Cooling Our Baking Cities

By Moll, Gary | American Forests, September-October 1989 | Go to article overview

Cooling Our Baking Cities


Moll, Gary, American Forests


July. We expect it to be hot, especially in the city. But do you Easterners remember last February? It was warm too, fooling the forsythias into behaving as if it were April. Rising like a golden sunrise out of the pavement near Logan Circle in Washington, DC, the flowers added some color to the winter cityscape, but I got the feeling we would pay for this oddity later on, in the summer. Winter temperatures were a little more moderate in the suburbs, mild but not warm enough to pull one over on the bloomin' plants. The forsythia bloom doesn't tell us much about global conditions, but it does help us focus on a related issue-the warming of cities. The 80s have brought us five of the warmest years on record, as well as an awareness of an atmospheric phenomenon called global warming (see "ReLeaf for Global Warming" in the November/December 1988 issue). In a way, what's happening in cities depicts global warming in miniature. Many of the same factors are involved-fewer trees, more pavement and manmade structures, carbon dioxide and pollutants trapped within the atmosphere.

Researchers at the Lawrence Berkeley Laboratory (LBL) in California report that average temperatures in the city can five to nine degrees Fahrenheit) higher than those in the surrounding suburbs. Imagine yourself as a satellite looking down on the globe and zeroing in on a region of the U. S. with your infrared scanner as an eyeball. Whatever region of the country you choose, you'll note urban hotspots that usually (depending on the type of cameras used) appear blue while cooler vegetation zones show up red. The hotspots are often called "heat islands."

But what makes a city form a climate so much warmer than the surrounding countryside? The fundamental laws of science producing heat islands are easy to demonstrate. Did you ever walk barefoot across an asphalt road on a warm summer day? If the road had a white line on it, you were lucky: maybe it saved you from burning your feet. Proceeding toward a grassy spot along the side of the road and then stopping under the shade of a tree, you experienced a gradation of climates" within a limited area. Dark surfaces soak up more heat than light ones, so naturally they get hotter. And while living plants absorb heat, they also transpire water, which keeps them cool. The city is an artificial environment low in green plant material and water, and high in manmade materials that soak up heat, stagnate the air, and reduce moisture. City centers can be especially hot because buildings, asphalt, and concrete comprise 70 to 90 percent of the area. As grass and trees infiltrate the city space, the temperature cools.

Because urban heat islands affect our health, our economy, and our environment, they have become an increasingly important facet of scientific study. Researchers Hashem Akbari and Art Rosenfeld at LBL have used computer models to estimate the cost of rising temperatures in urban areas. Their research shows that with each degree of increase in temperature within a heat island, Americans spend $1 million per hour in cooling expenses. The hotter the temperature, the more cooling units operate and the less efficient they become. Trees operate as nature's air conditioners (evaporative coolers), but as we will see later, a tree here and there within a heat island can easily be overwhelmed.

There has been an occasional news story suggesting that trees contribute to-rather than offset-pollution. Intuitively, most of us know that trees don't pollute like cars and factories do, and research is proving it. The Akbari and Rosenfeld studies indicate that 20 to 30 percent of air pollution in cities may be the direct result of warm temperatures. The heated air acts like a bubbling crucible that aids the chemical mixing of unwanted pollutants.

Most cities not meeting EPA clean-air standards get into trouble when temperatures are as high as the 90s, but no violations have occurred below 74 degrees. …

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