concluded that if we were serious about climate change and not cooking in our cities, we should be getting rid of cars and tearing up the asphalt to plant trees. So perhaps we should put Rhodes and his 1970 air conditioners aside and look at how trees can keep our cities cool.

Roland Ennos, a professor of biomechanics at the University of Hull, writes in The Conversation about how trees keep us cool.

"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. All up, the shade provided by trees can reduce our physiologically equivalent temperature (that is, how warm we feel our surroundings to be) by between seven and 15°C, depending on our latitude." 

Trees also cool the air and fight the urban heat island effect through evapotranspiration. Ennos continues:

"Urban trees can counter this process by intercepting the radiation before it reaches the ground, and using the energy for evapotranspiration. 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."

But it isn't just a few trees here and there; you need decent canopy coverage. Carly Ziter of Concordia University led a study with the mouthful of a name: "Scale-dependent interactions between tree canopy cover and impervious surfaces reduce daytime urban heat during summer." She and her team mounted mobile weather stations on bicycles and rode them around Madison, Wisconsin, taking readings every second, and translating them into a five-meter grid.

“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,” she told Concordia News. “So if your neighborhood has less than 40 percent canopy cover, you’ll get a little bit of cooling, but not very much. Once you tip over that threshold, you really see large increases in how much you can cool areas off.”

According to the study, the difference in temperature averaged 3.5 degrees Celsius (6.3 degrees Fahrenheit):

"Temperature decreased nonlinearly with increasing canopy cover, with the greatest cooling when canopy cover exceeded 40%. The magnitude of daytime cooling also increased with spatial scale and was greatest at the size of a typical city block (60–90 m). Daytime air temperature increased linearly with increasing impervious cover, but the magnitude of warming was less than the cooling associated with increased canopy cover. Variation in nighttime air temperature averaged 2.1 °C (range, 1.2–3.0 °C), and temperature increased with impervious surface. Effects of canopy were limited at night; thus, reduction of impervious surfaces remains critical for reducing nighttime urban heat. Results suggest strategies for managing urban land-cover patterns to enhance resilience of cities to climate warming."

Indeed, 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%. And, as I've suggested before, we should get rid of the cars that filled that parking too.

Correction—July 25, 2022: A previous version said Jon Burke is a London city councilor—he is a former councilor.