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 Addressing
Reversibility (Duration) for Projects
Duration (also referred to as permanence) refers to the potential for
carbon sequestered in agriculture and forestry to be partially or completely
reversible. The carbon that accumulates in vegetation and soils can
be released back to the atmosphere either through natural events
(e.g., forest fires) or human activities (e.g., harvesting of trees,
change in tillage practice for croplands).
The duration (or potential reversibility) of carbon benefits is characteristic
of agricultural and forestry projects, due to the dynamic biological
processes of the carbon cycle involved. Addressing duration can help
contribute to the environmental credibility of agricultural
and forestry sequestration projects. In particular, the risk of losing
carbon sequestration benefits may need to be addressed over two kinds
of timeframes: 1) during the project timeframe; and 2) after the project
timeframe ends.
There is currently no standard method for addressing the duration of sequestration
projects. Proposed ideas to date include the use of insurance mechanisms,
project diversification to mitigate the risk of losing benefits, the issuance
of temporary credits, and discounting credits. References to analyses
that examine the use of these options to address duration are provided
below.
Closely related to the duration issue is the fact that carbon sequestration
eventually saturates. As trees mature, for example, their growth and
thus
carbon sequestration rates slow and eventually level off. If left undisturbed,
carbon saturation in trees may not occur until at least 80 years in
the
U.S. For agricultural soils, carbon sequestration rates may saturate
after about 15-30 years (see the Practices section
of this Web site for more information on typical saturation periods).
The implication for projects is that, even after carbon saturation has
been reached, activities may need to be maintained to prevent the
accumulated
carbon from being released back to the atmosphere (e.g., mature trees
may need to be protected, or conservation tillage practices may need
to
continue).
The papers and links listed below are provided for information and discussion.
The opinions and findings expressed in the information below do not necessarily
represent those of the Environmental Protection Agency.
General/Overview references:
More specific references:
Note: All the below links will take you outside the
EPA Web site. 
- Sedjo,
R. and G. Marland (2003) Inter-trading permanent emissions credits
and rented temporary carbon emissions offsets: some issues and alternatives.
Climate Policy 3(4): 435:444.
- Subak,
S. (2003) Replacing Carbon Lost from Forests: an assessment of insurance,
reserves and expiring credits. Climate
Policy, 3(2): 107-122.
- McCarl,
B.A., B.C. Murray and U.A. Schneider (2001) Influences of Permanence
on the Comparative Role of Biological Sequestration versus Emissions
Offsets. CARD Working Paper 01-WP 282. (PDF, 20 pp., 59 KB, About
PDF)
- Marland,
G., K. Fruit and R. Sedjo (2001) Accounting for sequestered carbon:
the question of permanence. Environmental Science and Policy, 4(6):
259-268.
- Herzog,
H., K. Caldeira and J. Reilly (2003) An Issue Of Permanence: Assessing
The Effectiveness Of Temporary Carbon Storage. Climatic Change,
59: 293-310. (PDF, 19 pp., 340 KB, About
PDF)
- 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.
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