So in terms of contributing to greenhouse gas emissions which is worse -- coal or natural gas?
Two studies, two different answers.
A Carnegie Mellon study just published in Environmental Research Letters (8/5/11) did a lifecycle comparison of greenhouse gas emissions from coal and natural gas for electrical generation purposes and finds shale gas extracted from the Marcellus is better than coal in terms of overall greenhouse gas emissions.
Cornell published a study earlier this year in a letter to the journal Climate Change that found in terms of overall greenhouse gas emissions -- shale gas is not much better than coal.
Intrigued by these discrepant findings...read on.
The CMU Findings...
Life cycle greenhouse gas emissions of Marcellus shale gas
Mohan Jiang1, W Michael Griffin2,3, Chris Hendrickson1, Paulina Jaramillo3, Jeanne VanBriesen1 and Aranya Venkatesh1
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 (excluding combustion) and a 3% increase relative to the life cycle emissions when combustion is included. The life cycle GHG emissions of Marcellus shale natural gas are estimated to be 63–75 g CO2e/MJ of gas produced with an average of 68 g CO2e/MJ of gas produced. Marcellus shale natural gas GHG emissions are comparable to those of imported liquefied natural gas. Natural gas from the Marcellus shale has generally lower life cycle GHG emissions than coal for production of electricity in the absence of any effective carbon capture and storage processes, by 20–50% depending upon plant efficiencies and natural gas emissions variability. There is significant uncertainty in our Marcellus shale GHG emission estimates due to eventual production volumes and variability in flaring, construction and transportation.
Source: Environmental Research Letters (8/5/11)
These findings run counter to a Cornell study published earlier this year which claimed natural gas releases almost as much greenhouse gas emissions as coal when you take into account the full lifecycle.
The Cornell study used different assumptions for drilling and power plant combustion efficiencies. It selected a shorter, 20-year timeframe to calculate the global warming potential of methane emissions. CMU study numbers were based on a 100-year window, spreading out the impact. The Cornell study used data that showed the natural gas drilling, flaring, processsing and infrastructure aging process to be much "leakier" than the CMU study, which relied on more conservative figures.
NB: LCA's or lifecycle analysis studies are based on "averaged" national data. The well production rates, and methane leak rates are not yet out for Marcellus-related developement and production. Well production data, how much methane is leaked versus how much is produced for cleaner burning energy generation, will clearly affect how coal compares to natural gas. These horizonatal wells will have to produce a lot of methane to compensate for the early ineffecienices and losses...
The Cornell Findings ...
Methane and the greenhouse-gas footprint of natural gas from shale formations
by Robert W. Howarth, Renee Santoro and Anthony Ingraffea
We evaluate the greenhouse gas footprint of natural gas obtained by high-volume hydraulic fracturing from shale formations, focusing on methane emissions. Natural gas is composed largely of methane, and 3.6% to 7.9% of the methane from shale-gas production escapes to the atmosphere in venting and leaks over the life-time of a well. These methane emissions are at least 30% more than and perhaps more than twice as great as those from conventional gas. The higher emissions from shale gas occur at the time wells are hydraulically fractured—as methane escapes from flow-back return fluids—and during drill out following the fracturing. Methane is a powerful greenhouse gas, with a global warming potential that is far greater than that of carbon dioxide, particularly over the time horizon of the first few decades following emission. Methane contributes substantially to the greenhouse gas footprint of shale gas on shorter time scales, dominating it on a 20-year time horizon. The footprint for shale gas is greater than that for conventional gas or oil when viewed on any time horizon, but particularly so over 20 years. Compared to coal, the footprint of shale gas is at least 20% greater and perhaps more than twice as great on the 20-year horizon and is comparable when compared over 100 years.
Source: Climate Change Letters (4/12/2011)
Don Hopey of the Pittsburgh Post Gazette provides a short comparative review of the two studies study in Gas less polluting than coal in the long run, CMU study says
As does Scientific American Climate Footprint of Marcellus Shale Gas Could Be Less Than That of Coal
Also see this earlier post on Marcellus Media and a new NRDC report on the "leakiness" association with current natural gas development practices.