Environmental Engineering Reference
In-Depth Information
These amounts (about 32 and 7 gigatons for forests and soils, respectively) are proba-
bly not attainable by deliberate sequestration because restoration on this scale would
displace a large percentage of U.S. agriculture and disrupt many other present-day
activities. Decisions about terrestrial carbon sequestration require careful consider-
ations of priorities and trade-offs among multiple resources. For example, convert-
ing farmlands to forests or wetlands may increase carbon sequestration, enhance
wildlife habitat and water quality, and increase flood storage and recreational poten-
tial—but the loss of farmlands would decrease crop production. Converting existing
conservation lands to intensive cultivation, while perhaps producing valuable corps
(for example, for biofuels), may diminish wildlife habitat, reduce water quality and
supply, and increase CO
2
emissions. Scientists are working to determine the effects
of climate and land-use change on potential carbon sequestration and ecosystem
benefits and to provide information about these effects for use in resource planning.
SIDEBAR 9.1. Urban Forests and Carbon Sequestration
*
The urban environment presents important considerations for terrestrial carbon sequestration
and global climate change. Over half of the world's population lives in urban areas (Population
Reference Bureau, 2012). Because cities are more dense and walkable, urban per capita emis-
sions of greenhouse gases (GHGs) are almost always substantially lower than average per
capita emissions for the countries in which they are located (Cities Alliance, 2007; Romero-
Lankao, 2008). Urban areas are more likely than non-urban areas to have adequate emergency
services and so may be better equipped to provide critical assistance to residents in the case
of climate-related stress and events such as heat waves, floods, storms, and disease outbreaks
(Myers et al., 2013). However, cities are still major sources of GHG emissions (Dodman,
2009). Studies suggest that cities account for 40 to 70% of all GHG emissions worldwide due
to resource consumption and energy, infrastructure, and transportation demands (USEPA,
2009). Highly concentrated urban areas, especially in coastal regions and in developing coun-
tries, are disproportionately vulnerable to extreme weather and infectious disease.
The term
urban forest
refers to all trees within a densely populated area, including trees
in parks, on streetways, and on private property. Although the composition, health, age,
extent, and costs of urban forests vary considerably among different cities, all urban forests
offer some common environmental, economic, and social benefits. Urban forests play an
important role in climate change mitigation and adaptation. Active stewardship of a commu-
nity's forestry assets can strengthen local resilience to climate change while creating more
sustainable and desirable places to live. Trees in a community help to reduce air and water
pollution, alter heating cooling costs, and increase real estate values. Trees can improve
physical and mental health and strengthen social connections, and they are associated with
reduced crime rates. Trees, community gardens, and other green spaces get people outside,
helping foster active living and neighborhood pride.
Like any forest, urban forests help mitigate climate change by capturing and storing atmo-
spheric carbon dioxide during photosynthesis and by influencing energy needs for heating
and cooling buildings. Trees typically reduce cooling costs, but they can increase or decrease
winter heating use depending on their location around a building and whether they are ever-
green or deciduous. In the contiguous United States alone, urban trees store over 708 million
tons of carbon (approximately 12.6% of annual carbon dioxide emissions in the United
States) and capture an additional 28.2 million tons of carbon (approximately 0.05% of annual
*
Adapted from Safford, H. et al.,
Urban Forests and Climate Change
, U.S. Department of Agriculture,
Washington, DC, 2013.
