The heat wave that hit the United States and Europe in late July helped push the world toward its hottest month ever. To temper manmade climate change and make cities more livable, we can cool urban “heat islands” in places like Washington D.C. and New York City — with urban tree planting.

What is an “Urban Heat Island”?

Urban heat island is a term for metropolitan areas that create and hold more heat than the surrounding rural areas. Human activities – driving, walking, cooking, construction, etc. – emit energy known as “waste heat.” This heat, coupled with energy from the sun, is absorbed and held by buildings and pavement, leading to higher daytime and nighttime temperatures (Washington D.C. experienced overnight lows in the 80’s during the July heatwave). As cities continue to grow, replacing natural areas with pavement and buildings, construction activity releases more heat and the infrastructure creates more insulation, perpetuating a hot, vicious cycle.

In a 2016 study entitled “Planting Healthy Air,” The Nature Conservancy evaluated the return on investment of tree plantings in 245 urban areas across the globe. While the study identified 12 benefits of natural intervention (tree planting), it focused mainly on its ability to purify the air and reduce air temperature. In short, it found that trees can – and do – play a significant role in reducing urban air temperatures and particulate matter (PM) levels, although the return on investment varies greatly from city to city.

The impacts of urban plantings

So how do trees benefit urban air? The first major impact is by absorbing and diluting PM concentrations. Basically, tree canopies act as air filters by removing PM from the air as it passes through – much like the air filter in your car or furnace. As the PM-heavy air passes through the canopy, a fraction of the pollutants is absorbed by the tree and cleaner air continues downwind. As the cleaner air travels, it mixes with other air in a process called redilution which reduces the overall PM concentration over a larger area. According to the report, redilution can significantly improve the air within 30 meters of the tree and have diminishing impacts for up to 300 meters.

The report cites a study by Maher et al. which “found that a row of roadside street trees lowered indoor PM10 concentrations in houses along the street by 50 percent.” Other studies found a single tree lowered downwind PM concentration by 15 to 20 percent.

Deciduous trees cool in summer while cooling less in winter, appropriate for cities with strong seasonal changes… Photo by Bundo Kim on Unsplash

As tree canopies filter the air, they also cool air temperatures by shading heat-absorbing surfaces and through a process called transpiration. Finding shade is referred to several times in the National Weather Service’s Heat Safety Tips and Resources. Shade not only provides temporary relief on hot days; it reduces the amount of heat absorbed and later released by buildings and pavement.

This principle can be applied on a micro-level by strategically planting trees on the south and west sides of homes. The shade reduces the amount of heat absorbed by the house and, in turn, the need to run energy-consuming air conditioners.

The second way trees cool the air is through a process called transpiration. Basically, a portion of the sun’s energy goes toward evaporating the water held in a tree’s foliage instead of heating the ground and air. This is also true of shrubs, vines – any plant that is covering pavement or buildings. While the overall cooling effect varies based on the species of tree and its location, the study found meaningful impacts within 30 meters of a tree.

Overall, natural intervention impacts are quite meaningful within a limited distance. One row of trees can substantially reduce the temperature and improve the air quality on one city block and have no effect on the next block over. Effectively mitigating the impacts of heat waves would require significant tree planting campaigns, like the MillionTreesNYC initiative that finished two years early under New York City Mayors Michael Bloomberg and Bill DeBlasio. The campaign increased the number of trees in the city by 20% and the total acreage of woodlands to over 6,000.

While tree planting campaigns alone are not enough to reverse climate change and the urban heat island effect, it can go a long way.  If you know healthy trees that are being removed in your area, or believe there is a street where more could be planted, seek out the public organizations in your area that might have some knowledge about that.  In the local Seattle area, for example, the government solicits suggestions on tree protection, maintenance, and private planting/removal questions at treesforseattle@seattle.gov.

Featured photo by chuttersnap on Unsplash