It’s interesting to see Ray Pierrehumbert and Richard Muller on the same side of a debate. In a case like that, taking the contrary position is not really a good bet!
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.
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.
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. … If you are considering switching from coal to methane, 4.5 is the value to use, not 25.
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.
Pierrehumbert’s conclusion is interesting, particularly in the light of the Shakhova controversy which it somewhat resembles in this respect:
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.