More researchers and policy analysts have had a chance to digest a new study analyzing the climate impacts of unconventional natural gas development, published this week in the journal Climatic Change Letters. Many of the reactions echo earlier complaints that some of the methane leak data on which the study’s conclusions were based are thin (a fact that the authors, including the lead author, Robert Howarth of Cornell University, conceded).
Others critics continue to suggest that the authors unduly amplified the greenhouse gas footprint of unconventional gas development by measuring the global warming potential of leaked methane over a 20-year time frame, rather than the 100 years more commonly used by the Intergovernmental Panel on Climate Change.
That choice, the critics say, jacks up methane’s global warming footprint unnecessarily, allowing the authors to reach their controversial conclusion: that unconventional natural gas development is worse than coal.
Indeed, Melanie Kenderdine, the executive director of the M.I.T. Energy Initiative, told CNBC on Tuesday that “there are major scientific organizations that think we should actually extend that hundred-year period, not shorten it.”
Dan Lashof, the director of the Climate Center at the Natural Resources Defense Council, writing at the group’s Switchboard blog, took a more balanced view. He suggested that “there is no one right answer for the appropriate time horizon to consider, so the I.P.C.C. publishes [global warming potential] values for 20, 100 and 500 years.”
He continued:
For methane the I.P.C.C. values from its most recent report are 72, 25, and 7.6, respectively. Howarth does not consider the 500 year G.W.P. and relies on a more recent study by Shindell et al. that suggests that the indirect warming impact of methane (through chemical interactions in the atmosphere) could raise its G.W.P. to 105 over 20 years and 33 over 100 years. While these higher figures were produced by well respected researchers, they have not yet been subject to the level of review and scrutiny conducted by the I.P.C.C. for its estimates. Moreover, while I can see an argument for using a time horizon shorter than 100 years, I personally believe that the 20-year G.W.P. is too short a period to be appropriate for policy analysis because it discounts the future too heavily. I calculate that over a 50-year period, the G.W.P. of methane would be in the range of 42-56, based on the I.P.C.C. and the Shindell et al. analyses.”
Mr. Howarth, responding to Ms. Kenderdine’s comments, said that the majority of peer-reviewed papers on the greenhouse gas footprint of conventional gas have tended to consider both the long- and short-term time scales. He also said he was unaware of any “major scientific organizations” supporting and extension of the hundred-year period in this context.
“This flies in the face of the I.P.C.C. 2007 report, their most recent, as well as all climate change science of which I am aware,” he said.
Meanwhile, a novel critique came Friday from Michael A. Levi, the David M. Rubenstein Senior Fellow for Energy and Environment at the Council on Foreign Relations. Mr. Levi repeats some of the concerns raised by others, but he also adds this one:
Howarth’s gas-to-coal comparisons are all done on a per energy unit basis. That means that he compares the amount of emissions involved in producing a gigajoule of coal with the amount involved in producing a gigajoule of gas. (Don’t worry if you don’t know what a gigajoule is – it doesn’t really matter.) Here’s the thing: modern gas power generation technology is a lot more efficient than modern coal generation, so a gigajoule of gas produces a lot more electricity than a gigajoule of coal. The per kWh comparison is the correct one, but Howarth doesn’t do it. This is an unforgivable methodological flaw; correcting for it strongly tilts Howarth’s calculations back toward gas, even if you accept everything else he says.
I also reached out to Mr. Howarth to see if he had any response to this. Here’s what he had to say:
The per kWh is the “correct” analysis only if the question is simply one of generating electricity from natural gas vs. coal.However, 70 percent of natural gas in the U.S. is used for purposes other than for generating electricity, such as home and commercial heating, hot water heating, transportation, and industrial energy uses such as making nitrogen fertilizer and distilling ethanol. For these purposes, natural gas has no efficiency advantage over using other fossil fuels (oil or coal). And for these uses, it would be inappropriate to use a per Kwh basis (Kwh is used only for electricity).For the 30 percent of natural gas used to generate electricity, there is indeed an efficiency advantage for natural gas over coal. We state this explicitly in the paper, in section 6: “Our analysis does not consider the efficiency of final use. If fuels are used to generate electricity, natural gas gains some advantage over coal because of greater efficiencies of generation (see Electronic Supplemental Materials). However, this does not greatly affect our overall conclusion: the GHG footprint of shale gas approaches or exceeds coal even when used to generate electricity (Table in Electronic Supplemental Materials). Further, shale-gas is promoted for other uses, including as a heating and transportation fuel, where there is little evidence that efficiencies are superior to diesel oil.”In the on-line only materials, we run through the calculation from electricity generation. …Our analysis was designed to show the importance of considering methane as part of the greenhouse gas footprint of shale gas, which had never been done before. And we designed it so that other scientists could examine it in the context for any end use, as Hughes has done for electricity. We believe the singular focus on electricity is inappropriate, given how gas is actually used, and how it is being promoted for other uses. …One other detail: Kwh is not a standard international metric unit, and therefore is not approved for use in a science journal such as the one in which we published. The joule is the approved unit, and the one we used. But that is a technical detail: the important point is the extent to which one focuses on electricity vs. other uses of energy.
Mr. Levi, in an e-mail Monday morning, replied thusly:
I’m glad to see that we agree that the per kWh approach is the “correct” one for discussions of electricity generation from gas vs. coal. But this isn’t nearly as marginal as Howarth seems to believe. Gas vs. coal is the key debate to which the paper appears to attempt to speak. The fact that 70 percent of natural gas is used for things like home heating is beside the point — no one is weighing gas against the option of shoveling coal into the boiler in their basement. Howarth also claims to have addressed the issue in the paper. But it is inconsistent for him to claim that, with the adjustment, “the GHG footprint of shale gas approaches or exceeds coal” while also saying that it “does not greatly affect our overall conclusion” — after all, the paper’s overall conclusion is that gas is unequivocally worse than coal, which is qualitatively different.
Stand by for more points and counterpoints. They are almost certain to come.
