Environmental Engineering Reference
In-Depth Information
emissions) per year (Nowak et al., 2013; USEPA, 2014). The value of urban carbon seques-
tration is substantial: approximately $2 billion per year, with a total current carbon storage
value of over $50 billion (Nowak et al., 2013). Shading and reduction of wind speed by trees
can help to reduce carbon emissions by reducing summer air conditioning and winter heating
demand and, in turn, the level of emissions from supplying power plants (Nowak et al., 2010).
Shading can also extend the useful life of street pavement by as much as ten years, thereby
reducing emissions associated with the petroleum-intensive materials and operation of heavy
equipment required to repave roads and haul away waste (McPherson and Muchnick, 2005).
Establishing 100 million mature trees around residences in the United States would save
an estimated $2 billion annually in reduced energy costs (Akbari et al., 1992; Population
Reference Bureau, 2012). However, this level of tree planting would only offset less than 1%
of U.S. emissions over a 50-year period (Nowak and Crane, 2002).
The sustainable use of wood, food, and other goods provided by the local urban forest
may also help mitigate climate change by displacing imports associated with higher levels
of carbon dioxide emitted during production and transport. Urban wood is a valuable and
underutilized resource. At current utilization rates, forest products manufactured from
felled urban trees are estimated to save several hundred million tons of CO
2
over a 30-year
period. Furthermore, wood chips made from low-quality urban wood may be combusted for
heat and power to displace an additional 2.1 million tons of fossil fuel emissions per year
(Sherrill and Bratkovitch, 2011).
Urban forests enable cities to better adapt to the effect of climate change on temperature
patterns and weather events. Cities are generally warmer than their surroundings (typically
by about 1 to 2°C, although this difference can be as high as 10°C under certain climatic
conditions), meaning that average temperature increases caused by global warming are
frequently amplified in urban areas (Bristow et al., 2012; Kovats and Akhtar, 2008). Urban
forests help control this “heat island” effect by providing shade, by reducing urban albedo
(the fraction of solar radiation reflected back into the environment), and through cooling
evapotranspiration (Bristow et al., 2012; Novak et al., 2010; Romero-Lankao, 2008). Cities
are also particularly susceptible to climate-related threats such as storms and flooding.
Urban trees can help control runoff from these by catching rain in their canopies and
increasing the infiltration rate of deposited precipitation. Reducing stormwater flow reduces
stress on urban sewer systems by limiting the risk of hazardous combined sewer overflows
(Fazio, 2010). Furthermore, well-maintained urban forests help buffer high winds, control
erosion, and reduce drought (Cullington and Gye, 2010; Fazio, 2010; Nowak et al., 2010).
Urban forests provide critical social and cultural benefits that may strengthen community
resilience to climate change. Street trees can hold spiritual value, promote social interac-
tion, and contribute to a sense of place and family for local residents (Dandy, 2010). Overall,
forested urban areas appear to have potentially stronger and more stable communities (Dandy,
2010). Community stability is essential to the development of effective long-term sustainable
strategies for addressing climate change (Williamson et al., 2010). For example, neighbor-
hoods with stronger social networks are more likely to check on the elderly and other vulner-
able residents during heat waves and other emergencies (Klinenberg, 2002).
Urban forests help control the causes and consequences of climate-related threats; how-
ever, forests may also be negatively impacted by climate change. Although increased carbon
dioxide levels and water temperature may initially promote urban tree growth by accelerating
photosynthesis, too much warming in the absence of adequate water and nutrients stresses
trees and retards future development (Tubby and Webber, 2010). Warmer winter temperatures
increase the likelihood of winter kill, in which trees, responding to their altered environment,
prematurely begin to circulate water and nutrients in their vascular tissue. If rapid cooling fol-
lows these unnatural warm periods, tissues will freeze and trees will sustain injury or death.
