Planting trees in urban areas can help mitigate carbon dioxide levels because trees can sequester carbon and offset some energy use for cooling, as some studies have illustrated in California.
The California Global Warming Solutions Act of 2006 (AB32) requires a reduction in greenhouse gas emissions to 1990 levels by 2020. This amounts to a reduction of 173 million metric tons from the level projected for 2020.
Aerial photography revealed 242 million potential sites for planting individual trees in the 21 California cities considered (McPherson and Simpson 2003). If 50 million trees were planted, they would sequester about 4.5 million metric tons of carbon dioxide (as 1.2 million metric tons of carbon) every year.
If they were planted strategically to shade east and west walls of residential buildings, they would reduce energy use from air conditioning by 6,408 gigawatt-hours, equivalent to reducing carbon dioxide emission by 1.8 million metric tons annually. This assumes a 75 percent survival rate of seedlings after 15 years.
Combining the carbon sequestration by trees with the energy savings that strategic planting would allow, the estimated total carbon dioxide reduction is 6.3 million metric tons a year. This amounts to 3.6 percent of the state’s goal of reducing the emissions of this greenhouse gas by 173 million metric tons. That is about the same as would be obtained from retrofitting homes with energy-efficient electrical appliances.
The annual cooling related to California’s existing 177 million existing urban trees already provide enough cooling that the annual energy savings would power about 730,000 homes. This includes reducing summer peak demand by about 10 percent (about 5,000 megawatts), which helps to avert potential power outages.
McPherson, G., and J.R. Simpson, 2003. Potential energy savings in buildings by an urban tree planting programme in California. Urban Forestry and Urban Greening. 2: 73-86.
Related to Climate Mitigation by Urban Forests:
Preparers: Greg McPherson, Jim Simpson, Dan Marconett, Paula Peper, and Elena Aguaron, Center for Urban Forest Research, Pacific Southwest Research Station.
Adapted from: McPherson, E.G.; J.R. Simpson, P.J. Peper, and E. Aguaron. 2008. Urban Forestry and Climate Change. Albany, CA: USDA Forest Service, Pacific Southwest Research Station. http://www.fs.fed.us/ccrc/topics/urban-forests/ . Adapted by Melanie Lenart, University of Arizona