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Can't Take the Heat? Plant a Tree

THE ancient Chinese proverb "No shade tree? Blame not the sun, but yourself," offers practical advice for improving our urban areas. As with many proverbs, the message is both profound and obvious.

Last week, while looking at the weather reports from New York City, I was reminded of just how profound this one is. While the temperature in Manhattan was listed at 90 degrees, in Times Square it was more like 100 degrees, and in Central Park, it was probably closer to 80.

Temperature variations like this play out daily all over the world. Cities heat up more than rural areas because they contain more heat-trapping materials. They have less of the vegetation cover that dissipates heat. And our activities -- from driving to cooking to cooling our homes -- emit heat. As a result, many cities are about 6 to 8 degrees warmer than the forested areas around them. Improving the energy efficiency of these urban heat islands is not trivial. With the temperature hovering at more than 90 degrees and air-conditioners running marathons, Americans are again sweating over energy bills.

More trees would help, because trees are nature's air-conditioners. When the sun shines on buildings, roads and bridges, they give off that solar energy as heat -- just think of the cliché about it being so hot you could cook eggs on the sidewalk. Trees not only provide shade, but also absorb the sun's energy to make biomass, the living matter that trees are made of. Trees also release water, which cools things down. The water they absorb passes to their leaf surfaces where it evaporates, cooling the air much the way perspiration cools our skin. With enough vegetation, evaporative cooling lowers air temperatures.

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