When and if the United States chooses to implement a domestic GHG reduction program and/or joins in any international efforts to mitigate climate change, it will be necessary to decide whether carbon sequestration policies should be part of the domestic portfolio of compliance activities. The potential opportunities and associated costs of carbon sequestration will presumably be a major criterion in determining its role and so it is important to assess the cost of supplying forest-based carbon sequestration in the United States. Failure to include carbon sequestration as a mitigation option in economic models will lead to over-estimation of the cost of reducing net GHG emissions. However, including carbon sequestration in a naïve manner could produce misleading results as well.
In this report, we have surveyed major previous studies of sequestration, examining the factors that have affected their cost estimates and synthesizing their results. The assumptions that stand out as being particularly important in previous cost estimates include those concerning biological factors such as species, forestry practices, and carbon yield patterns; the opportunity cost of land; management practices; methods of disposition of biomass; relevant prices; and policy instruments used to achieve carbon sequestration.
We identified eleven previous analyses of carbon sequestration costs in the United States as particularly good candidates for comparison and synthesis and normalized their findings to narrow the useful range of estimated costs and allow for consistent comparisons. The normalization included adjustments for constant year dollars, use of identical discount rates, adjustments to scale for identical (national) geographic scope, and consistent reporting in equivalent annual costs. As anticipated, normalizing results across studies led to a significant narrowing of the range of estimated marginal cost functions (Figure 4). This range was subsequently narrowed further by excluding regional studies, since we judged the extrapolation from regional results to national estimates to be problematic. After excluding the three regional studies, our analysis shows that at 300 million tons of annual carbon sequestration nearly all supply functions fall within a marginal cost range of $25 - $75 per short ton of carbon ($7.50 - $22.50 per metric ton of CO2-equivalent). Not surprisingly, the range increases somewhat—to $30 - $90 per ton—for programs sequestering 500 million tons annually (Figure 5).
To make our results more transparent and accessible, we also used econometric techniques to estimate the central tendency of these marginal cost functions; the resulting “best fit” cost curve is presented as an additional output of our analysis. Graphically (Figure 6), it approximates a straight line up to 500 million tons of annual sequestration at which point each additional ton of carbon sequestration costs a bit more than $70 per ton.
Three conclusions emerge from our analysis:
(1) there is a broad range of possible forest-based carbon sequestration supply functions whose shape and magnitude depend on what is assumed about underlying biological and economic factors, as well as on the analytical methods used to estimate costs and supply;
(2) by limiting the set of supply functions to those that come from national studies and that lend themselves to quantitative normalization, the results from previous analyses can be rendered more comparable and the range of estimated supply functions can be narrowed considerably;
(3) when a transparent and accessible approach is employed to estimate econometrically the central tendency of the individual studies making up this range of results, the resulting marginal cost function indicates that the cost of supplying forest-based carbon sequestration in the United States is nearly, though not exactly, linear up to 500 million tons per year, where marginal costs reach a bit more than $70 per ton.
The results presented in this report represent a synthesis of the best existing cost studies, not the final word on the topic. Future research could benefit from further attention to important issues of programmatic leakage (or countervailing forces) that might diminish the positive impacts of a program and thus raise the social cost of sequestration, the impermanence or reversibility of forest carbon sequestration, the broader impacts of a forest carbon sequestration program on the agriculture and forestry sectors and on public finance and tax systems, and the potential secondary costs and benefits of a carbon sequestration program with respect to, for example, natural resources such as water quality and wildlife habitat. Moreover, additional exploration is needed of the interaction between different policy mechanisms to promote sequestration (whether offset trading, agricultural subsidies for specific practices, command-and-control, or direct government production) and the ultimate opportunity costs of sequestration. In general, there may be a tradeoff between the power of incentives directly linked to desired outcomes (in this case the quantity of carbon sequestered) and the costs of implementing and monitoring a program. The optimal program design for promoting sequestration, and how that design affects the issues delineated above, merits more attention.
It is important to understand the magnitude of the hypothetical programs under consideration in this study. The amount of agricultural land involved is huge—approximately 27 million acres for a program achieving 50 million tons of sequestration per year and 148 million acres for a program achieving 300 million tons of sequestration per year. Total annual costs, based on the cost estimates developed here, would be approximately $840 million and $7.2 billion, respectively, for 50 and 300 million ton programs. Because much of this cost would occur upfront, the total social cost in present value terms may be thought of as similar to incurring a one-time cost of $17 billion to $143 billion. Needless to say, this would be a large amount for the U.S. or any other economy to absorb—financially, physically, and administratively—and so a program of this size would probably need to be implemented gradually over many years.
The estimate of carbon sequestration potential discussed in this report (i.e., up to 500 million tons per year) would require a very significant sequestration program, equivalent to about one-third of annual U.S. carbon emissions. Given that available sequestration cost estimates (at these quantity levels) are not very far above typical cost estimates for emissions abatement through fuel switching and energy efficiency improvements, it follows that a domestic carbon sequestration program (assuming such a program can be designed and implemented) ought to be included in a cost-effective portfolio of compliance strategies if and when the United States chooses to implement a domestic GHG reduction program.