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Natural gas, which is primarily CH4, is an attractive option (relative to other fossil fuels) for reducing CO2 emissions from electricity generation. Existing natural gas power generation technologies are now in widespread use, including natural gas combined cycle (NGCC) units, which can achieve thermal efficiencies of over 50% [29]. Natural gas is less carbon intensive than coal or petroleum, and can therefore reduce the CO2 per unit of electric energy output, but has a higher global warming potential (GWP) of about 20 times that of CO2. In addition, use of natural gas can reduce emissions of NOx, PM, and other pollutants as well.

Natural gas can also be used as a transportation fuel, and has considerable potential for reducing GHG emissions. Wang and Huang evaluated life cycle emissions associated with increased natural gas use as a replacement for petroleum fuels, and estimated that, compared to gasoline-fueled vehicles, GHG emissions could be decreased by 20% for dedicated liquefied natural gas (LNG) vehicles, by nearly 50% for spark ignition LNG hybrid electric vehicles, and as much as 90% for spark ignition hybrid electric vehicles operating on gas diverted from natural gas flares [30].

In most cases studied by Wang and Huang, life cycle emissions of volatile organic compounds (VOCs), NOx, and PM were reduced when using natural gas compared to emissions from gasoline-fueled vehicles, although NOx emissions were estimated to increase (as much as 30%) for natural-gas-fired flex fuel vehicles and dedicated compressed natural gas vehicles. There has been concern over emissions of some hazardous air pollutants, particularly aldehydes, in comparison to petroleum-fueled engines, especially older diesel engine [31]. As newer engine and control systems are put into service, these differences are likely to decrease.

The production and use of natural gas results in some loss of CH4 into the atmosphere, making it important to evaluate life cycle GHG emissions rather than focusing only on CO2 emission reductions. Recent studies focused on the U.S. estimated leakage and other fugitive emissions to be between 1.1% and 1.4% [13, 32]. These and additional studies that have estimated life cycle GHG emissions indicate that total emissions across the full life cycle are substantially lower than those from the production and use of coal [33, 34]. Globally, the available data are not as detailed. Estimates for fugitive GHG emissions from petroleum and natural gas production are approximately 2.3% of total world natural gas production, which will be higher than emissions from natural gas production alone. The Energy Information Administration estimates that about 125 x 1012 cu ft (2.21 x 106 Gg at standard temperature and pressure) of gross natural gas was produced globally in 2004 [35], compared to approximately 50,000 Gg of world-wide CH4 emissions estimated by EPA as fugitive emissions from combined petroleum and natural gas production and transport [36]. Global emissions vary considerably by country, with emissions from some countries being considerably higher than the 2.3% global average [36]. In general, however, increasing production of natural gas has the potential to result in additional fugitive CH4 emissions, although measures can be employed to reduce those emissions.

Natural gas may also contain trace amounts of mercury, although there is considerable uncertainty with regard to the level of Hg in natural gas and the distribution of such contamination across different natural gas fields [37]. In general, it is very likely that displacement of coal by natural gas for electricity production will result in lower total Hg emissions, but local concentrations could increase in natural gas production fields and around processing facilities.

Additional potential impacts such as drilling fluid contamination of water bodies, increased vehicle traffic into production areas, and disturbance of ecosystems are likely to occur with increased exploration and drilling operations. If new or increased pipeline capacity is required, similar impacts will occur related to natural gas transport. There has also been considerable concern, including on the part of the U.S. Congress, regarding the increasing production of natural gas in "unconventional" formations such as the Marcellus shale formations [38]. Production of natural gas from these formations often relies on the use of hydraulic fracturing, which has the potential for contamination of water supplies by the materials used in the fracturing process.

There has been considerable discussion regarding the availability of natural gas reserves, leading to interest in methane hydrates as a possible additional source of natural gas. Methane hydrates are typically found in permafrost and deep-sea beds, and proposals for exploitation of these materials has led to concern regarding the potential for catastrophic releases of methane to the atmosphere and the subsequent increase in warming potential [39-42]. When evaluating the potential risks and benefits of such approaches, it may be appropriate to incorporate the potential for catastrophic change by using risk analysis approaches that specifically incorporate such possibilities [43].

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