The reason that Carnegie Mellon's report claims than natural gas has lower carbon footprint that coal compared to the Howarth Cornell report which makes an opposite conclusion lies in some fundamental difference in the factors used by these researchers.
Carnegie Mellow authors use the 100 year time period and a factor of 25 for the heat trapping effect of methane greenhouse gas equivalent to CO2 based on the IPCC's 4th Assessment Report issued in 2007.
Cornell's Howarth uses a factor of 33 for 100 years and a factor of 105 for 20 years.
The global warming potential factors for methane of 33 for 100 years and 105 for the 20 year time period are factors that have been peer reviewed (Lelieveld, et. al. 2005) and a number of experts expect these factors to become the IPCC's factors in their 2012 report.
Carnegie Mellons' report cites methane industry leakage at 2% based on EPA 1996. This year EPA updated it's estimates of methane emissions from the industry which significantly increased the expected rates of methane release from a variety of sources. (See Greenhouse Gas Reporting from the Petroleum and Natural Gas Industry, EPA, 2011)
For example, in 1996 EPA had had an emission factor for well completions from both conventional gas and non-conventional gas (i.e. shales and tight sands that use horizontal wells and fracking) of .02 metric tons of CH4 per well per year. EPA then changed this emission factor this year to .71 tons/year for a conventional well and a whopping 177 tons of CH4 per year from a non-conventional gas well -- such as a fracked Marcellus well.
Cornell's Howarth calculates total industry methane leakages between 3.6%-7.9% based on these reseachers' rigorous evaluation of data including pipeline compression leakage obtained from Russia on pipelines built in the last decade This rate is from pipelines that are newer than US interstate gas lines which have unknown but likely higher leakage rates than these newer facilities built post-Soviet collapse. Professor Howarth used these Russian sources since neither the US industries nor the Dept. of Energy had provided data on pipeline compression leaks in the U.S.
Neither the Carnegie Mellon nor the Cornell Howarth study estimates leakage in the aging system of distribution lines to the end consumer nor the gas/soil flux that may be coming from the shale gas exploration and development in the field. Such leaks could originate from from either an improper well cementing job or up through the overlying rock formations. (Under EPA's 2011 GHG reporting rules, EPA will not require the oil/gas industry to monitor soil/gas flux in any gas production field.)
Duke University's study indicates that 85% of tested domestic wells with one kilometer of fracked Marcellus wells had thermongenic methane gas, but those researchers did not determine if the pathway was from an improper well cementing job or up through the rock formations from the hydraulically-fractured gas production zone.
Carnegie Mellon's study says it assumes produced water from Marcellus gas in disposed by deep well injection. There are no deep wells in Pennsylvania to my knowledge as the deep metamorphic geologic section there is unsuitable for injection wells due to their lack of permeability and porosity.
Instead the Marcellus Shale produced water in Pa. is disposed of in sewage plants (which either Pa. or EPA may stop) and by disposal on land farm and roads or hauled to centralized waste facilities. While this is not an important factor to calculate GHG life cycle comparisons with coal, it calls into the question the Carnegie Mellon author's understanding of current practices used to develop the Marcellus Shale.
The cover story offered in the blog by John Hanger , the former director of the Pennsylvania DEP, states: "The researchers found that there was virtually no difference between greenhouse emissions from Marcellus shale gas and conventional gas production." This conflicts with what EPA has reported in its 2011 update of emission factors as noted above. In fact the very abstract from Carnegie Mellon does comport with Mr. Hanger's claim as it states that shale gas has an 11% net higher emissions that conventional gas.
"Abstract. This study estimates the life cycle greenhouse gas (GHG) emissions from the production of Marcellus shale natural gas and compares its emissions with national average US natural gas emissions produced in the year 2008, prior to any significant Marcellus shale development. We estimate that the development and completion of a typical Marcellus shale well results in roughly 5500 t of carbon dioxide equivalent emissions or about 1.8 g CO2e/MJ of gas produced, assuming conservative estimates of the production lifetime of a typical well. This represents an 11% increase in GHG emissions relative to average domestic gas..." (emphasis added)