Recent reports in the scientific literature and popular press have created some confusion about the GHG emissions profile and the subsequent climate implications of natural gas. On the one hand, a shift to natural gas is promoted as climate change mitigation because it has a lower CO 2 emissions intensity than either oil or coal (i.e., it is commonly stated that natural gas has half the CO 2 emissions intensity of coal). On the other hand, methane—the primary constituent of natural gas—is itself a more potent GHG than CO 2 per unit of mass, and some contend that methane leakage from the production, transport, and use of natural gas has the potential to offset the GHG emissions benefits of switching. Debate continues as to whether the increased production and use of natural gas brings net benefits to the general economy, including jobs, investments, infrastructure, national security, human health, safety, and the environment. Specifically, it presents a comparative analysis of the potential climate implications of switching from coal to natural gas in the domestic electric power generating sector. 2295 , S. 15 , S. 411 , S. 1196 , and S. 1276 ); Bills that would streamline the approval, permitting, and/or construction of natural gas infrastructure (including H.R. Many of these proposals promote technology and infrastructure investments that could be significant and long lasting. For this reason, some stakeholders recommend a thorough analysis of the costs and benefits of these proposals as well as a full assessment of the economic and environmental impacts of increased natural gas development. Some see a comparative analysis of the GHG emissions from the production and use of natural gas and other fossil fuels to be a significant component in this assessment. The net climate impact of replacing other fossil fuels with natural gas depends upon a number of analytic choices, including the following: 
The fuel being replaced (e.g., coal, fuel oil, gasoline, diesel), The end-use sector (e.g., electricity generation, transportation, home heating), The equipment or facility within the sector (e.g., all existing power plants, only the least efficient existing power plants, new power plant configurations), The rate and extent to which a sector will be converted, The time period over which the impacts will be estimated, The fuel cycle (e.g., combustion cycle, production cycle, "cradle-to-grave") and specific production processes modeled (e.g., conventional vertical wells, hydraulically fractured horizontal wells), and The GHGs modeled (e.g., CO 2 , methane, nitrous oxide). Natural gas, when combusted at different types of existing U.S. power plants, produces anywhere from 42% to 63% of the CO 2 emissions of coal, depending upon the power plant technology. However, in order to more fully assess the climate impacts of a fuel employed in the power sector, analyses aim to aggregate emissions across the entire supply and utilization chain (i.e., from extraction to end use). Such analyses are referred to as life-cycle assessments (LCAs). Due to its potency as a GHG, methane lost to the atmosphere during the production and transport of fossil fuels can greatly impact the life-cycle GHG emissions estimates for power generation. DOE and EPA currently estimate a FER of around 1% in natural gas systems; a number of academic studies estimate FERs in the range of 2%-4%. Further, due to its chemical composition, methane's climate impacts are significantly more pronounced in the short term as compared to the long term. Thus, when considering existing power plants, a natural-gas-fired combined cycle power plant produces approximately 50% of the life-cycle GHG emissions of a coal-fired steam generator, both in the short and the long terms, given a FER of around 1% in natural gas systems. However, when considering other existing natural-gas-fired technologies (e.g. single cycle), or advanced technologies, the comparative life-cycle emissions benefits of natural gas are reduced. Further, when considering the possibility of higher fugitive emissions rates for natural gas production and transport activities (e.g., 2%-4%), the life-cycle GHG emissions of existing natural-gas-fired technology could be comparable to coal-fired power generation initially (within 5%-35%) and could remain within range of the coal plant's life-cycle emissions over the first 20 after the emissions (within 20%-40%). A full assessment of the costs and benefits of fuel-switching strategies would demand an integrated analysis across all issues.
Recent expansion in natural gas production has made the resource an increasingly significant component in the U.S. energy market. Further, a number of policies recently proposed and/or promulgated at the federal, state, and local levels may serve to accelerate this development. Examples of federal policies include U.S. Environmental Protection Agency air standards for power plants and vehicles, as well as bills introduced in the 114th Congress to promote increased natural gas production on federal lands, amend provisions in the tax code to incentivize natural gas production and use, and streamline the approval, permitting, and/or construction of natural gas infrastructure. Many of these proposals promote technology and infrastructure investments that could be significant and long lasting. For this reason, some stakeholders recommend a thorough analysis of the costs and benefits of these proposals as well as a full assessment of the economic and environmental impacts of increased natural gas development.
Fuel-switching strategies from other fossil fuels to natural gas have the potential to impact many segments of the general economy, including jobs, investments, infrastructure, national security, human health, safety, and the environment. A full assessment of the costs and benefits of these strategies would demand an integrated analysis across all issues. Some contend that an important component of this assessment would be a comparative analysis of the various fuels' greenhouse gas (GHG) emissions. However, reports in the scientific literature and popular press have created some confusion about the climate implications of natural gas. On the one hand, a shift to natural gas is promoted as climate change mitigation because natural gas combustion has a lower carbon dioxide (CO2) emissions intensity than either oil or coal. On the other hand, methane, the primary constituent of natural gas, is itself a more potent GHG than CO2, and some contend that methane leakage from the production, transport, and use of natural gas has the potential to offset the GHG emissions benefits of switching.
The net climate impact of replacing other fossil fuels with natural gas depends upon a number of analytic assumptions, including the choice of fuel, end-use sector, equipment, and processes modeled. This report presents a comparative analysis of the potential climate implications of switching from coal to natural gas in the domestic electric power generating sector. The findings include the following:
Natural gas, when combusted at different types of existing U.S. power plants, produces anywhere from 42% to 63% of the CO2 emissions of coal, depending upon the power plant technology. However, in order to more fully assess the climate impacts of a fuel employed in the power sector, analyses aim to aggregate emissions across the entire supply and utilization chain (i.e., from extraction to end use). Such analyses are referred to as life-cycle assessments (LCAs). Due to its potency as a GHG, methane lost to the atmosphere during the production and transport of fossil fuels (i.e., fugitive emissions) can greatly impact the life-cycle GHG emissions estimates for power generation. The Department of Energy currently estimates a fugitive emissions rate (FER) of around 1% in natural gas systems; a number of academic studies estimate rates in the range of 2%-4%. Further, due to its chemical composition, methane's climate impacts are significantly more pronounced in the short term as compared to the long term. Thus, when considering existing power plants, the average natural-gas-fired combined cycle technology produces approximately 50% of the life-cycle GHG emissions of coal-fired steam generation, both in the short and the long terms, given a FER of around 1%. However, when considering other existing natural-gas-fired technologies (e.g. single cycle) or advanced technologies, the comparative life-cycle emissions benefits of natural gas are reduced. Further, when considering the possibility of higher FERs (e.g., 2%-4%), the life-cycle GHG emissions of both existing and advanced natural-gas-fired technology may be comparable to coal-fired technology in the short term and could remain within range of coal-fired technology for several decades after emissions.