Constructed Climates

People in developed countries use large amounts of energy. Of course, using a lot of energy is, today, one mark of a developed country, and the challenge of the next few generations involves reducing energy use or finding a sustainable or inexhaustible energy source without environmental hazards. The direct problem, again, reflects our large population and high energy use, with too many vehicles and too little mass transit. This transportation situation demands too many impervious surfaces — roads and parking lots — leading to too much driving, too much urban heat, and too much air pollution. The urban heat island directly results from large expanses of impervious surfaces and the concomitant lack of vegetative cover, though it is somewhat moderated by city architecture. Sadly, urban nature plays no consequential role in offsetting our fossil-fuel use through carbon sequestration, nor does it provide the all-encompassing energy source for America’s energy-rich lifestyles. Even a naturally vegetated and sustainably harvested United States can’t satisfy that demand. Americans simply use too much fossil fuel to hope that planting a tree in everyone’s backyard will address the carbon and energy problems.

Vegetation and trees certainly shade impervious surfaces by absorbing and reflecting light, and cool the air by evaporating water. These natural solutions cool cities. Other solutions include painting impervious surfaces white, reducing their heat absorption; in fact, compared to trees, tall grass does a much better job at cooling through both shading the ground and evaporating water. Replacing impervious asphalt with grassy paving provides both types of cooling, allowing some stormwater infiltration while also providing parking. At least in areas where cooling takes precedence over heating, strategically placed deciduous trees can make a difference, but with ample water, long grass might do a better job.

A well-placed tree, one situated, say, in an urban canyon, efficiently and beautifully interrupts the exchange of radiant energy between thermally massive structures. But these spots tend to be in high-traffic areas that stress even the toughest trees and demand constant maintenance. Near homes, a well-placed tree might have some hopes of reducing energy use, yet, more often than not, the energy reductions promoted in summer cooling simply offset energy increases in winter heating, and the use of fossil fuels in pruning, maintenance, and careful removal when dying trees are too close to buildings adds costs. A well-insulated house minimizes much of the potential benefit, not to mention having a cold house in the winter and a warm house in the summer.

Coal-burning power plants and petroleum-powered traffic add reactive nitrogen to urban air, providing a critical link between cities and harmful air pollution. In particular, I presented the steps that lead to high ozone concentrations, but many other pollutants, including the nitrogen itself, cause problems.

Vegetation reduces these pollution levels through two paths: first by slowing down air as it moves through branches and leaves, letting heavy particles fall out; and second by absorbing pollutants into their tissues through open stoma. Well-chosen urban trees in the right spot can provide some pollution-reduction benefits. But can we consider vegetation as pollution scrubbers any more than we can the lungs of people and animals as they breathe city air? All organisms sequester many of these toxic chemicals and suffer adverse consequences, but do organisms efficiently remove from the atmosphere what came out of our automobiles’ tailpipes? Empirical studies just don’t support large increases in air quality due to trees, other than displacing impervious surfaces and reducing traffic. Similar to our carbon problem, directly reducing emissions from burning fossil fuels likely provides the best strategy. Effective legislation has produced cleaner air over the last few decades, and further regulations demanding cleaner energy sources seemingly make the best sense.