The most comprehensive analysis of the potential for sequestering carbon in trees in the United States is that undertaken by Moulton and Richards (1990) of the U.S. Forest Service. This is a detailed analysis of the land available in the United States that could support trees, the carbon uptake that might be expected, and actual costs for each type of land to be managed. The analysis assumes that trees could be planted on economically marginal and environmentally sensitive pasture and croplands and that forest management programs could increase carbon uptake on many nonfederal forestlands. After analyzing the potential carbon uptake and cost per ton for 70 region and land-type classes, Moulton and Richards arrange these in order by cost per ton and assemble a supply curve for carbon sequestering. The analysis concludes that up to 56.4 percent of U.S. emissions could be sequestered in domestic trees at costs ranging from $5.80 to $47.75/t of carbon (C) (there is no adjustment for the small energy and CO2 cost of implementing such a program).1
The analysis has a 40-year time horizon and so does not confront the consequences of declining growth rates as trees approach maturity or of the long-term possibilities for tree maintenance or harvest. Two sets of numbers that heavily influence the numeric results but are likely to elicit some discussion are land rental rates and the ratio between carbon uptake in marketable timber and total ecosystem carbon uptake. Both sets of numbers are carefully laid out in the analysis and could be manipulated by an analyst with different notions. The ecosystem carbon ratios, for example, ranges from 1.8 to 8.4 and include considerable carbon accumulation in soils. The result of using high ecosystem carbon ratios is that average carbon accumulation for the full program amounts to 5.3 t/hectare(ha)/yr. Trexler (1990) suggests that values nearer 3.7 might be more within expectation for a
program of this magnitude. Any comparison with current forest growth rates must acknowledge that most of the increased growth envisioned in the Moulton and Richards analysis would be on what is characterized as ''marginal cropland" but which would in fact be very good forestland. The analysis does not include a start-up period for tree planting and establishment but assumes "instant trees" with all establishment costs in the first year.
The structure of the Moulton and Richards report is such that one can examine the program incrementally. Table P.1 shows the implications of planting enough treesin order of increasing dollars per ton of carbonto sequester carbon at rates of 10 percent, 20 percent, and 56.4 percent of current U.S. total CO2 emissions. The table shows that at the 10 percent level most of the uptake would be accomplished by changing forest management practices on current forestlands and planting on marginal pasturelands, but that in order to get very much of the maximum potential, large-scale inclusion of marginal croplands would be required.
Recognizing that the Moulton and Richards analysis suggests that 56.4 percent of U.S. CO2 emissions could perhaps be offset with a massive commitment to a reforestation program, the Mitigation Panel adopts a very conservative approach to estimating the carbon offset that might be envisioned. This analysis accepts that the 10 percent objective described by Moulton and Richards is a reasonable initial target and that reforestation of economically marginal or environmentally sensitive pasture and croplands and nonfederal forestlands to a total 28.7 Mha could take place at costs as described in their analysis. The carbon sequestering rate is then divided by 2 to ensure that only carbon that is truly taken into long-term storage is counted. This baseline then suggests that 240 Mt CO2 could be sequestered at costs between $3 and $10/t CO2 (average cost is $7.20/t CO2). Demonstration projects could verify the lower costs and higher targets for total sequestration projected by some.
TABLE P.1 Reforestation Program Costs by Percentage Reduction
1. Tons (t) are metric; 1 Mt = 1 megaton = 1 million tons.
Moulton, R. J., and K. R. Richards. 1990. Costs of sequestering carbon through tree planting and forest management in the United States. U.S. Forest Service Report. Draft. U.S. Department of Agriculture, Washington, D.C.
Trexler, M. C. 1990. Minding the carbon store: Weighing U.S. forest strategies to slow global warming. Draft. World Resources Institute, Washington, D.C.