Shade trees reduce building energy use and CO2 emissions from power plants
Introduction
World energy use is the main contributor to atmospheric CO2. In 1997, about 6.4 Giga metric ton of carbon (GtC) were emitted internationally by combustion of gas, liquid, and solid fuels (CDIAC, 2001), 2–5 times the amount contributed by deforestation (Brown et al., 1988). The share of atmospheric carbon emissions for the United States from fossil fuel combustion was 1.46 GtC. Increasing use of fossil fuel and deforestation together have raised atmospheric CO2 concentration some 25% over the last 150 years. According to global climate models and preliminary measurements, these changes in the composition of the atmosphere have already begun raising the earth's average temperature. If current energy trends continue, these changes could drastically alter the earth's temperature, with unknown but potentially catastrophic physical and political consequences. Since the first OPEC embargo in 1973 and the oil price shocks in 1979, increased energy awareness have led to conservation efforts and leveling of energy consumption in the industrialized countries. An important byproduct of this reduced energy use is a lowering of CO2 emissions.
In the United States, of all electricity generated, about one-sixth [400 tera-watt-hours (TWh), equivalent to about 80 million metric tons of carbon (MtC) emissions, and translating to about $40 billion (B) per year] is used to air-condition buildings. Of this $40 B/year, about half is used in cities classified as “heat islands” where the air-conditioning demand has risen 10% within the last 40 years. Metropolitan areas in the United States (e.g. Los Angeles, Phoenix, Houston, Atlanta, New York City) typically have pronounced heat islands that warrant special attention by anyone concerned with broad-scale energy efficiency (HIG, 2001).
Strategies that increase urban vegetation and the reflectance of roofs and paved surfaces not only assure cost savings to individual homeowners and commercial consumers, but also reduce energy consumption citywide. These strategies also serve to reduce smog, important in those cities such as Houston, Los Angeles, and Atlanta where air pollution is a significant health problem.
Trees affect the urban ecosystem in many different ways. McPherson et al. (1994) provide a good review of the impact of an urban forest in the city of Chicago. In this paper, we briefly review the benefits and costs associated with a large-scale urban tree-planting program. We specifically focus on discussing the benefits of such a program as they relate to shading of buildings and streets, evaporative cooling of ambient air, shielding buildings and inhabitants from cold winter and hot summer winds, the collective impact of tree shading, evaporative cooling, and wind shielding on building heating- and cooling-energy use, the impact of ambient cooling on smog reduction, and removal of PM10 (particulate matter less than 10 micron) pollutants and dry deposition. We also briefly discuss the potential cost associated with a large-scale tree-planting program.
Section snippets
Urban trees: an energy conservation strategy
In addition to their aesthetic value, urban trees can modify the climate of a city and improve urban thermal comfort in hot climates. Individually, urban trees also act as shading and wind-shielding elements modifying the ambient conditions around individual buildings. Considered collectively, a significant increase in the number of urban trees can moderate the intensity of the urban heat island by altering the heat balance of the entire city (Fig. 1).
Trees affect energy use in buildings
Design of an urban tree program and costs associated with trees
Two primary factors to be considered in designing a large-scale urban tree program is the potential room (space available) for planting trees, and the types of programs that utilize and employ the wide participation of the population. We recently studied the fabric (fraction of different land-uses) of Sacramento by statistically analyzing high-resolution aerial color orthophotos of the city, taken at 0.30-m resolution (Akbari et al., 1999; Fig. 2). On average, tree cover comprises about 13% of
Carbon sequestration of urban shade trees
Data for the rate of carbon sequestration by urban trees are scarce; most data are given in the units of tons per year of carbon per hectare of forested land. However, Nowak (1994b) has performed an analysis of carbon sequestration by individual trees as a function of tree diameter measured at breast height (dbh). He estimates that an average tree with a dbh of 31–46 cm (about 50 m2 in crown area) sequesters carbon at a rate of 19 kg/year. We also performed an analysis of the rate of carbon
Conclusion
We doubt that the direct savings noted in this paper are enough, in themselves, to induce a building owner to plant shade trees for energy-savings purposes. For LA, annual benefits of $270 M are possible, after 15–20 years of planting trees. Trees can potentially reduce energy consumption in a city and improve air quality and comfort. These potential savings are clearly a function of climate: in hot climates, deciduous trees shading a building can save cooling-energy use, in cold climates,
Acknowledgements
This work was supported by the Assistant Secretary for Conservation and Renewable Energy, Office of Building Technologies, of the US Department of Energy, and the US Environmental Protection Agency, under contract No. DE-AC0376SF00098. This paper was presented at the USDA Forest Service Southern Global Change Program sponsored Advances in Terrestrial Ecosystem: Carbon Inventory, Measurements, and Monitoring Conference held 3–5 October 2000 in Raleigh, North Carolina.
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