How Clean is the Marcellus Shale?

My friend Tom Konrad at Clean Energy Wonk, writes that natural gas from the Marcellus Shale may not be as cheap, clean and abundant as advertised. Yes, natural gas does burn cleaner than coal, once it has been extracted. But Cornell professor Robert Howarth has written a draft paper that concludes that, when it comes to greenhouse gas emissions, extracting natural gas from the Marcellus Shale could be as dirty as coal power:What is likely to make the Marcellus Shale so damaging in terms of greenhouse gas emissions is the leaking of methane from the gas fields through the many wells and coal mines that have perforated the shale formation over the years. Methane may burn cleanly, but is a potent greenhouse gas if it escapes into the atmosphere.