Environmental Engineering Reference
In-Depth Information
Atmospheric CO 2
H 2 O
H 2 O
Decaying
and
Burning
Photosynthesis
Respiration
O 2
O 2
Organic C
Plants and Animals
FIGURE 9.1 The global carbon cycle. Carbon naturally moves, or cycles, between the
atmosphere and vegetation, soils, and the oceans over time scales ranging from years to mil-
lennia and longer. Human activities, primarily the burning of fossil fuels and clearing of
forests, have increased the transfer of carbon as CO 2 to the atmosphere. Although some of the
anthropogenic CO 2 is removed from the atmosphere by the natural uptake processes (“sinks”)
of the carbon cycle, much of it remains in the atmosphere and causes rising CO 2 concentra-
tions. The goal of deliberate carbon sequestration is to decrease the net flux of CO 2 to the
atmosphere by sequestering carbon in the oceans, vegetation, soils, and porous rock for-
mations. (From USGS, Carbon Sequestration to Mitigate Climate Change , U.S. Geological
Survey, Washington, DC, 2008.)
sequestration process: capturing and sequestering carbon dioxide. Further, to con-
trol atmospheric carbon dioxide requires deliberate mitigation with an approach that
combines reducing emissions by utilizing renewable sources and by increasing cap-
ture and storage.
The term carbon sequestration is used to describe both natural and deliberate
processes by which CO 2 is either removed from the atmosphere or diverted from
emission sources and stored in the ocean, terrestrial environments (vegetation, soils,
and sediments), and geologic formations. Before human-caused CO 2 emissions began
to occur, the natural processes that make up the global carbon cycle (see Figure 9.1)
maintained a near balance between the uptake of CO 2 and its release back to the
atmosphere. However, existing CO 2 uptake mechanisms (sometimes called CO 2 or
carbon sinks ) are insufficient to offset the accelerating pace of emissions related to
human activities. Annual carbon emissions from burning fossil fuels in the United
States are about 1.6 gigatons (billion metric tons), whereas annual uptake amounts
are only about 0.5 gigatons, resulting in a net release of about 1.1 gigatons per year
(see Figure 9.2 ) .
Scientists at the U.S. Geological Survey (USGS) and elsewhere are working to
assess both the potential capacities and the potential limitations of the various forms
of carbon sequestration and to evaluate their geologic, hydrologic, and ecological
consequences. The USGS is providing information needed by decision makers
and resource managers to maximize carbon storage while minimizing undesirable
impacts on humans and their physical and biological environment.
 
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