Two prominent analysts of human-driven global warming have offered fresh criticisms of the way Anthony Ingraffea, a Cornell University engineering professor, has been portraying the contribution of natural gas leaks to climate change.

The researchers are Raymond Pierrehumbert, a climate scientist at the University of Chicago, and Richard A. Muller, a physics professor at the University of California, Berkeley who is best known of late for his research corroborating the extent of recent climate warming. The sent separate “Your Dot” contributions building on the post I published earlier this week from Louis Derry, an associate professor in the earth and atmospheric sciences department at Cornell University.

In a July 28 Op-Ed article in The Times, Ingraffea, a prominent foe of the drilling technique best known as fracking, challenged the widespread view of natural gas as a “bridge” from coal to a cleaner energy future, saying it was a “gangplank to a warm future.” He also stresses this point in “Gasland Part 2,” the new anti-drilling film from Josh Fox.

Natural gas is mostly methane, a potent, but short-lived greenhouse gas. Careless gas drilling and development of pipelines and distribution systems can release substantial amounts of gas. Recognizing the pollution problems this poses, the Environmental Protection Agency is moving to tighten rules (too slowly, for sure). Both of these scientists say leakage is a problem, but feel the climate threat has been grossly overstated. Here are their posts:

Raymond Pierrehumbert:

Lou Derry’s post already hit a lot of the high points of what’s wrong with Ingraffea’s arguments concerning the supposed lack of climate benefits of natural gas fracking. I’d summarize Lou’s main point as being that it’s meaningless to sound off about how much more “potent” a greenhouse gas methane (natural gas) may be, compared to CO2, unless you also take into account how small methane emissions are relative to CO2. Beyond Lou’s analysis, I would like to emphasize the important implications of the short lifetime of methane as compared to CO2, which basically hangs around forever. Ingraffea and co-workers systematically abuse the concept of “global warming potential” in an effort to exaggerate the climate importance of methane leakage. The important thing to understand is that essentially all of the climate effects of methane emissions disappear within 20 years of cessation of emissions; in this sense, the climate harm caused by methane leakage is reversible. In contrast, CO2 accumulates in the atmosphere, ratcheting up the temperature irreversibly, at least out to several millennia. Therefore, if switching to natural gas from coal reduces the amount of CO2 you emit, you can tolerate quite a large amount of leakage and still come out ahead, because the warming caused by the leakage will go away quickly once you eventually stop using natural gas (and other fossil fuels), whereas the warming you would get from all the extra CO2 you’d pump out if you stuck with coal would stay around forever. Global warming potentials, which are meant to be a kind of “currency exchange rate” between amounts of methane and CO2 purporting to yield equal climate harm, do a very poor job of representing the reversible/irreversible dichotomy between methane and CO2 (as discussed in the Metrics section of my paper with Susan Solomon. I think the analysis that best captures this effect is the one done by Larry Cathles (see here and here), which concludes that even with 1 percent leakage, on the centennial time scale switching to natural gas gives you 40 percent of the benefit of switching to entirely carbon-free energy. It takes a far, far higher leakage rate to completely negate the benefits of switching, and even then you’d wind up coming out ahead from the switch a few decades after you stop using fossil fuels. There is a related flaw concerning time scales in the “2011 National Center for Atmospheric Research Study” cited in Ingraffea’s Op-Ed piece, (presumably this study by Tom Wigley). The reason Wigley concludes that switching from coal to natural gas yields little climate benefit for decades is that Wigley considers the sulfate aerosol pollution that accompanies coal burning to be a side benefit because the sunlight it reflects helps offset the warming effect of the extra CO2 emitted by coal. But this is silly, since the atmospheric lifetime of aerosols is just a matter of days, so once we stop burning coal, as we eventually must, the aerosols disappear quickly, unmasking the pent-up warming due to all the extra CO2 we emitted by not switching from coal to natural gas. And that warming, unlike the warming due to methane leakage from fracking, is essentially irreversible. The real argument against over-reliance on fracking as a solution to the climate problem is that its value as a bridge fuel has been oversold, as noted in Michael Levi’s analysis. The basic point is that, if your energy demand is growing exponentially, it doesn’t take many years of growth to eat up the factor of two advantage in carbon emissions you get by switching from coal to natural gas, if that’s all you do. Natural gas is better than coal, but that doesn’t mean it’s good enough. As Michael has put it, if you’re not careful, natural gas becomes a bridge from a coal-powered past to a coal-powered future. But it can unquestionably be helpful if the switch is accompanied by policies that promote energy efficiency and continued growth of renewables. Perhaps there could also be some carbon capture and storage for natural gas power plants in the mix. I’ve spent a certain amount of time around Ithaca, and I am well aware that there are a lot of people in the area that can’t abide the idea of fracking in any form. They are worried about industrialization of the rural landscape, heavy machinery disturbing once-quiet neighborhoods, and air pollution. Above all, they are worried about the safety of their drinking water supply. These are all legitimate concerns, and they should be addressed on their own merits. But what Ingraffea is doing in continuing to claim that natural gas is as bad as coal is not a matter of looking at the same data as everybody else and drawing different conclusions. It is more a matter of distorting science in order to support a preconceived political agenda. We don’t accept this sort of thing when it is done by self-professed climate skeptics like Pat Michaels or the Heartland Institute crowd, and we shouldn’t accept it from green-ish Cornell professors either.

Richard Muller sent the post below preceded by this note:

People are confused about the numbers, and in my mind, a primary goal of your column is to make sure readers get the numbers right. I fear that many readers looking at Pierrehumbert’s comments will simply decide that the issue is too complicated, but they will clearly remember Ingraffea’s statement that methane is 25 times worse than CO2 (or maybe 72 times worse), so it is a disaster if only a few percent leaks. In fact, it would be acceptable (although not good) to have 10 percent leak.

Here’s Muller’s piece:

The Op-Ed “Gangplank to a Warm Future” by Anthony Ingraffea contains a correct but highly misleading number – the value of the “global warming potential,” or GWP, for methane. It states the number is “at least 25 times” the greenhouse effect of CO2 even after a century. I will argue that the proper value to use when considering fugitive methane is not 25 but 3.3. This dramatically smaller value can directly affect the way people think about fugitive methane. The number 25 represents the relative greenhouse effect of equal kilograms of CH4 and CO2. But molecule per molecule, methane is only 9 times more potent than CO2. That’s because a molecule of CH4 weighs less than a molecule of CO2 by a factor of 4/11. If a methane molecule is burned, it produces one CO2 molecule. If it leaks, it adds one methane molecule to the atmosphere. The leakage causes 9 times as much greenhouse effect as it would have caused if burned. Let’s call the greenhouse potential for fugitive methane (GWPfm) 9. But even this is an overestimate if we are comparing methane to the most likely alternative, coal. Methane produces about 2 times as much energy as does coal for the same CO2. So although leaked methane is 9 times worse than burned methane, it is only 4.5 times worse than burned coal. If you are considering switching from coal to methane, 4.5 is the value to use, not 25. But even 4.5 is an overestimate if the fuel is used to produce electricity. A methane combined cycle plant is about 60 percent efficient, whereas the best coal plants operate at 44 percent. That reduces the greenhouse effect of leaked methane for electricity production down to a GWPfm of 3.3 for equal kilowatt hours produced by coal.

From this value you can show that even at a huge and unprofitable 14 percent leakage rate, the methane still offers a factor of two reduction in greenhouse effect compared to coal. The details of the calculation are posted on our Berkeley Earth website. The other GWP numbers (72 for a 25 year period; 33 as the new value about to be used by the IPCC for the 100 year period) should all be reduced by the factor of 4/11 x ½ x (44/60) = 0.13 when considering leakage danger. I am not claiming anything new or original. Everything I’ve said is well known to the experts. But if you want to make methane leakage sound scarier than it really is, you can present the short term GWPw (global warming potential per unit of weight) of 25 or 72, rather than the long term GWPfm of 3.3 or 10, and perhaps hope that the reader will do the mistaken calculation and come away overly frightened by the large value. I personally know many people working in the global warming field who have inadvertently fallen into this trap. In fact, no operating natural gas company would consciously throw away 14 percent of its profits by continuing to allow 14 percent leakage. The companies I spoke to said that it is their goal to keep leakage below 1 percent, not just for environmental reasons, but for financial ones. This is a fortunate situation in which the financial incentives match the environmental ones.

Update, Aug. 19, 12:01 p.m. | Lawrence Cathles, a professor of earth and atmospheric sciences at Cornell, has offered a third perspective, which I’ll run shortly as a fresh post.



Tom Bowman has posted a podcast of an interview with two of the authors of the paper finding a big global cost from Arctic methane emissions — Peter Wadhams and Chris Hope.