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The value of the North American lawn |
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Page 4 of 11
Lawns reduce urban heat island effect through cooling
"Heat islands" occurs when developed urban areas have significantly higher average temperatures than the surrounding countryside. On hot summer days, urban air can be 2-10oF hotter than the surrounding rural areas. Heat islands form as vegetation is replaced by roads (asphalt and concrete), buildings, and other structures necessary to accommodate growing populations. These surfaces absorb rather than reflect, the sun's heat, causing surface temperatures and overall ambient temperatures to rise. One of the most extreme examples of this phenomenon is the city of Los Angeles, where the average high temperature in summer has increased by nearly one degree per decade. Among other things, this has contributed to the city's legendary smog problem, since ozone forms more readily at higher temperatures.
Heat island effects should not be confused with global warming. Heat islands and global warming are two distinct phenomena, and the presence of elevated urban temperatures does not imply that the global climate is changing. Indeed, increased air temperatures associated with heat islands are primarily a result of urban paving, not global warming. However, hotter local temperatures can contribute to global climate change by increasing energy demand. As temperatures increase due to the heat island effect, more electricity is required for indoor air conditioning and other cooling purposes. As fossil fuels are burned to produce this cooling energy, power plant pollution, which includes ozone precursors and greenhouse gasses, increases. Thus, local temperature increases resulting from the heat island effect can have an indirect impact on global climate.
Heat islands raise urban temperatures in the summertime, adversely affecting human health and the environment. Additionally, heat islands reduce livability by decreasing urban ventilation, increasing air pollution levels, and increasing the risk of heat-related illness and death. In fact, heat islands increase ground-level ozone pollution. Ozone is an odorless, colorless gas that has been referred to as good or bad depending on its location in the atmosphere. The good ozone occurs naturally in the stratosphere, approximately 4-15 miles above the earth's surface, where it forms a protective layer that shields us from harmful ultra-violet radiation. Stratospheric ozone forms what is known as the ozone layer. The bad ozone exists in the troposphere, close to the earth's surface. It is formed by a chemical reaction between nitrogen oxides and volatile organic compounds in the presence of heat and sunlight. Ground-level ozone is a harmful pollutant and is the main constituent of atmospheric smog. Because heat and sunlight increase the formation of ground-level ozone and its precursor compounds, the heat island effect can increase ozone pollution to levels that threaten human health; especially that of children, the elderly, asthmatics, and other vulnerable groups.
What do lawns have to do with the heat island effect? All vegetation, including trees, shrubs, and grasses produce natural cooling effects through a process called evapotranspiration. In evapotranspiration, water transpires from a leaf's surface and evaporates into the atmosphere, reducing ambient temperature. Although trees and shrubs are generally appreciated for producing significant cooling due to their visible shade effects, lawns substantially contribute to this phenomenon. A study has shown that the average size front lawns of eight homes have the cooling effect of about 70 tons of air conditioning, while the average home has an air-conditioner with only 3 or 4 ton capacity (Johns and Beard, 1985). For this reason, the lawns are referred to as the natural air-conditioners.
Thus, well-maintained lawns have a dual effect on the urban heat island effect. First the lawns reduce the negative impacts of the heat islands by lowering the actual ambient temperature. Second, by reducing energy demand for air conditioning, the cooling effect of lawns reduces the production of climate altering emissions which exacerbate concerns about global warming.
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