How Trees Combat the Urban Heat Island Effect

Trees are a natural way for cities to keep temperatures — and energy costs — low.

Brownstone street with trees.

 Brian Kennedy / Getty Images

You might consider urban areas to be the epitome of cool, but that couldn't be further from the truth. If you're a city dweller, you've probably found yourself stranded on an urban heat island, and you didn't even know it.

Heat island effect is a term that refers to higher temperatures and air pollution in urban areas, which is caused by the structures within the urban areas themselves. Urban areas are much warmer than surrounding rural areas and can be viewed as lonely islands filled with oppressive heat and extreme pollution.

Buildings, concrete, lack of soil — all these things contribute to the heat island effect. As it turns out, having a city literally go green by planting more trees is one of the best ways to combat the harmful environmental effects. Introducing more vegetation, like trees, into urban environments helps with everything from basic shade refuge to cleaner air to the reduction of energy costs.

One of the simplest ways trees in urban areas can help diminish heat is shade. The Environmental Protection Agency (EPA) reports that shaded areas can be up to 20-45 degrees cooler than areas that lack shade. The extreme temperature discrepancy between shaded and non-shaded areas plays a huge part in the need for higher energy costs. Strategically planting trees around non-shaded buildings helps reduce the need for air conditioning. Lower energy costs also means fewer pollutants and greenhouse gas emissions, so shade plays a role in maintaining healthy air quality in addition to keeping people cool.

The gates to the city of Rothenburg, Germany
By providing shade, trees can reduce temperatures and energy costs. Dudarev Mikhail/Shutterstock

According to the New York State Department of Environmental Conservation, trees also create cooler environments through the process of evapotranspiration. Evapotranspiration occurs when trees transpire, and trees transpire water to cool themselves much the way humans sweat to cool off. When the transpired water evaporates, the area surrounding the tree cools as well. The EPA notes that evapotranspiration and shade can help to lessen peak summer temperatures by 2 to 9 degrees.

Aside from shade and cooler temperatures, trees offer other ways to help clear the air of pollutants often found in abundance in urban areas. Trees absorb harmful pollutants like nitrogen oxide, nitrogen dioxide, and sulfur dioxide, while simultaneously releasing oxygen into the environment, says the N.Y. State Department of Environmental Conservation. Essentially, the leaves of trees "breathe in" the bad stuff and "breathe out" what we need.

Trees can also contribute to the well being of a city by controlling water quality. The trees and surrounding soil act as a natural water purifier because they act as a sort of filtration system. According to American Forests, rainwater is absorbed by the trees and is naturally filtered through the soil, meaning less water filtration is required in areas in trees than in areas without them. Planting trees in urban environments also helps reduce water runoff created by storms.

So if you need some relief from the urban heat this summer, find some shade — and then thank a tree.