Natural gas-fired baseload power production has life cycle greenhouse gas emissions 42to 53 percent lower than those for coal-fired baseload electricity, after accounting for a widerange of variability and compared across different assumptions of climate impact timing. Thelower emissions for natural gas are primarily due to differences in the current fleets' averageefficiency - 53 percent for natural gas versus 35 percent for coal, and a higher carbon contentper unit of energy for coal than natural gas. Even using unconventional natural gas, from tightsands, shale and coal beds, and compared with a 20-year global warming potential (GWP),natural gas-fired electricity has 39 percent lower greenhouse gas emissions than coal perdelivered megawatt-hour (MWh) using current technology.In a life cycle analysis (LCA), comparisons must be based on providing an equivalentservice or function, which in this study is the delivery of 1 MWh of electricity to an end user.This life cycle greenhouse gas inventory also developed upstream (from extraction to deliveryto a power plant) emissions for delivered energy feedstocks, including six different domesticsources of natural gas, of which three are unconventional gas, and two types of coal, and thencombines them both into domestic mixes. These are important characterizations for the LCAcommunity, and can be used as inputs into a variety of processes. However, these upstream,or cradle-to-gate, results are not appropriate to compare when making energy policydecisions, since the two uncombusted fuels do not provide an equivalent function. Theseresults highlight the importance of specifying an end-use basis-not necessarily powerproduction-when comparing different fuels.Despite the conclusion that natural gas has lower greenhouse gases than coal on adelivered power basis, the extraction and delivery of the gas has a large climate impact -32percent of U.S. methane emissions and 3 percent of U.S. greenhouse gases (EPA, 2011b). AsFigure ES-2 shows, there are significant emissions and use of natural gas-13 percent at thecity or plant gate-even without considering final distribution to small end-users. The vastmajority of the reduction in extracted natural gas -64 percent cradle-to-gate-are notemitted to the atmosphere, but can be attributed to the use of the natural gas as fuel forextraction and transport processes such as compressor operations. Increasing compressorefficiency would lower both the rate of use and the CO2 emissions associated with thecombustion of the gas for energy. Note that this figure accounts for the total mass of naturalgas extracted from the earth, including water, acid gases, and other non-methane content. But, with methane making up 75 to 95 percent of the natural gas flow, there are manyopportunities for reducing the climate impact associated with direct venting to theatmosphere. A further 24 percent of the natural gas losses can be characterized as pointsource, and have the potential to be flared-essentially a conversion of GWP-potent methaneto carbon dioxide. The conclusions drawn from this analysis are robust to a wide array of assumptions.However, as with any inventory, they are dependent on the underlying data, and there aremany opportunities to enhance the information currently being collected. This analysis showsthat the results are both sensitive to and impacted by the uncertainty of a few key parameters:use and emission of natural gas along the pipeline transmission network; the rate of naturalgas emitted during unconventional gas extraction processes such as well completion andworkovers; and the lifetime production of wells, which determine the denominator overwhich lifetime emissions are placed.This analysis inventoried both average and marginal production rates for each natural gastype, with results shown in Table ES-1. The average represents natural gas produced from allwells, including older and low productivity stripper wells. The marginal production raterepresents natural gas from newer, higher productivity wells. The largest difference was foronshore conventional natural gas, which had a 41 percent reduction in upstream greenhousegas emissions from 20.1 to 34.2 lbs CO2e/MMBtu when going from marginal to averageproduction rates. This change has little impact on emissions from power production.This inventory and analysis are for greenhouse gases only, and there are many otherfactors that must be considered when comparing energy options. A full inventory ofconventional and toxic air emissions, water use and quality, and land use is currently underdevelopment, and will allow comparison of these fuels across multiple environmentalcategories. Further, all options need to be evaluated on a sustainable energy basis, consideringfull environmental performance, as well as economic and social performance, such as theability to maintain energy reliability and security. There are many opportunities fordecreasing the greenhouse gas emissions from natural gas and coal extraction, delivery andpower production, including reducing fugitive methane emissions at wells and mines, andimplementing advanced combustion technologies and carbon capture and storage.