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close this bookCarbon Counts: Estimating Climate Change Mitigation in Forestry Projects (WRI, 1997, 32 pages)
close this folderII. Leakage
View the document(introduction...)
View the documentCauses of Leakage
View the documentLeakage Index
View the documentApplication of the Index


Leakage is defined here as the unexpected loss of estimated net carbon sequestered. In some cases leakage may be positive - that is, more net carbon reductions were achieved than was expected, but it is the loss of greenhouse gases that most concern us. Leakage can be the result of incorrectly estimating the project's impact or of unexpected effects such as a population increase in the project region, during the course of a project. Therefore, correctly estimating a project's impact and designing projects that avoid leakage are critical.

Figure 1 illustrates the land-use dynamics and potential leakage over time in a project region characterized by subsistence agriculture, where population growth and demand for agricultural land drive deforestation. As population grows, marginal upland is brought into agricultural production. As this land eventually degrades into pasture, people cultivate increasingly higher, and more easily degraded slopes. Each of the three scenarios depicted in Figure 1 features permanent agriculture on the lowland areas and a combination of cattle grazing and farming in the upland areas. The project goals are to move from scenario 1 (the baseline) to scenario 3, and sequester carbon by relieving pressure on the forest through the introduction of higher productivity agroforestry and tree plantations, which would provide fuelwood. However, as shown by scenario 2, leakage occurs. Agroforestry is more productive than the pasture and agriculture it replaces, but the movement of cattle to upland areas results in forest conversion and degradation, albeit at a slower pace than indicated in the baseline. Ideally, leakage can be anticipated, or mitigated, by the measures listed in scenario 3-halting encroachment, allowing sustainable use of forests, and incorporating cattle into the woodlots via silvopastoral systems.

In response in part to uncertainties regarding correct net carbon calculation, investors are focussing on simple afforestation projects, because their impacts appear to be easier to quantify than those of other types of forestry carbon sequestration projects. In fact, tree planting is the only forestry activity the U.S. Department of Energy defines as a "standard" rather than "reporter-designed" project, meaning that enough credible data has been assembled to estimate the project's carbon benefits. (6) Of the greenhouse gas mitigation projects reported to the Department of Energy, tree planting projects are the most popular. (7) The carbon storage portfolio sponsored by the Netherlands' state utility consists entirely of tree planting on degraded lands. (8)

The popularity of tree planting may also be explained in part by the notion that growing biomass accumulates carbon while mature forests are merely stable. However, these "stable", mature forests have usually accumulated much more carbon in biomass than tree plantations are likely to, as Table 1 illustrates. The biomass accumulation of the mature forests in the northwestern United States, Malaysia's Sabah region, and Cameroon represents only carbon in living biomass; the total carbon of tree plantations with rotations varying by species, represents projected accumulations of carbon in living biomass and forest products after 300 years. (9)

Table 1 compellingly illustrates that tree plantations are less effective than natural forests at storing carbon, even when carbon storage in wood products are included in the estimate. Furthermore, a recent study of a tropical rain forest in Brazil indicates that undisturbed forests are continually sequestering carbon. The study estimated that the forest sequestered one metric ton of carbon per hectare per year. (10) This finding underscores the greenhouse gas benefits of projects that prevent deforestation.

It is important to note that when the authors in this publication refer to projects that prevent deforestation, they are not necessarily referring to forest preservation projects that set aside forestland in a park or protected area, which can have significant leakage problems. Rather they are referring to those projects that successfully prevent deforestation by addressing underlying land-use dynamics and demands on resources in and around the project site. Satisfying demands for these resources can, and often does, involve tree planting, as noted later in the case studies.

Figure 1. - Expanding Agricultural Frontier

1. Baseline

2. With Project (leakage)

3. With Project (no leakage)

Table 1. Comparison of Potential Biomass

Forest Type

Tons of Carbon per Hectare

Mature Douglas Fir, Northwestern United States

611 (1)

Mature Closed Forest, Sabah, Malaysia

348 (2)

Mature Primary Moist Forest, Cameroon

279 (3)

Industrial Black Locust Plantation, Europe


Industrial Slash Pine Plantation, Brazil


Afforestation of Tropical Wasteland, Borneo


Industrial Poplar Plantation, Europe


1. Mark E. Harmon, William K. Ferrell, and Jerry F. Franklin, "Effects on Carbon Storage of Conversion of Old-Growth Forests to Young Forests," Science, 9 February 1990, 699.

2. Francis E. Putz and Michelle Pinard, "Reduced Impact Logging as a Carbon Offset Method," Conservation Biology, vol. 7 no. 4, December 1993, 755-757.

3. Sandra Brown, Andrew J.R. Gillespie, and Ariel E. Lugo, "Biomass Estimation Methods for Tropical Forests with Applications to Forest Inventory Data," Forest Science, vol. 35, no. 4, December 1989, 895.

The urge to pursue simple projects in an effort to avoid difficult issues is understandable but unnecessary. It is possible to evaluate projects, identify conditions likely to result in leakage, determine factors contributing to this problem, and recommend actions to avoid or at a minimum account for it.

The analysis of specific carbon sequestration projects indicates that leakage can and should be incorporated into project design and that projects addressing the drivers of land use-change will maximize project benefits, reduce risks and costs, and minimize the potential for leakage.