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From the Editors of E/The Environmental Magazine

Urban Heat Islands

Rising temps heat up city dwellers and our world climate

What is an urban heat island and does it have anything to do with global warming

—Max, via e-mail

An urban heat island is a metropolitan area significantly warmer than its surrounding rural areas. Unlike global warming, which entails a worldwide rise in temperatures, heat islands occur at the local level. Many cities and suburbs have air temperatures up to 10 degrees warmer than their neighboring areas, according to the U.S. Environmental Protection Agency.

Heat islands form as cities replace their natural land cover with pavement, buildings and other infrastructure. These changes contribute to higher urban temperatures in a number of ways. For one, displacing trees and removing soil and vegetation takes away the natural cooling effects offered by shading and water evaporation from soil and leaves. Meanwhile, tall buildings and narrow streets can heat the air trapped between them and reduce airflow. Waste heat from vehicles, factories and air conditioners adds warmth to the surroundings, exacerbating the heat island effect.

The intensity of a heat island will also depend upon its topography and proximity to water bodies, as well as local weather and climate. Urban heat islands can also impact local weather, altering local wind patterns, spurring the development of clouds and fog, increasing the number of lightning strikes and influencing the rates of precipitation.

During summer, urban heat islands can contribute to global warming. The increased air conditioning and refrigeration needed to cool indoor spaces in a heat-island city, for example, results in the release of more of the heat-trapping greenhouse gases that cause global warming. Furthermore, the poor air quality that results from this increased energy usage can affect our health, aggravating asthma and promoting other respiratory illnesses.

Costs are impacted, too. The Heat Island Group, a research and advocacy organization that works to educate the public and policy makers about the heat island effect, estimates that the city of Los Angeles spends about $100 million per year in extra energy costs to offset its heat-island effect.

The heat-island effect can be reduced by using white and light-colored construction materials (including white roofing materials) in buildings, for light colors work to reflect the sun’s heat skyward rather than absorbing it, as dark surfaces tend to do. Also, preserving or creating pockets of green space and vegetation helps to cool areas naturally. A national program called Cool Communities, coordinated by American Forests and supported by the U.S. Department of Energy, encourages building owners and local policy makers to adopt just such practices. Another useful practice is green roofs — or rooftop gardens — in which roofs are partially or completely covered with vegetation and soil, or a growing medium, planted over a waterproofing layer.

For more information:

• EPA Heat Island Effect Information: www.epa.gov/heatislands.

• Heat Island Group: eetd.lbl.gov/HeatIsland.

• American Forests: www.americanforests.org.

Got an environmental question? Send it to: EarthTalk, c/o E/The Environmental Magazine, P.O. Box 5098, Westport, CT 06881; submit it at www.emagazine.com/earthtalk/thisweek: or e-mail [email protected]. Read past columns at: www.emagazine.com/earthtalk/archives.php.

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Urban Heat Islands

Rising temps heat up city dwellers and our world climate