Greg McPherson, Jim Simpson, Dan Marconett, Paula Peper, Elena Aguaron, Center for Urban Forest Research, Pacific Southwest Research Station
The current version of this paper (2013) is available at www.fs.usda.gov/ccrc/topics/urban-forests
Urban forests have a role to play in reducing levels of carbon dioxide and other greenhouse gases (GHG) in the atmosphere. Urban trees reduce atmospheric carbon dioxide (CO2) through sequestration and reducing GHG emissions by conserving energy used for space heating and cooling (Figure 1). Carbon sequestration is the process by which CO2 is transformed into above- and belowground biomass and stored as carbon. During photosynthesis, atmospheric CO2 enters the leaf through stomata, combines with water, and is converted into cellulose, sugars, and other materials in a chemical reaction catalyzed by sunlight. Most of these materials become fixed as wood, although some are respired back as CO2 or used to make leaves that are eventually shed by the tree.
Tree shade reduces summer air conditioning demand, but can increase heating energy use by intercepting winter sunshine. Lowered air temperatures and wind speeds from increased tree cover can decrease both cooling and heating demand. Air conditioning and heating savings result in reduced GHG emissions from power plants. Reduced emissions can be substantial, especially in regions with large numbers of air-conditioned buildings, long cooling seasons, and where coal is the primary fuel for electric power generation.
Once trees die or are cut down, they begin to decompose and return stored carbon to the atmosphere. The rate of decomposition differs greatly based on the fate of the wood. Wood that is chipped and applied as mulch decomposes relatively quickly, while wood salvaged for use in wood products can survive 50 years or more, before gradually decomposing. The combustion of gasoline and diesel fuels by vehicle fleets, and by equipment such as chainsaws, chippers, stump removers, and leaf blowers is a GHG emission source. Typically, CO2 released due to tree planting, maintenance, and other program-related activities is about 2 to 5% of annual CO2 reductions obtained through sequestration and reduced power plant emissions.
Our initial research suggests that planting lots of trees in California communities can make a difference when it comes to fighting climate change. The California Global Warming Solutions Act of 2006 (AB32) requires a reduction in GHG emissions to 1990 levels by 2020. This amounts to a reduction of 173 Mt (million metric tons) from the predicted level in 2020. Using aerial photography, we found 242 million empty tree planting sites in California cities (McPherson and Simpson 2003). If 50 million trees were planted, they would sequester about 4.5 Mt CO2 (million tons) annually. If they were planted strategically to shade east and west walls of residential buildings, they would reduce air conditioning energy use by 6,408 GWh, equivalent to an average annual CO2 equivalent emission reduction of 1.8 Mt. The estimated total CO2 reduction of 6.3 Mt annually is 3.6% of the 173 Mt statewide goal, about the same as would be obtained from retrofitting homes with energy-efficient electric appliances.
Urban Forest Project Protocol [PDF 3.8 MB]
The Urban Forest Project Protocol provides detailed guidance to insure that tree projects meet eligibility requirements, produce GHG reductions that are additional to a baseline, are sustained for at least 100 years, and do not detract from management of existing trees. Also, it describes how to calculate and report carbon storage by project trees, as well as emissions associated with their maintenance.
The protocol was first adopted by the California Air Resources Board and the California Climate Action Registry (CCAR) in 2008 and was updated by the Climate Action Reserve (CAR) in March 2010. Key elements of the protocol are outlined in a new Protocol Summary [PDF 42 KB]. Urban forest projects anywhere in the U.S. can follow the new protocol and be reported to Climate Action Reserve, which will register and serialize GHG reductions after independent verification. If these offsets are sold or retired, the Climate Action Reserve will track their transaction, adding confidence and credibility to the voluntary carbon market.
Important changes to the revised protocol include:
- Project start date must be within 6 months prior to project submission rather than any project started from 2001 to the present
- Project verification procedures are now included within the Protocol
Adoption of the Urban Forest Protocol sets the stage for investment in large-scale tree planting and stewardship projects because projects that adhere to the protocol’s guidance will generate real, reliable, additional, and permanent GHG reductions. Registered carbon reductions are "quality offsets" that pose less risk to investors than unregistered offsets. The market for quality offsets is growing as corporations, utilities, and individuals purchase them to offset their emissions or become carbon neutral.