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Urban Forests and Climate Change
Preparers: Greg McPherson, Jim Simpson, Dan Marconett, Paula Peper, Elena Aguaron, Center for Urban Forest Research, Pacific Southwest Research Station
Newer (2013) version of this paper is available here.
Figure 1. Trees sequester carbon dioxide as they grow, and reduce GHG emissions from power plants through energy conservation. Carbon dioxide is released through decomposition of removed wood and tree care activities that consume gasoline and diesel fuels. (drawing by Mike Thomas)
The CUFR Tree Carbon Calculator is the only tool approved by the Climate Action Reserve's Urban Forest Project Protocol for quantifying carbon dioxide sequestration from GHG tree planting projects. The CTCC is programmed in an Excel spreadsheet and provides carbon-related information for a single tree located in one of sixteen United States climate zones. Once the user enters information on the climate region and tree's size or age the CTCC produces information on:
- Carbon dioxide stored in the tree due to its growth over many years
- Carbon dioxide sequestered during the past year
- Dry weight of aboveground biomass that could be utilized if the tree was removed
If trees are strategically located to shade buildings and reduce energy consumed for heating and cooling, additional inputs are required. CTCC outputs include:
- Annual energy savings in kWh of electricity and MBtu of heating per tree
- Carbon dioxide equivalents of these energy savings
- The CTCC can be used to estimate GHG benefits for an existing tree or to forecast future benefits for a planting project.
Tree size and growth data are developed from samples of about 650-1000 street trees representing approximately 20 predominant species in each of the sixteen regional reference cities. Biomass equations, many derived from volumetric measurements of open-grown city trees, are used to derive total CO2 stored and sequestered. To determine effects of tree shade on building energy performance, over 12,000 simulations were conducted for each reference city using different combinations of tree sizes, locations, and building vintages.
Users should recognize that conditions vary within regions, and data from the CTCC may not accurately reflect their rate of tree growth, microclimate, or building characteristics. When conditions are different it may be necessary to apply biomass equations manually using adjusted tree growth data and perform building energy simulations with modified weather and tree data to more accurately depict effects of trees on GHGs.
The CTCC is intended as "proof of concept" software that is in the testing phase. It is provided "as is" without warranty of any kind. In 2011, this version will be replaced by a Web-based version with greater functionality.
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.
Resources and Documents
California urban woody green waste utilization [PDF 1.18 MB]
Physical properties and moisture relations of wood [PDF 500 KB]
Urban tree utilization and why it matters [PDF 646 KB]
Inventory and monitoring
Carbon storage by urban soils in the United States [PDF 1.82 MB]
Planning, management, assessment
Atmospheric carbon reduction by urban trees [PDF 400 KB]
Guidelines for Developing and Evaluating Tree Ordinances [PDF 1.65 MB]
Urban ecosystems and the North American carbon cycle [PDF 181.37 KB]
Leafy, green and good.[PDF 20 KB]
Urban Forest Project Protocol [PDF 1.20 MB]
Sequestration and pollutant uptake
Allometric scaling theory applied to FIA biomass estimation . [PDF 824 KB]
Physical properties and moisture relations of wood [PDF 500 KB]
Climate Action Reserve Urban Forest Project Protocol. – This site contains the protocol and appendices, provides a brief description, and alerts you to changes to its status.
U.S. Carbon Dioxide Emissions from Energy Sources 2007 – Provides relatively current estimates of U.S. carbon dioxide emissions by end-use sector, such as residential, transportation, and electric power.
U.S. Emissions of Greenhouse Gases Report 2006 – Presents annual U.S. emissions for different GHGs and provides a global perspective.
Mayors Climate Protection Center – Describes the Climate Protection Agreement signed by over 850 mayors, and presents best practices, including tree planting and stewardship projects in selected cities.
U.S. Environmental Protection Agency Heat Island – presents basic information on urban heat islands, describes research and demonstration projects, and what can be done using trees.
A Guide to Street Tree Inventory Software. – presents a collection of publications to guide urban forestry professionals in the selection of a street tree inventory software program and describes those that are commercially available. [Note: info not limited to street trees].
i-Tree Software Suite v. 2.0 Users Manual- is a tool for assessing and managing community forests.
McPherson, E.G.; Simpson, J.R.; Peper, P.J.; Aguaron, E. 2008. Urban Forestry and Climate Change. Albany, CA: USDA Forest Service, Pacific Southwest Research Station.