We all know how hot it can get during the summer months in North America. It is not uncommon to be able to see heat waves radiating off of the asphalt on city streets or concrete sidewalks. Thankfully, nature provides us with an excellent and all-natural form of air conditioning – the shade from trees.

Trees play an essential part in keeping the temperatures lower in cities and towns. The decreased temperatures are the result of the cooling effect trees have by shading the surrounding environment.

One study conducted by researchers from the University of Wisconsin-Madison found that an adequate amount of tree covering in an area was able to lower summertime temperatures by a minimum of 10 degrees Fahrenheit. Ten degrees makes a significant impact on comfortability when you think about how it feels to be in 85-degree heat versus 95 degrees.

One of the coauthors of the study, Monica Turner, told Science Daily, “We knew that cities are warmer than the surrounding countryside, but we found that temperatures vary just as much within cities. Keeping temperatures more comfortable on hot summer days can make a big difference for those of us who live and work there.”

So how exactly do trees keep our cities cool? Well, the shade from trees provides us with a barrier from the direct rays of the sun, lessening their intensity. The leaves of trees are capable of transpiring and releasing water into the air which also keeps temperatures cooler.

Unfortunately, summer heatwaves are getting more and more powerful due to climate change. Heatwaves increase energy costs and demands and also have significant impacts on human health. Trees appear to be one of our best assets in lowering city temperatures.

Researchers believe that to receive the maximum cooling effect from trees, the tree coverage of an area must be at least 40 percent. In terms of cities, this means that one city block would need to be almost half-way covered by trees and other tall plants. The most considerable amount of cooling occurs when the 40 percent threshold is crossed for over one city block.

“Tree canopy cover can actually do more than offset the effects of impervious surfaces . . . [in a day] an equivalent amount of canopy cover can cool the air down more than pavement will warm it up,” said researcher Carly Ziter of the University of Wisconsin-Madison to Science Daily.

There is a phenomenon that occurs in most large cities known as the “urban heat island effect.” Satellites often hover over states and note areas of concentrated heat where there are big cities. Ziter used her bicycle to capture the ground surface temperature readings in Madison, Wisconsin in the summer of 2016. Ziter collected temperature data every 5 meters of the city.

She noted small islands of heat intermixed throughout the city within cooler shaded areas. Otherwise, known as a “heat archipelago” instead of an island. The data Ziter collected demonstrates how vital trees are when it comes to keeping cities cool.

It is also essential to think about how trees are planted in a city. City planners need to strategically plant trees in order to maximize their cooling effect.

Ziter believers that city planners should look at areas that are close to the 40 percent tree coverage threshold and focus their efforts on planting more trees there. Trees also need to be placed in places where people are active outside of just in parks.

Also, there needs to be coordination in cities surrounding planting trees. It is not uncommon that the city may be in charge of planting trees along the streets, the parks department may oversee the plantings of trees in parks, whereas homeowners plant the trees in their yards. Everyone needs to be on the same page and working together in order to achieve the 40 percent coverage threshold.

“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 . . . 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,” said Ziter to Science Daily.

Additionally, the areas with lower tree canopy coverage are often the neighborhoods of lower-income and minority peoples. Therefore, it is essential to add trees to those areas as well in order to have equitable distribution of the cooling properties trees provide.

As global warming continues to cause urban temperature spikes, it is important that we start planting more trees now. “The trees we plant now or the areas we pave now are going to be determining the temperatures of our cities in the next century,” Ziter told Science Daily.

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