Urban Forests = Cleaner, Cooler Air

Watch an animation from ASLA’s “Designing Our Future: Sustainable Landscapes” online exhibition that explains how urban forests fight air pollution and the urban heat island effect. See how cities can add in millions of trees, while ensuring the trees themselves live long, healthy lives.

Poor air quality has led to an explosion of asthma cases and other health problems among vulnerable populations including children, the elderly, and low-income residents. Each year bad air causes two million deaths worldwide. Also, in the U.S., there have been 8,000 premature deaths from excessive heat over the past 25 years. Urban heat islands, which are caused, in part, by sunlight being absorbed by paved surfaces and roofs, lead to higher surface temperatures, up to 90 degrees. Atmospheric air temperatures are also higher: in the day by up to 6 degrees, and at night, by up to 22 degrees. Vulnerable populations also face greater risks of heat exhaustion.
(Sources:  Heat Island Impacts, U.S. Environmental Protection Agency (E.P.A.), World Health Organization (WHO))

Increasing the tree canopy in cities is one way to fight both poor air quality and urban heat islands. Research shows significant short-term improvements in air quality in urban areas with 100 percent tree cover. There, trees can reduce hourly ozone by up to 15 percent, sulfur dioxide by 14 percent, and particulate matter by 13 percent. U.S. trees remove some 784,000 tons of pollution annually, providing $3.8 billion in value. Furthermore, a single large healthy tree can remove greater than 300 pounds of carbon dioxide from the atmosphere every year. In fact, New York City’s urban forest alone removes 154,000 tons of CO2 annually. Through their leaves, trees also provide evaporative cooling, which increases air humidity. Shaded surfaces may be 20-45 degrees cooler, and evapotranspiration can reduce peak summer temperatures by 2-9 degrees. (Sources: Heat Island Mitigation: Trees and Vegetation, U.S. Environmental Protection Agency (E.P.A.), “Sustaining America’s Trees and Forests,” David J. Nowak, Susan M. Stein, Paula B. Randler, Eric J. Greenfield, Sara J. Comas, Mary A. Carr, and Ralph J. Alig, U.S. Forest Service.)

Some other benefits: Urban forests reduce energy use by providing shade in the summer and wind breaks in the winter, reduce stormwater runoff, remediate soils, and provide animal and plant habitat. Trees have economic benefits: they increase property value. Lastly, trees have positive cognitive effects and may even help improve moods. (Sources: Does Looking at Nature Make People Nicer?The Dirt, “The Restorative Effects of Nature in Cities,” The Dirt, “Sustaining America’s Trees and Forests,” David J. Nowak, Susan M. Stein, Paula B. Randler, Eric J. Greenfield, Sara J. Comas, Mary A. Carr, and Ralph J. Alig, U.S. Forest Service.)

In the U.S., cities take up just three percent of land but contain 80 percent of the population. Cities may take up a relatively small share of all land now, but are projected to consume an area the size of Montana between 2000 and 2050. Two-thirds of the planet is expected to live in cities by 2050. With rapid urban growth, it’s essential that trees remain, whether along streets, in small pocket parks, or big green spaces. A 40 percent tree canopy is a challenging but worthy goal for every city to reach. (Sources: American Forests Tree Canopy Goals, “Projected Urban Growth (2000-2050) and Its Estimated Impact on the U.S. Forest Resource,” David J. Nowak and Jeffrey T. Walton, U.S. Forest Service, “Sustaining America’s Trees and Forests,” David J. Nowak, Susan M. Stein, Paula B. Randler, Eric J. Greenfield, Sara J. Comas, Mary A. Carr, and Ralph J. Alig, U.S. Forest Service.)

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