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U.S. Emissions of Greenhouse Gases in Perspective Appendix A: Estimation Methods Appendix B: Carbon Coefficients Used in this Report Appendix C: Uncertainty in Emissions Estimates Appendix D: Emissions Sources Excluded Appendix E: Emissions of Energy-Related Carbon Dioxide in the United States, 1949-1997 Appendix F: Common Conversion Factors
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Overview Land use patterns play an important role in greenhouse gas emissions and sequestration in the United States. Land use types that affect greenhouse gas emissions and sequestration include forest land, grassland, pasture, rangeland, cropland, wetlands, and urban land.(73) The effects of land use on national emissions estimates are complicated by two issues:
This chapter focuses primarily on the second issue: describing the candidate sources of emissions and sequestration in a U.S. context and analyzing the ways in which changing definitions of what should be included or excluded can alter the estimate of net emissions and sequestration. Specifically:
Substantial quantities of greenhouse gases are removed from the atmosphere by natural processes in terrestrial systems each year. A similarly substantial, although smaller quantity is released back into the atmosphere by the reverse of these natural processes each year. In the United States, annual aggregate sequestration of greenhouse gases attributable to all land use types is, and is likely to be for decades, greater than emissions. Carbon dioxide, nitrous oxide, and methane are both emitted and absorbed by soils and vegetation in the United States. Changes in land use can produce large changes in the balance between emissions and absorption. Carbon dioxide accounts for most of the emissions and absorption, primarily by forests, but with lesser amounts absorbed or emitted by grasslands, pasture, and rangeland. Methane and nitrous oxide are also emitted and absorbed in smaller quantities by bacterial action in soils and wetlands. As discussed in Chapter 4, anthropogenic soil fertilization with nitrogen in the form of fertilizer or manure enhances natural bacterial action that causes emissions of nitrous oxide. The Role of Land Use in Greenhouse Gas Emissions and Reductions Sources and sinks of greenhouse gases related to land use are difficult both to quantify and to categorize. Quantification ultimately relies on direct measurements and extrapolations therefrom. Because the land area of the United States equals 2.26 billion acres (Table 36), direct measurements of even a sample of representative land use types are expensive, complex, and time-consuming. Categorization of land use types is difficult, because land frequently shifts from one category to another, blurring definitions. For example, grassland recently converted from cropland is categorized as grassland, even though its soil carbon content, the most important carbon sink in grasslands, more closely matches that of cropland, which typically contains far less soil carbon. Similarly, a dense forest stand may be reclassified as urban land as the result of surrounding urban development, even if the forest, and its carbon dynamics, are unchanged. Forest may also be reclassified as park land, even if no forest is removed in the transition. The result may be the appearance of a decrease in forest area in national inventories when, in fact, there has been no change. Calculating Greenhouse Gas Emissions and Reductions Under the Kyoto Protocol The IPCC Guidelines For purposes of estimating greenhouse gas emissions and reductions attributable to land use, the IPCC has released guidelines on greenhouse gas accounting for land use change and forestry (the "IPCC guidelines").(74) Under the Kyoto Protocol, participating nations must follow the IPCC guidelines when calculating greenhouse gas emissions and reductions. The guidelines consist of specific instructions for estimating greenhouse gas emissions and reductions from all potential anthropogenic and natural sources and sinks, and they call for national estimates of 1990 aggregate carbon dioxide equivalent emissions of greenhouse gases, which will serve as a baseline against which national emissions during the period 2008-2012 will be compared. One of the ways in which the United States and other countries may reach their emissions reduction goals, if they ratify the Kyoto Protocol, is by accounting for and, potentially, enhancing net sequestration of greenhouse gases from the atmosphere by natural processes in forests, grasslands, pasture, and rangelands. The IPCC guideline on "Land Use Change and Forestry" calls for greenhouse gas accounting under four categories:
The categories are intended to capture all greenhouse gas emissions and sequestration caused by land uses and the processes that occur on them. The greenhouse gas effects in each category can be estimated for the United States; however, the data cannot support estimates of all dimensions of possible change in carbon flux. It is uncertain whether all four of the IPCC categories for "Land Use Change and Forestry" will be allowable under accounting procedures for the Kyoto Protocol. The Kyoto Protocol specifically calls for use of the IPCC guidelines;(75) the latest IPCC methods include all four categories listed above. However, the Protocol specifically states that allowable land use change and forestry activities should be limited to ". . . afforestation, reforestation, and deforestation,"(76) potentially excluding grassland conversion, abandonment of managed lands, and certain changes in soil carbon. No decision has been made by the parties to the Kyoto Protocol as to which IPCC categories or parts of categories will be allowed and which will be excluded. Table 37 presents some relevant components of forestry and land use change and indicates whether they are included in, or excluded from, the Kyoto Protocol and IPCC guidelines, respectively. A total of 11 components are shown, organized into the four "Land Use Change and Forestry" categories of the IPCC guidelines. For the first six components, labeled B through G to correspond with the same components in Table 38, carbon flux estimates and projections have been produced by the U.S. Forest Service. The last five components are not included in Table 38 (and hence are unlabeled), because estimates of the U.S. carbon flux for these components are not currently available. The purpose of Table 37 to highlight some of the key areas where the IPCC guidelines differ from the Kyoto Protocol in terms of the type of land use change and forestry data included in each. The IPCC guidelines call for a greater range of data types than does the Kyoto Protocol. Because the IPCC guidelines are intended to serve as the reporting mechanism under the Kyoto Protocol, such important differences would have to be clarified before reporting could commence in the year 2008. Baseline and Commitment Period The Kyoto Protocol calls for inclusion of land use and forestry-related greenhouse gas emissions and removals in baseline 1990 emissions only from "Those parties . . . for whom land use change and forestry constituted a net source of greenhouse gas emissions in 1990."(77) In the United States, land use and forestry-related greenhouse gas emissions and sequestration constituted a net sink in 1990. Therefore, the United States could include net sequestration from forests in 2008-2012 as an offset from other greenhouse gas emissions during the commitment period, but would not have to make a similar offset against baseline emissions in 1990. In other words, any sequestration applicable under the Kyoto Protocol would reduce the stringency of the U.S. target for other greenhouse gases. Further, to verify whether the United States is meeting its greenhouse gas reduction goal, it would be necessary to report on aggregate emissions and removals of greenhouse gases during each year of the commitment period 2008-2012. Participants are required to ". . . have in place, no later than one year prior to the start of the first commitment period, a national system for the estimation of anthropogenic emissions by sources and removals by sinks of all greenhouse gases."(78) Data necessary for such reporting may not be available in the year 2007 or for specific years in the 2008-2012 commitment period. The most important statistics on forest and other land necessary for estimates of greenhouse gas fluxes in the United States are updated by the USDA Forest Service on average only once every 5 years, and full reports containing the data are released only once every 10 years.(79) However, one of the provisions under consideration in Congress in the current version of the Farm Bill is a new USDA Forest Service mandate to collect and report national forest statistics on an annual basis.(80) If this measure is ultimately included in the Farm Bill, it would satisfy the Kyoto Protocol requirement for annual reporting during the 5-year period 2008-2012. Changes in Forest and Other Woody Biomass Stocks There is no established method for applying the IPCC guidelines to available data on forest land and the fate of wood and paper product pools for purposes of the category "Changes in Forest and Other Woody Biomass Stocks." The figure in any given year would depend on a variety of factors that influence biomass-related carbon sequestration and emissions. The following seven factors, labeled A through G, may be included or excluded, depending on the final method ultimately adopted by the United States and other signatories to the Kyoto Protocol (see also Table 38):
Estimates for all these carbon sources and sinks (Table 38) were obtained from a report by USDA Forest Service researchers Richard Birdsey and Linda Heath.(81) A linear interpolation between Birdsey and Heath's 1992 and 2000 net flux estimates was used to derive the estimates for 1997. Projections for the fluxes in 2010 are taken directly from their report. Depending on the final accounting methods adopted for the category "Changes in Forest and Other Woody Biomass Stocks," aggregate net carbon flux due to land use change and forestry could vary considerably. At the high end, aggregate net carbon flux could be calculated as net flux on all U.S. forest land (factor A), plus carbon stored in wood products and landfills (factors D and E), for a total of 227 million metric tons sequestered in both 1997 and 2010. At the low end, if flux for land uses other than timberland (factor C) were excluded, as well as carbon storage in wood products and landfills (factors D and E), and if emissions from wood burning and decay were included (factors F and G), aggregate net carbon flux would equal 53 million metric tons sequestered in 1997 and 52 million metric tons sequestered in 2010. From the standpoint of net carbon emissions and removals from the atmosphere in any given year, it could be argued that carbon emitted from wood burning for energy (factor F) and from wood decay and non-energy burning (factor G) should be subtracted from the net flux from timberland (factor B) to arrive at the estimate for "Changes in Forest and Other Woody Biomass Stocks." This would be in keeping with the emphasis in the IPCC guidelines on estimating all emissions and removals in the inventory year, regardless of whether such emissions and removals were influenced by processes that occurred before the inventory year. The justification is as follows: flux on timberland could be included because the IPCC guidelines call for estimates of fluxes on all land managed for wood products; carbon emissions from wood burned for energy and from decaying wood and non-energy burning could be included, because they represent releases of carbon to the atmosphere in the inventory year; fluxes on reserved and other forest land could be excluded, because the lands are not managed for wood products; and finally, carbon storage in wood products and landfills could be excluded, because such storage represents merely a transfer of previously sequestered carbon from forests to product and waste pools, rather than a sequestration of carbon from the atmosphere during the inventory year. Adherence to this accounting method would result in the low-end net carbon sequestration rate estimate of 53 million metric tons sequestered in 1997 and a projected 52 million metric tons sequestered in 2010. Previous estimates of net carbon flux on U.S. forest land in this report have represented factor A only (net carbon flux on all U.S. forest land). That estimation method may or may not match the method that ultimately would be adopted by the United States for purposes of reporting aggregate net carbon-dioxide-equivalent emissions of greenhouse gases resulting from land use change and forestry under the Kyoto Protocol. The IPCC's category "Changes in Forest and Other Woody Biomass Stocks" includes all emissions and sequestration of greenhouse gases from forests managed for wood products and from wood products and waste pools. Forests not managed for wood products are not included, because they are not considered to be either an anthropogenic source or sink of greenhouse gases.(82) If the probable methodology described above is adopted (factor B minus factors F and G), the substantial quantity of carbon sequestered on reserved and other forest land (e.g., land in parks, wilderness, and conservation areas) would be excluded from the accounting under the category "Changes in Forest and Other Woody Biomass Stocks." However, it might be possible to include those lands under another IPCC category, "Abandonment of Managed Lands," discussed below. More than 50 percent of the carbon in forest ecosystems is found in the soil.(83) However, the IPCC guideline category "Change in Soil Content" includes only soil carbon on non-forest land. The role of soil carbon in forests has yet to be determined under the Kyoto Protocol. Forest and Grassland Conversion The IPCC's category "Forest and Grassland Conversion" includes conversion of forests and grasslands to other land uses, such as highways, urban development, and agricultural land. Although included under the IPCC's "Land Use Change and Forestry" section, there is uncertainty as to whether this category will be allowable under national greenhouse gas accounting for purposes of the Kyoto Protocol, because grassland conversion is not mentioned in the Protocol. Of the three IPCC categories under the "Land Use Change and Forestry" section, this category is the most complex and difficult to quantify. The greenhouse gas effect of converting a forest or grassland to another land use depends on many factors, including the history of the land area, the type of land use after conversion, temperature and humidity, and other factors. For example, a forest converted to cropland could have previously contained either large or small quantities of carbon stored above and below the ground. The magnitude of the stored quantity could significantly affect the amount of carbon dioxide released to the atmosphere after conversion. The amount of time that has elapsed since the conversion is also a key factor, because initial loss of carbon tends to be significantly higher than loss in subsequent years. Such distinctions often must be made on a site-by-site basis, requiring a tremendous research effort for the aggregation of data on a national scale. The IPCC method requires a sequence of steps involving calculations of:
Converting forests or grasslands to other land uses can have large per-acre effects on greenhouse gas emissions and removals. Forest conversion causes greater changes in greenhouse gas emissions and reductions than grassland conversion. When forests are cleared for development or agriculture, large amounts of carbon dioxide are typically emitted to the atmosphere. For example, average carbon storage (above and below ground) in forests of the Pacific Coast is 205,000 pounds per acre.(84) In contrast, typical estimates of carbon storage in cultivated lands range from a far lower 9,000 to 71,000 pounds per acre.(85) When grasslands are converted to agricultural use, two effects occur simultaneously: above-ground carbon in the form of biomass may increase, decrease, or remain roughly stable; and below-ground carbon decreases. The net change in carbon in above-ground biomass depends on the type of crop replacing the grassland species. The default assumption in the IPCC guidelines is that above-ground biomass remains stable when grasslands are converted to crops or pasture.(86) Below-ground carbon, however, virtually always decreases, because soil disturbances during such practices as row-cropping aerate the soil, oxidizing carbon, which is released as carbon dioxide.(87) Conversion of forest and grasslands to other land uses can also change emissions and absorption of methane and nitrous oxide. The scientific literature suggests that grass and forest lands are both weak natural sinks for methane and weak natural sources of nitrous oxide, although adequate research to establish accurate estimates of aggregate methane and nitrous oxide emissions and sequestration is lacking. Natural soils apparently serve as methane sinks: well-aerated soils contain a class of bacteria called "methanotrophs," which use methane as food and oxidize it into carbon dioxide in small but unquantified amounts. Experiments indicate that cultivation reduces methane uptake by soils and increases nitrous oxide emissions.(88) Exactly how much methane is absorbed by natural soils, and how much nitrous oxide is emitted, is difficult to estimate, although total amounts are very small. It is known that conversion of forests and grasslands to cropland accelerates nitrogen cycling and increases nitrous oxide emissions from the soil. It is not known with certainty by how much (see Appendix A).(89) Estimates of forest and grassland conversion in 1997, including estimates of post-conversion land use, are not currently available. The latest national estimates for acreages of different land use types extend only until 1992 (Table 36). Even those estimates lack information called for by the IPCC method; the estimates are limited to land uses upon survey, leaving unanswered the question of what the land was converted from. For example, land classified as "urban" increased by 9 million acres between 1980 and 1990;(90) however, the proportions of the land that were previously forest land, grassland, cropland, or some other land use type are unknown. Without such data, it is impossible to quantify accurately greenhouse gas flux due to the conversion to urban land. Similar methodological problems are associated with conversions to other land use types, such as cropland. An evaluation of the limited data that do exist suggests that forests and grasslands are not being converted to other uses in large quantities. In fact, forest land increased in extent, and grassland remained stable, between 1987 and 1992, the last year for which data are available. Some forest land has been cleared for urban development, but the quantity is small and unquantified. Agricultural land is often more desirable for urban development than forest land because of its proximity to existing population centers and (usually) well-drained, level topography. Abandonment of Managed Lands The IPCC category "Abandonment of Managed Lands" includes conversion of cropland, pastureland, or other managed land to unmanaged land. Although included under the IPCC's "Land Use Change and Forestry" section, there is uncertainty as to whether this subcategory will be allowable under national greenhouse gas accounting for purposes of the Kyoto Protocol. Abandoned managed land may revert to forest, grassland, land dominated by shrubs, or any combination of those land types. Under the Kyoto Protocol, only land that reverts to forest is currently reportable, and only if the change to forest is achieved through afforestation or reforestation since 1990. Land that reverted to grassland or other nonforest land would presumably not be reportable under the Kyoto Protocol. The IPCC guidelines allow for inclusion of abandoned lands that serve as a net sink for carbon accumulation.(91) In the United States, the two most important contributors to this category are lands that have been converted to forested parks, wildlife, and conservation areas; and cropland that has been idled. It may be possible to count an estimated net 36 million metric tons of carbon sequestered by land included in the "Abandonment of Managed Lands" category in 1997, almost all of which was sequestered by reserved forest land (in parks, conservation areas, etc.) and forested idled cropland. The precise split in this carbon estimate between reserved and other forests versus idled cropland is not known; however, almost all of the estimated 36 million metric tons was sequestered on reserved forest land, because the forested portion of idled cropland accounted for only 2 million acres in 1992, compared with an estimated 247 million acres of reserved forest land in that year.(92) Although the 2 million acres of forest land in the form of idled cropland would be included in estimates under the IPCC guidelines, the 36 million tons of carbon sequestered on reserved forest land might or might not be included. Another land use not explicitly included in the Kyoto Protocol but discussed in the IPCC guidelines is wetland drainage. Wetlands emit methane to the atmosphere as a result of anaerobic decomposition of organic matter. The range of observed methane fluxes from U.S. wetlands is enormous. One survey of experiments conducted in the United States found estimates ranging from a negative flux (methane absorption) to a flux of 213 grams of methane per square meter per year, largely dependent on habitat type.(93) Thus, it is difficult to extrapolate from experimental data to large-scale emissions estimates. Comparisons of estimates of methane fluxes from global wetlands versus temperate zone wetlands alone tend to indicate that methane emissions from temperate-zone wetlands are minimal--typically between 5 and 10 million metric tons of methane per year for worldwide temperate-zone wetlands (which include U.S. wetlands)--when compared with estimated global wetlands emissions of 110 million metric tons.(94) The U.S. share of all temperate-zone wetlands is about 57 percent, and U.S. wetlands lost during the 1980s accounted for about 0.5 percent of the extent of wetlands at the beginning of the decade. Consequently, the reduction in natural methane emissions from U.S. wetlands lost might be on the order of 10,000 to 20,000 metric tons annually over the decade. Conversely, destruction of wetlands may increase atmospheric concentrations of carbon dioxide, which is released after wetland conversion due to aerobic decomposition of organic matter that was formerly under saturated conditions.(95) Changes in Soil Carbon The IPCC category "Changes in Soil Carbon" includes changes in soil organic carbon stocks attributable to changes in agricultural land use and management systems.(96) This category may or may not be allowable under the Kyoto Protocol, which limits reporting to activities deemed "measurable and verifiable" and related to ". . . afforestation, reforestation, and deforestation . . . since 1990." The restrictions potentially exclude some or all changes in soil carbon, either because soil carbon is not deemed measurable and verifiable, or because a portion of the accumulation is occurring on land that has not been subject to afforestation, deforestation, or reforestation since 1990, or both. Because forest soils are such an important carbon sink, serving as the repository for more than half of all carbon on forest lands, USDA Forest Service estimates and projections of soil carbon flux are included under carbon flux on forest land. The ultimate treatment of soil carbon under the Kyoto Protocol is thus a matter of some importance in assessing the potential effects of U.S. adherence to the Protocol. Changes in soil carbon attributable to changes in agricultural land use and management systems can take on many forms and can vary widely in magnitude. In general, forests contain the most soil carbon, followed by grasslands, pasture, and rangeland, with cropland containing the least soil carbon. Aside from changes in forest land soil carbon (discussed above), the most significant changes in soil carbon occur when grassland, pasture, or rangeland is converted to or from cropland. Grasslands, pasture, and rangelands all contain significantly more soil carbon than does cropland.
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