Uncertainty Uncertainties in the flux estimates for mineral and organic soils result from both the activity data and the carbon stock and adjustment factors. Each of the datasets used in deriving the area estimates has a level of uncertainty that is passed on through the analysis, and the aggregation of data over large areas necessitates a certain degree of generalization. The default IPCC values used for estimates of mineral soil carbon stocks under native vegetation, as well as for the base, tillage and input factors, carry with them high degrees of uncertainty, as these values represent broad regional averages based on expert judgment. Moreover, measured carbon loss rates from cultivated organic soils vary by as much as an order of magnitude. In addition, this methodology does not take into account changes in carbon stocks and land-use trends that occurred over longer time periods Uncertainties in the estimates of emissions from liming stem primarily from the methodology, rather than the underlying activity data. It can take several years for agriculturally-applied lime to degrade completely. The IPCC method assumes that the amount of mineral applied in any year is equal to the amount that degrades in that year, so annual application rates can be used to derive annual emissions. Further research is required to determine applied limestone degradation rates. Moreover, soil and climatic conditions are not taken into account in the calculations. Changes in Non-Forest Carbon Stocks in Landfills As is the case with landfilled forest products, carbon contained in landfilled yard trimmings can be stored indefinitely. In the United States, yard trimmings (ie., grass clippings, leaves, branches) comprise a significant portion of the municipal waste stream. In 1990, the EPA estimated discards of yard trimmings to landfills at over 21 million metric tons. Since then, programs banning or discouraging disposal, coupled with a dramatic rise in the number of composting facilities, have decreased the disposal rate for yard trimmings; the 1998 landfill disposal was about 10 million metric tons. The decrease in the yard trimmings landfill disposal rate has resulted in a decrease in the rate of landfill carbon storage from about 4.9 MMTCE in 1990 to 2.3 MMTCE in 1998 (see Table 6-9). Yard trimmings comprise grass, leaves, and branches and have long been a significant component of the U.S. waste stream. In 1990, discards (i.e., landfilling plus combustion) of yard trimmings were about 27.9 million metric tons, representing 17.9 percent of U.S. disposal of municipal solid waste (EPA 1999). Unlike most of the rest of the waste stream, yard trimmings disposal has declined consistently in the 1990s-generation has declined at 3.3 percent per year, and recovery (e.g., composting) has increased at an average annual rate of 15 percent. Laws regulating disposal of yard trimmings now affect over 50 percent of the U.S. population, up from 28 percent in 1992 (EPA 1999). By 1997, discards were about 15 million metric tons, representing 10 percent of U.S. municipal waste disposal. Methodology The methodology for estimating carbon storage is based on a life cycle analysis of greenhouse gas emissions and sinks associated with solid waste management (EPA 1998). According to this methodology, carbon storage is the product of the mass of yard trimmings disposed, on a wet weight basis and a storage factor. The storage factor is based on a series of experiments designed to evaluate methane generation and residual organic material in landfills under average conditions (Barlaz 1997). These experiments analyzed grass, leaves, branches, and other materials, and were designed to promote biodegradation by providing ample moisture and nutrients. For purposes of this analysis, the composition of yard trimmings was assumed to consist of 50 percent grass clippings, 25 percent leaves, and 25 percent branches. A different storage factor was used for each component. The weighted average carbon storage factor is 0.19 Gg carbon per Gg of yard trimmings, as shown in Table 6-10. Results, in terms of carbon storage, are also shown. Data Sources The yard trimmings discard rate was taken from the EPA report Characterization of Municipal Solid Waste in the U.S.: 1998 Update (EPA 1999), which provides estimates for 1990 through 1997 and forecasts for 2000 and 2005. Yard trimmings discards for 1998 were projected using the EPA (1999) forecast of generation and recovery rates (decrease of 6 percent per year, increase of 8 percent per year, respectively) for 1997 through 2000. This report does not subdivide discards of individual materials into volumes landfilled and combusted, although it does provide an estimate of the overall distribution of solid waste between these two man Table 6-10: Composition of Yard Trimmings (%) in MSW and Carbon Storage Factor (Gg Carbon/Gg Yard Trimmings) agement methods (76 percent and 24 percent, respectively) for the waste stream as a whole. 10 Thus, yard trimmings disposal to landfills is the product of the quantity discarded and the proportion of discards managed in landfills (see Table 6-11). The carbon storage factors were obtained from EPA (1998). Uncertainty The principal source of uncertainty for the landfill carbon storage estimates stem from an incomplete understanding of the long-term fate of carbon in landfill environments. Although there is ample field evidence that many landfilled organic materials remain virtually intact for long periods, the quantitative basis for predicting long-term storage is based on limited laboratory results under experimental conditions. In reality, there is likely to be considerable heterogeneity in storage rates, based on (1) actual composition of yard trimmings (e.g., oak leaves decompose more slowly than grass clippings) and (2) landfill characteristics (e.g., availability of moisture, nitrogen, phosphorus, etc.). Other sources of uncertainty include the estimates of yard trimmings disposal rates--which are based on extrapolations of waste composition surveys, and the extrapolation of a value for 1998 disposal from estimates for the period from 1990 through 1997. Storage 10 Note that this calculation uses a different proportion for combustion than an earlier calculation in the waste combustion section of Chapter 6. The difference arises from different sources of information with different definitions of what is included in the solid waste stream. 70-630 D-01--46 7. Waste aste management and treatment activities are sources of greenhouse gas emissions (See Figure 7-1) of the U.S. total. Waste combustion is the second largest source in this sector, emitting carbon dioxide (CO2) and nitrous oxide (NO). Smaller amounts of methane are emitted from wastewater systems by bacteria used in various treatment processes. Wastewater treatment systems are also a potentially significant source of N2O emissions; however, methodologies are not currently available to develop a complete estimate. Nitrous oxide emissions from the treatment of the human sewage component of wastewater were estimated, however, using a simplified methodology. Nitrogen oxide (NO), carbon monoxide (CO), and non-methane volatile organic compounds (NMVOCs) are emitted by each of these sources, and are addressed separately at the end of this chapter. A summary of greenhouse gas emissions from the Waste chapter is presented in Table 7-1 and Table 7-2. Overall, in 1998, waste activities generated emissions of 65.4 MMTCE, or 3.6 percent of total U.S. greenhouse gas emissions. Landfills Landfills are the largest anthropogenic source of methane (CH) emissions in the United States. In 1998, landfill emissions were approximately 58.8 MMTCE (10,268 Gg). Emissions from municipal solid waste (MSW) landfills, which received about 61 percent of the total solid waste generated in the United States, accounted for about 93 percent of total landfill emissions, while industrial landfills accounted for the remainder. Landfills also emit non-methane volatile organic compounds (NMVOCs). There are over 2,300 landfills in the United States (BioCycle 1999), with the largest landfills receiving most of the waste and generating the majority of the methane. Landfills also store carbon from biogenic sources, due to incomplete degradation of organic materials such as wood products and yard trimmings, as described in Chapter 6. Waste 7-1 Methane emissions result from the decomposition of organic landfill materials such as paper, food scraps, and yard trimmings. This decomposition process is a natural mechanism through which microorganisms derive energy. After being placed in a landfill, organic waste is initially digested by aerobic (i.e., in the presence of oxygen) bacteria. After the oxygen supply has been depleted, the remaining waste is attacked by anaerobic bacteria, which break down organic matter into substances such as cellulose, amino acids, and sugars. These substances are further broken down through fermentation into gases and short-chain organic compounds that form the substrates for the growth of methanogenic bacteria. Methane-producing anaerobic bacteria convert these fermentation products into stabilized organic materials and biogas consisting of approximately 50 percent carbon dioxide (CO2) and 50 percent methane, by volume.2 Methane production typically begins one or 2 two years after waste disposal in a landfill and may last from 10 to 60 years. Between 1990 and 1998, methane emissions from landfills were relatively constant (see Table 7-3 and Table 7-4). The roughly constant emissions estimates are a result of two offsetting trends: (1) the amount of MSW in landfills contributing to methane emissions increased, thereby increasing the potential for emissions; and (2) the amount of landfill gas collected and combusted by landfill operators also increased, thereby reducing emissions. Methane emissions from landfills are a function of several factors, including: (1) the total amount of MSW in landfills, which is related to total MSW landfilled annually for the last 30 years; (2) composition of the waste-in-place; (3) the amount of methane that is recovered and either flared or used for energy purposes; and (4) the amount of methane oxidized in landfills instead The percentage of CO2 in biogas released from a landfill may be smaller because some CO2 dissolves in landfill water (Bingemer and Crutzen 1987). Box 7-1: Biogenic Emissions and Sinks of Carbon For many countries, CO, emissions from the combustion or degredation of biogenic materials is important because of the significant amount of energy they derive from biomass (e.g., burning fuelwood). The fate of biogenic materials is also important when evaluating waste management emissions (e.g., the decomposition of grass clippings or combustion of paper). The carbon contained in paper and grass trimmings was originally removed from the atmosphere by photosynthesis, and under natural conditions, it would eventually degrade and cycle back to the atmosphere as CO2. The quantity of carbon that these degredation processes cycle through the Earth's atmosphere, waters, solis, and biota is much greater than the quantity added by anthropogenic greenhouse gas sources But the focus of the United Nations Framework Convention on Climate Change is on anthropogenic emissions-emissions resulting from human activities and subject to human control-because it is these emissions that have the potential to alter the climate by disrupting the natural balances in carbon's biogeochemical cycle, and enhancing the atmosphere's natural greenhouse effect. Thus, if CO2 emissions from biogenic materiais (e.g., paper, wood products, and yard trimmings) result from materials grown or a sustainable basis, then those emissions are considered to mimic the closed loop of the natural carbon cycle—that is, they return to the atmosphere CO2 that was originally removed by photosynthesis. Conversely, CO2 emissions from burning fossi fuels or products such as plastics derived from fossil sources would not enter the cycle were it not for human activity (ie., they were removed from permanent fossil deposits). Likewise, CH, emissions from landfilled waste would not be emitted were it not for the man-made anaerobic conditions conducive to CH, formation that exist in landfills. However, the removal of carbon from this cycling of carbon between the atmosphere and biogenic materials—which occurs when wastes of sustainable, biogenic origin (e.g., yard trimmings) are deposited in landfills-sequesters carbon. When wastes of sustainable, biogenic origin are landfilled, and do not completely decompose, the carbon that remains is effectively removed from the global carbon cycie. Landfilling of forest products and yard trimmings results in long-term storage of about 19 MMTCE and 2 to 5 MMTCE per year, respectively. Carbon storage that results from forest products and yard trimmings disposed in landfills is accounted for in Chapter 6 to comport with IPCC inventory reporting guidance regarding the tracking of carbon flows. Box 7-2: Recycling and Greenhouse Gas Emissions and Sinks U.S. waste management patterns changed dramatically in the 1990s in response to changes in economic and regulatory factors. Perhaps the most significant change from a greenhouse gas perspective was the increase in the national average recycling rate, which climbed from 16 percent in 1990 to 28 percent in 1997 (EPA 1999). This change had an important effect on emissions in several areas, primarily in regard to emissions from waste and energy activities, as well as forestry sinks. The impact of increased recycling on greenhouse gas emissions can be best understood when emissions are considered from a life cycle perspective (EPA 1998). When a material is recycled, it is used in place of virgin inputs the manufacturing process, rather than being disposed and managed as waste. The substitution of recycled inputs for virgin inputs reduces three types of emissions throughout the product life cycle. First, manufacturing processes involving recycled inputs generally require less energy than those using virgin inputs. Second, the use of recycled inputs leads to reductions in process non-energy emissions. Third, recycling reduces disposal and waste management emissions, including methane from landfills and nitrous oxide and non-biogenic carbon dioxide emissions from combustion. In addition to greenhouse gas emission reductions from manufactur ing and disposal, recycling of paper products-which are the largest component of the U.S. waste stream-results in increased forest carbon sequestration. When paper is recycled, fewer trees are needed as inputs in the manufacturing process; reduced harvest levels result in older average forest ages, with correspondingly more carbon stored. |