Quantifying the Greenhouse Gas Benefits of Urban Parks
The expansion of green spaces in urban areas can reduce the energy use and CO2 emissions associated with water delivery by providing a medium for wastewater recycling and increased stormwater retention. Through the planting of trees, urban green space also provides the opportunity to not only sequester substantial quantities of carbon pulled from the air and soil, but also reduce local energy consumption by providing cooler surfaces and additional shade for buildings.
Groth, P. (2008). Quantifying the Greenhouse Gas Benefits of Urban Parks. San Francisco, California: The Trust for Public Land.
With governments at all levels looking at ways to reduce greenhouse gas (GHG) emissions, increasing attention is being paid to the relationship between land use patterns and GHG emissions. Parkland and recreational space is an important element of land use planning that deserves consideration for its potential to reduce net GHG emissions.
Urban green space serves diverse purposes, ranging from neighborhood and city parks to river parkways, bike paths, and street trees, which in turn can produce different types of GHG benefits.
The goal of this paper is to help inform local planning decisions by discussing the potential GHG benefits of adding green space to an urban area and introducing methodologies for estimating potential GHG reductions. We are not attempting to provide GHG inventory or accounting methodologies, as those methodologies are already well-established and address a broader range of GHG sources and sinks. Instead, this is an illustration of the types of GHG benefits that warrant further exploration when designing an urban park or when making larger policy decisions about land use. For example, the study provides several of the types of calculations that could be used when determining quantitative benefits to GHG emissions. We look at potential groundwater recharge, reduction of vehicle trips, promotion of bicycling and walking, mitigation of the urban heat island effect, and the carbon sequestration expected from the addition of trees.
The expansion of green spaces in urban areas has been identified as a pathway for reducing the energy use and CO2 emissions associated with water delivery by providing a medium for wastewater recycling and increased stormwater retention (Anderson, 2003; Kramer and Dorfman, 2000). The delivery and treatment of water require a significant amount of energy. Pumping and delivery of water accounted for approximately eight percent of California’s total electricity use in 2004. The water-related energy use is not evenly distributed throughout the state, however. In water districts that import much of their water supply from elsewhere in the state or from out of state, the energy use associated with obtaining water is much greater than for areas that are able to get water from local groundwater aquifers.
The most direct and quantifiable impact on water resources is through the increase in groundwater recharge that is associated with the high permeability of green spaces, compared with the low permeability surfaces of densely developed areas. The benefit to water resources is dependent on the spatial area and the “type” of green space. If the primary purposes of adding green space are to aid in water conservation, mitigation of the urban heat island effect, and the reduction of greenhouse gases, a larger fraction of the ground cover should be highly permeable surfaces.
More hydrologically-beneficial urban green spaces include community gardens, stormwater ponds/wetland buffers, and neighborhood parks.
Through the planting of trees, urban green space also provides the opportunity to not only sequester substantial quantities of carbon pulled from the air and soil, but also reduce local energy consumption by providing cooler surfaces and additional shade for buildings. As trees grow, they remove carbon dioxide from the atmosphere and store it in the form of biomass carbon in the leaves, roots, branches, and trunk. A young sapling can sequester anywhere from 1.0 to 1.3 lbs. carbon each year, while a 50 year old tree can sequester over 100 lbs. annually (DOE 1998). With the sequestration of many trees put together, urban trees can be a significant sink for carbon dioxide. The rate of net sequestration per area of tree cover can be as high as 0.29 kg C/sq. m tree cover (EPA 2008). Indeed, the sequestration by urban trees in the city of New York is estimated to be 38,374 MT annually, and other cities can also claim similar GHG benefits. In total, urban trees in the US sequestered an estimated 95.5 MMTCO2 in 2006 (EPA 2008).
The trees and vegetation provided by urban parks also provide an effective way to reduce urban heat islands. On an individual level, carefully selected and planted trees can reduce the energy consumption for individual buildings. Trees achieve this effect by providing shade and evapotranspiration to cool buildings during summer, thereby reducing the need to run air conditioners and consume electricity (EPA, 2007). Researchers have demonstrated that trees and other heat island reduction measures can combine to reduce building carbon emissions by 5-20 percent (Akbari and Konopacki, 2003).
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