methane has a global warming potential (GWP) that is 105-fold greater than carbon dioxide when viewed over a 20-year period and 33-fold greater when viewed over a 100-year period, compared mass-to-mass. Several studies which have estimated lifecycle methane leakage from shale gas development and production have found a wide range of leakage rates, from less than 1% of total production to nearly 8%. A 2011 study published in Climatic Change Letters claimed that the production of electricity using shale gas may lead to as much or more life-cycle GWP than electricity generated with oil or coal. In the peer-reviewed paper, Cornell University professor Robert W. Howarth, a marine ecologist, and colleagues claimed that once methane leak and venting impacts are included, the life-cycle greenhouse gas footprint of shale gas is far worse than those of coal and fuel oil when viewed for the integrated 20-year period after emission. On the 100-year integrated time frame, this analysis claims shale gas is comparable to coal and worse than fuel oil. However, other studies have pointed out flaws with the paper and come to different conclusions. Among those are assessments by experts at the U.S. Department of Energy, peer-reviewed studies by Carnegie Mellon University and the University of Maryland, and the Natural Resources Defense Council, which claimed that the Howarth et al. paper's use of a 20-year time horizon for global warming potential of methane is "too short a period to be appropriate for policy analysis." In January 2012, Howarth's colleagues at Cornell University, Lawrence Cathles et al., responded with their own peer-reviewed assessment, noting that the Howarth paper was "seriously flawed" because it "significantly overestimate[s] the fugitive emissions associated with unconventional gas extraction, undervalue[s] the contribution of 'green technologies' to reducing those emissions to a level approaching that of conventional gas, base[s] their comparison between gas and coal on heat rather than electricity generation (almost the sole use of coal), and assume[s] a time interval over which to compute the relative climate impact of gas compared to coal that does not capture the contrast between the long residence time of CO2 and the short residence time of methane in the atmosphere." The author of that response, Lawrence Cathles, wrote that "shale gas has a GHG footprint that is half and perhaps a third that of coal," based upon "more reasonable leakage rates and bases of comparison." In April 2013 the U.S. Environmental