One of the drivers regarding favorable tax and regulatory treatment of natural gas as an automobile fuel is that it is perceived as releasing less greenhouse gases per unit of energy output than gasoline or diesel. The other major driver is that natural gas from local sources may reduce dependence upon foreign oil.
Recently, there has been reference to a study by Robert Horwarth (of Cornell’s Dept of Ecology and Evolutionary Biology) that “ A complete consideration of all emissions from using natural gas seems likely to make natural gas far less attractive than other fossil fuels in terms of the consequences for global warming.“
This is a serious challenge to the natural gas industry, largely (in my opinion) because the public has already been exposed to the fallacies of selective greenhouse analysis for ethanol as a fuel source.
Howarth concludes that greenhouse gas emissions from hydrofracking-obtained natural gas are estimated to be 60% more than for diesel fuel and gasoline. This conclusion rests heavily upon estimates of methane leakage during production, transport, and processing. Methane is a 72-times more powerful than carbon dioxide as an atmospheric greenhouse gas.
My personal feeling is that this analysis needs to be confirmed by an independent source.
Presentation of assumptions and uncertainties behind estimates:
Considering the release during combustion alone, greenhouse gas emissions from burning natural gas average 13.7 g C of CO2 per million joules of energy compared to 18.6 for gasoline, 18.9 for diesel fuel, and 24.0 for bituminous coal (U. S. Department of Energy: http://www.eia.doe.gov/oiaf/1605/coefficients.html ). Additional emissions of greenhouse gas occur during the development, processing, and transport of natural gas (due to the use of fossil fuels to build pipelines, truck water, drill wells, make the compounds used in drilling and fracturing, and treat wastes, and the loss of carbon-trapping forests). I am aware of no rigorous estimate for these additional greenhouse gas emissions, but they appear likely to equal at least one third of those released during combustion (4.5 g C of CO2 per million joules of energy). For comparison, the greenhouse gas emissions from obtaining, processing, and transporting diesel fuel and gasoline are in the range of 8% (Howarth et al. 2009: http://cip.cornell.edu/biofuels/ ), or perhaps 1.5 g C of CO2 per million joules of energy. Note that as fossil fuel energy resources become more diffuse and difficult to obtain (as is gas in the Marcellus Shale), the energy needed to extract them and the greenhouse gas emissions associated with this effort go up substantially.
The leakage of methane gas during production, transport, processing, and use of natural gas is probably a far more important consideration. Methane is by the far the major component of natural gas, and it is a powerful greenhouse gas: 72-times more powerful than is CO2 per molecule in the atmosphere (Table 2.14 in the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4), Climate Change 2007: The Physical Science Basis. http://www.ipcc.ch/publications_and_data/publications_ipcc_fourth_assessment_report_wg1_report_the_physical_science_basis.htm ). Note that this comparison of the global warming potential of methane with CO2 is based on a 20-year assessment time; the factor decreases to 25-fold for for an 100-year assessment time. The shorter time with the higher relative global warming potential is the appropriate one, if one is concerned about the effects of methane during the time a natural gas field is developed, and for the few decades after production in the field ends. Since methane is such a powerful greenhouse gas, even small leakages of natural gas to the atmosphere have very large consequences on global warming. The most recent data I could find for the US (from 2006) suggest a leakage rate from the oil and gas industry of an amount of methane equal to 1.5% of the natural gas consumed (based on leakage data reported in EPA (2008) Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990 – 2006 and consumption data from the U.S. Department of Energy: http://www.eia.doe.gov/pub/oil_gas/natural_gas/data_publications/natural_gas_monthly/current/pdf/table_02.pdf ). This leakage rate is roughly equal to that estimated by the EPA in 1997 ( http://p2pays.net/ref/07/06348.pdf ). However, as noted by Andrew Revkin in the New York Times in October 2009, the actual leakage is not well known, as monitoring is quite limited, and “government scientists and some industry officials caution that the real figure is actually higher” ( http://www.nytimes.com/2009/10/15/business/energyenvironment/15degrees.html?_r=2&scp=1&sq=natural%20gas%20leaks%20tanks&st=cse ).
If we assume a 1.5% leakage rate, this would have a greenhouse gas warming potential equal to 14.8 g C of CO2 per million joules of energy. This would be additive to the emissions during combustion (13.7 g C of CO2 per million joules of energy) and to the emissions associated with obtaining and transporting the natural gas (very roughly estimated above as 4.5 g C of CO2 per million joules of energy). Total greenhouse gas emissions from natural gas from hydraulic fracturing may, therefore, be equivalent to 33 g C of CO2 per million joules of energy. For diesel fuel or gasoline, the total greenhouse gas emissions are equivalent to approximately 20.3 g C of CO2 per million joules of energy.
The comparison with coal is difficult, as the energy needs and greenhouse gas emissions from mining and transporting the coal are not well known. As a first cut, it may make sense to assume that these are roughly equal to one third of the emissions from direct combustion, as we have done with natural gas. If so, total emissions from coal would be equivalent to 31.9 g C of CO2 per million joules of energy, or very slightly less than the estimate for the natural gas.
