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

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 that is significantly warmer than its surrounding rural areas. Unlike global warming, which entails a worldwide rise in temperatures, heat islands occur at the local level. According to the U.S. Environmental Protection Agency (EPA), many cities and suburbs have air temperatures up to 10 degrees Fahrenheit warmer than their neighboring areas.

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 that shading and water evaporation from soil and leaves ordinarily provide. Meanwhile, tall buildings and narrow streets can heat the air trapped between them and reduce airflow. And waste heat from vehicles, factories and air conditioners adds warmth to the surroundings, further exacerbating the heat island effect.

The intensity of a heat island will also depend upon its topography, its proximity to water bodies, and 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.

And although urban heat islands are distinctly different from the phenomenon of climate change, during the summer months they can contribute to global warming. The increased use of 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 policymakers 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 through the use of white and light-colored construction materials (including white roofing materials) in buildings, which will work to reflect the sun’s heat skyward rather than absorb it, as dark surfaces tend to do. Also, preserving or creating pockets of green space and vegetation help 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 policymakers to adopt just such practices. Another useful practice is the creation of “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.

CONTACTS: EPA Heat Island Effect, Heat Island Group; American Forests