![](https://webarchive.library.unt.edu/web/20090510054648im_/http://www.epa.gov/epafiles/images/epafiles_misc_space.gif) |
![](https://webarchive.library.unt.edu/web/20090510054648im_/http://www.epa.gov/epafiles/images/epafiles_misc_space.gif) |
Frequent Questions
1. What is terrestrial carbon sequestration?
2. Why are agricultural and forestry sequestration activities
important?
3. Which agricultural and forestry practices sequester carbon?
4. How much carbon can agricultural and forestry practices
sequester?
5. How well can carbon sequestration be measured?
6. How much carbon sequestration occurs in the U.S.?
7. What is the potential for additional sequestration to offset
greenhouse gas emissions?
8. Do sequestration practices affect greenhouse gases other
than CO2?
9. What are the other environmental effects of sequestration
practices?
10. How could carbon sequestration be affected by climate
change?
11. How do sequestration activities compare with greenhouse
gas reductions in other sectors?
12. Are there other means of storing carbon beyond agricultural
and forestry activities?
Terrestrial carbon sequestration is the process through which carbon dioxide
(CO2) from the atmosphere is absorbed by trees, plants and crops
through photosynthesis, and stored as carbon in biomass (tree trunks, branches,
foliage and roots) and soils. The term "sinks" is also used to refer to
forests, croplands, and grazing lands, and their ability to sequester carbon.
Agriculture and forestry activities can also release CO2 to the
atmosphere. Therefore, a carbon sink occurs when carbon sequestration is
greater than carbon releases over some time period.
Forests and soils have a large influence on atmospheric levels of carbon
dioxide (CO2)—the most important global warming gas
emitted by human activities. Tropical deforestation is responsible
for about 20% of the world's annual CO2 emissions (IPCC
Special Report on LULUCF (2000) ).
On a global scale, however, these emissions are more than offset by
the uptake of atmospheric CO2 by forests and agriculture.
Therefore, agricultural and forestry activities can both contribute
to the accumulation of greenhouse gases in our atmosphere, as well
as be used to help prevent climate change, by avoiding further emissions
and by sequestering additional carbon. Sequestration activities can
be carried out immediately, appear to present relatively cost-effective
emission reduction opportunities, and may generate environmental co-benefits.
At the same time, it is important to recognize that carbon sequestered
in trees and soils can be released back to the atmosphere, and that
there is a finite amount of carbon that can ultimately be sequestered.
![Go to top](images/back_to_top.gif)
There are three general means by which agricultural and forestry practices
can reduce greenhouse gases:
(1) avoiding emissions by maintaining existing carbon storage in trees
and soils;
(2) increasing carbon storage by, e.g., tree planting, conversion from conventional
to conservation tillage practices on agricultural lands;
(3) substituting bio-based fuels and products for fossil fuels, such as
coal and oil, and energy-intensive products that generate greater quantities
of CO2 when used.
For more detailed information on individual agricultural and forestry
practices, visit the Practices section
of this Web site.
Carbon sequestration rates vary by tree species, soil type, regional climate,
topography and management practice. In the U.S., fairly well-established
values for carbon sequestration rates are available for most tree species.
Soil carbon sequestration rates vary by soil type and cropping practice
and are less well documented but information and research in this area is
growing rapidly.
Pine plantations in the Southeast can accumulate almost 100 metric tons
of carbon per acre after 90 years, or roughly one metric ton of carbon
per acre per year (Birdsey 1996). Changes in forest management (e.g.,
lengthening the harvest-regeneration cycle) generally result in less carbon
sequestration on a per acre basis. Changes in cropping practices, such
as from conventional to conservation tillage, have been shown to sequester
about 0.1 – 0.3 metric tons of carbon per acre per year (Lal et
al. 1999; West and Post 2002). However, a more comprehensive picture of
the climate effects of these practices needs to also consider possible
nitrous oxide (N2O) and methane (CH4) emissions.
(See also FAQ #8)
Carbon accumulation in forests and soils eventually reaches a saturation
point, beyond which additional sequestration is no longer possible. This
happens, for example, when trees reach maturity, or when the organic matter
in soils builds back up to original levels before losses occurred. Even
after saturation, the trees or agricultural practices would need to be
sustained to maintain the accumulated carbon and prevent subsequent losses
of carbon back to the atmosphere.
For more information on carbon sequestration and saturation rates for
individual practices, visit the Practices section
of this Web site. Full references cited in this answer are provided
below.
![Go to top](images/back_to_top.gif)
Several methods can be used to measure the carbon and—more importantly
for the atmosphere—the changes in carbon in above-ground and below-ground
biomass, soils, and wood products. Statistical sampling, computer modeling
and remote sensing can be used to estimate carbon sequestration and emission
sources at the global, national and local scales. Current forest carbon
estimates are generally more accurate and easier to generate than soil
estimates. Estimating changes in soil carbon over time is generally more
challenging due to the high degree of variability of soil organic matter—even
within small geographic scales like a corn field—and because changes
in soil carbon may be small compared to the total amount of soil carbon.
More information on these carbon accounting methodologies can be found
in the Land-Use Change and Forestry chapter of the Inventory
of U.S. Greenhouse Gas Emissions and Sinks (
PDF, 52 pp., 9 MB, About
PDF), and in the IPCC
(2000) Special Report on Land Use, Land-Use Change, and Forestry section
on methods.![Exit disclaimer](https://webarchive.library.unt.edu/web/20090510054648im_/http://www.epa.gov/epafiles/images/epafiles_misc_exitepadisc.gif)
The U.S. landscape acts as a net carbon sink—it sequesters more
carbon than it emits. Two types of analyses confirm this: 1) atmospheric,
or top-down, methods that look at changes in CO2 concentrations;
and 2) land-based, or bottom-up, methods that incorporate on-the-ground
inventories or plot measurements. Net sequestration (i.e., the difference
between carbon gains and losses) in U.S. forests, urban trees and agricultural
soils totaled almost 840 teragrams (Tg) of CO2 equivalent
(or about 230 Tg or million metric tons of carbon equivalent) in 2001
(Inventory
of U.S. Greenhouse Gas Emissions and Sinks). This offsets
approximately 15% of total U.S. CO2 emissions
from the energy, transportation and other sectors. However, the overall
sequestration level in the U.S. has been declining and is projected
to continue declining, due to increasing harvests, land-use changes
and maturing forests. More information on U.S. carbon sequestration
estimates and historical trends can be found under the National
Analysis
section of this Web site.
At the global level, the IPCC
Third Assessment Report
estimates
about 100 billion metric tons of carbon over the next 50 years could be
sequestered through forest preservation, tree planting and improved agricultural
management. This would offset 10-20% of the world's projected fossil fuel
emissions. For the U.S., some analyses (e.g., McCarl and Schneider 2001)
suggest that between 50 and 150 million metric tons of additional carbon
sequestration per year could be achieved through changes in agricultural
soil and forest management, tree planting, and biofuel substitution. These
particular results consider the financial incentive to improve land-use
practices at prices of, respectively, $10 and $50 per metric ton of additional
carbon stored. For more information on analyses of the potential for additional
sequestration in the U.S., visit the National
Analysis section of this Web site.
![Go to top](images/back_to_top.gif)
Yes. Methane (CH4) and nitrous oxide (N2O) are potent
greenhouse gases that are also important to consider for forests, croplands
and grazing lands. Practices that maintain and sequester carbon can have
both positive and negative effects on CH4 and N2O
emissions. The relationship among practices that affect CO2,
CH4, and N2O can be especially complex in cropping
and grazing systems. For example, if nitrogen-based fertilizers are applied
to crops to increase yields, this would likely enhance soil carbon but the
benefit could be partially or completely offset by increased emissions of
N2O. The practice of rotational grazing can be beneficial across
all three major gases: soil carbon can be maintained and enhanced; livestock
CH4 emissions should decline due to improved forage quality;
and N2O emissions can be avoided by eliminating the need for
fertilizer applications on the pasture. These complex interactions among
gases mean that it is important to consider not only carbon but all the
greenhouse gas effects for certain sequestration practices.
For more information on levels of CH4 and N2O
emissions from U.S. agriculture, visit the National
Analysis section of this Web site.
Practices that aim to reduce carbon losses and increase sequestration
generally enhance the quality of soil, water, air and wildlife habitat.
Tree planting that restores fuller forest cover may not only sequester carbon
but could improve habitat suitability for wildlife. Preserving threatened
tropical forests may avoid losses in both carbon and biodiversity, absent
any leakage effects. And reducing soil erosion
through tree planting or soil conservation measures can sequester carbon
and improve water quality by reducing nutrient runoff. In certain cases,
there may be tradeoffs between carbon objectives and environmental quality.
Replacing diverse ecosystems with single-species timber plantations may
generate greater carbon accumulation, but could result in less biodiversity,
at least at the scale of the plantation. For more information on some of
the broader environmental effects of sequestration visit the Co-benefits
section of this Web site.
According to a National Academy of Sciences 2001 report, "Greenhouse gases
are accumulating in the Earth's atmosphere as a result of human activities,
causing surface air temperatures and subsurface ocean temperatures to rise."
In addition to temperature, human-induced climate change may also affect
growing seasons, precipitation and the frequency and severity of extreme
weather events, such as fire. These changes can influence forests, farming
and the health of ecosystems, and thus carbon sequestration. Some argue
that rising CO2 levels will enhance sequestration above normal
rates due to a fertilization effect. However, the concurrent changes in
temperature and precipitation, along with local nutrient availability and
harmful air pollutants, complicate this view. Furthermore, recent studies
of pine forests fumigated with elevated CO2 levels have shown
that this fertilization effect may only be short-lived (Schlesinger and
Lichter 2001; Oren et al. 2001). Current projections of business-as-usual
U.S. sequestration rates under various climate change scenarios show both
increases and decreases in carbon storage depending on various assumptions.
To date, few analyses of the potential for additional sequestration over
time have considered the future effects of climate change.
In terms of its global warming impact, one unit of CO2 released
from a car's tailpipe has the same effect as one unit of CO2
released from a burning forest. Likewise, CO2 removed from the
atmosphere through tree planting can have the same benefit as avoiding an
equivalent amount of CO2 released from a power plant. However,
the climate benefits of sequestration practices can be partially or completely
reversed because terrestrial carbon can be released back to the atmosphere
through decay or disturbances. Trees that sequester carbon are subject to
natural disturbances and harvests, which could suddenly or gradually release
the carbon back to the atmosphere. And if carbon sequestration practices
in agriculture, such as reduced tillage, are abandoned or interrupted, most
or all of the accumulated carbon can be quickly released. Some sequestration
practices, like tree planting and improved soil management, also reach a
point where additional carbon accumulation is no longer possible. For example,
mature forests will not sequester additional carbon after the trees have
fully grown. At this point, however, the mature trees or practices still
need to be sustained to maintain the level of accumulated carbon. Addressing
the issues of reversibility (or duration) and carbon saturation is important
if sequestration benefits are to be compared to other greenhouse gas reductions.
For more information on how the duration
issue might be addressed for carbon sequestration projects, visit the
Project Analysis section of this Web
site.
There is growing interest in storing carbon in underground geologic
formations and possibly in the oceans. The concept is to prevent CO2 emissions
from power plants and industrial facilities from entering the atmosphere
by separating and capturing the emissions and then securely storing CO2 for
the long term. Research and demonstration projects are underway to separate
and capture the CO2 from fossil fuels (pre combustion) and
from flue gases (post combustion). The theoretical potential for both
underground and deep oceanic storage is very large. Today, more than
750 billion gallons of hazardous and non-hazardous fluids are disposed
of safely through underground injection. EPA’s Underground Injection
Control (UIC) Program ensures that these fluids are disposed of safely
and cost effectively while fulfilling its mission to protect ground water
resources. However, because geologic and oceanic sequestration are emerging
climate mitigation optionskey issues still need to be addressed, including
the costs, energy requirements, long-term effectiveness, and ecological
consequences, especially for oceanic storage. For more information on
these options and EPA’s UIC program, visit DOE’s
Office of Science, Office
of Fossil Energy, National
Energy Technology Laboratory, and EPA’s Office
of Ground Water and Drinking Water.
![Go to top](images/back_to_top.gif)
References in this FAQ section where links are not available:
Birdsey, R.A. (1996) Regional Estimates of Timber Volume
and Forest Carbon for Fully Stocked Timberland, Average Management After
Final Clearcut Harvest. In Forests and Global Change: Volume 2,
Forest Management Opportunities for Mitigating Carbon Emissions,
eds. R.N. Sampson and D. Hair, American Forests, Washington, DC.
Lal, R. et al. (1999) The
Potential of U.S. Cropland to Sequester Carbon and Mitigate the Greenhouse
Effect. Lewis Publishers.
McCarl, B. and U. Schneider (2001) Greenhouse Gas Mitigation
in U.S. Agriculture and Forestry. Science,
294:2481-2482.
Oren, R. et al. (2001) Soil fertility limits carbon
sequestration by forest ecosystems in a CO2-enriched atmosphere.
Nature, 411:469-472.
Schlesinger, W. and J. Lichter (2001) Limited carbon
storage in soil litter of experimental forest plots under increased atmospheric
CO2. Nature, 411:466-469.
West, T.O. and W.M. Post (2002) Soil Carbon Sequestration
by Tillage and Crop Rotation: A Global Data Analysis. Soil Science Society
of America Journal. Available at DOE CDIAC site.
|