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Biomass energy can be used to avoid greenhouse gas emissions from fossil fuels by providing equivalent energy services: electricity, transportation fuels, and heat. The avoided fossil fuel CO2 emissions of a biomass energy system are equal to the fossil fuels substituted by biomass energy services minus the fossil fuels used in the biomass energy system. These quantities can be estimated with a full fuel-cycle analysis of the system. The net effect on fossil fuel CO2 emissions is evident as a reduction in fossil fuel consumption.
For biomass energy to lead to an overall reduction in greenhouse gas emissions, land use and land-use change emissions of the biomass energy system must also be included (Marland and Schlamadinger, 1995). For example, if biomass is harvested and subsequently regrows without an overall loss of carbon stocks, there would be no net CO2 emissions over a full harvest/ growth cycle. In this way, land can be used continuously for the production of biomass energy to avoid fossil fuel CO2 emissions. By contrast, using land to grow carbon stocks to be conserved thereafter can only be a temporary measure to limit fossil fuel use.
Biomass can originate as a co-product of forestry or from crops grown expressly for biomass energy. For example, logging and paper mill residues are being widely used for heat production in Sweden (Gustavsson et al., 1995; Johansson and Lundqvist, 1999). In Brazil, sugar cane crops are used to provide ethanol for blending into motor vehicle fuels, and sugar cane residue (bagasse) is being used for electricity generation. The use of sugar cane biomass in Brazil led to the avoidance of fossil fuel CO2 emissions of 9.2 Mt C yr-1 through the blending of ethanol with fossil fuels in 1997 and 1998 (Macedo, 1998)-approximately 11 percent of Brazil's fossil fuel CO2 emissions (British Petroleum Company, 1999). During the next 10 years, the generation of an additional 3 GW of electrical power from sugar cane products is expected.
The use of land to hold stocks of carbon and to provide energy as a substitute for fossil fuels adds to the existing primary uses of land for agriculture, forestry, settlements, recreation, and conservation. The competition for land among these uses will partly determine the extent to which land can be used to reduce greenhouse gas concentrations. Analyses of scenarios of future development show that expanded use of biomass energy could lead to a significant reduction in atmospheric CO2 concentrations (Edmonds et al., 1996; Ishitani et al., 1996; Leemans et al., 1996). The development of technology for efficient production of biomass energy, as well as for competing land uses, will affect the amount of land available for alternative uses. Consideration of multiple uses of land to provide food and fiber while enhancing carbon stocks and producing energy may present further opportunities to reduce greenhouse gas concentrations with minimum use of resources.
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