A Tree (x 10 million) Grows in Los Angeles
One of our remedies for urban heat islands has an even greater benefit. Most policymakers and environmental activists concerned with the threat of global warming urge two strategies to combat it: cutting the use of fossil fuels; and planting trees, which sequester carbon dioxide in their wood. The planting of trees in cities does both of these, and is far more effective than planting trees in forests.
Any tree-whether in the forest or the city-removes CO2 from the air through photosynthesis. Typically, a tree sequesters a few kilograms of carbon per year in its wood. For a forest tree, that is the total benefit of the tree’s existence, from the standpoint of cutting CO2 levels. But a tree planted in a city also lowers fossil fuel usage, by cooling the city and thus reducing the amount of electricity consumed in air conditioning. A tree in Los Angeles, for example, will save an additional 3 kilograms of carbon per year by lowering the city’s overall need for air conditioning, plus 15 kilograms more if it directly shades a building.
Thus, present efforts by organizations concerned with greenhouse warming to plant trees in forests ought to be broadened to stimulate utilities in cities with growing air-conditioning demand to start shade-tree/cool-surfaces programs. Such programs would not only save more CO2 per tree than would forest trees, but would mitigate smog problems as well. A massive tree-planting campaign would be compatible with Southern California’s present water supply. Los Angeles gets enough rain to support trees without irrigation (except for their first few years). A tree shading a lawn actually saves municipal water, which would otherwise go to watering the lawn.
Not all trees are equally beneficial. It is better to plant deciduous trees, for example, which give shade in summer but do not block the warmth in winter. Also, some types of trees emit large amounts of the volatile organic hydrocarbons (VOCs) that combine with oxides of nitrogen to form smog. Ash and maple are among the more VOC-free trees, emitting only about 1 VOC unit (defined as one microgram per hour per gram of dry leaf). Eucalyptus trees, on the other hand, are a problem. They were introduced a century ago, are thriving, and emit 32 units; perhaps they should be replaced with more suitable native trees. Weeping willows top the emissions list, releasing a whopping 230 VOC units.