1.
In this guest post for the blog Dot Earth, Lawrence Cathles, a professor of earth and
atmospheric science at Cornell, adds this argument to others pointing to a substantial
climate benefit in a shift from coal to natural gas:
In a July 28 Op-Ed article in The New York Times, Cornell Professor Anthony Ingraffea reiterated his
previously widely criticized message that natural gas is not a bridge to a low carbon energy future
because it leaks. Previous comments to Dot Earth by Derry, Pierrehumbert and Muller refuted his
arguments again by pointing out, respectively, that:
 methane increases measured in the atmosphere attributable to unconventional gas leakage can
force less than 1 percent of the warming of that simultaneously measured CO2 increases can,
 sidestepping the reversible nature of methane leakage compared to the near permanence of
CO2 accumulation is deceptive and irresponsible, and
 stating that methane is a greenhouse gas 25 times more potent than CO2 is incorrect when the
issue is leakage, the comparison is to coal, and the proper comparative potency is 3.3.
To these points I would add that:
 the substitution of gas for coal has clear warming benefits even when considered within the
short term global warming potential framework used by Robert Howarth and Ingraffea, and
 from a resource availability perspective, coal poses the overwhelmingly dominant warming risk.
All agree that the comparative impact of methane depends on the leakage rate, electrical conversion
efficiency, and the time interval considered. Figure 1 is constructed using the same methods and similar
parameters as used by Howarth, Ingraffea and Santoro in their 2011 paper in Climatic Change where
they first stated that gas could be twice as bad as coal. The figure shows the crossover leakage at which
methane is as bad as coal in the short (20 year) and long (100 year) term when coal electrical plants with
a range of electrical conversion efficiencies (ece) are converted to gas plants with 60 percent ece. If the
time frame is 20 years, and the ece of both the coal and gas plants is 60 percent, the cross-over is at a
gas leakage of 2.8 percent of production. If the coal plant replaced has a 40 percent ece, the cross-over
is at 5.4 percent. For leakages less than this, substitution of gas for coal will reduced greenhouse
warming. Initially the coal plants replaced will be the oldest and most inefficient, and coal plants
probably can never have conversion efficiencies as high as 60 percent so the last case is the most
relevant at least initially.
According to E.P.A. report 430-R-13-01, the methane leakage
associated with gas production since 1990 has been less than
2.5 percent, and since 2007 has declined quite sharply, to 1.7
percent in 2011. Thus, by Ingraffea’s own methods,
substitution of coal for gas will reduce global warming and
move the world away from any tipping points, even on a 20
year timeframe. Even if “the next two decades are crucial,”
substituting gas for coal is one of the best steps we could
Figure 1. Benefit of substituting gas for coal. If
leakage is below lines, substitution will reduce
greenhouse warming.

2.
take. Since methane leakage from natural gas systems represents only 29 percent of the total U.S.
leakage (59 percent is from enteric fermentation, landfills, and manure management, according to the
E.P.A. report cited above), we can look to many places other than gas production to reduce methane
leakage if it is of concern.
My second point is that, from a resource abundance perspective, coal is the fossil fuel to avoid or
minimize. Measured in terms of the number of pre-industrial atmospheres worth of CO2 (PAL) that
burning the resource base (the resource that could be feasibly ever recovered) could add to the
atmosphere, and using the generous resource base estimates of Rogner (Ann. Rev. Energy and Env., 22,
1997, p217), natural gas could add 0.9 PAL, oil 1.2 PAL, and coal 6.6 PAL. Burning all the imaginable oil
and gas could (if no CO2 were removed) thus increases the atmospheric content of CO2 around twofold,
whereas burning all the coal would increase it 6.6 fold. We would be wise to minimize our eventual
consumption of the world’s coal resources. Here a second aspect of Rogner’s analysis is important:
Rogner emphasizes that once extraction of a resource begins, technological advances tend to keep the
cost of its extraction relatively constant until the resource base is exhausted. This means that if China
and India decide to recover coal and build big coal industries, their coal resources will likely be fully
extracted. But if they focus first on natural gas (which also has immense pollution, health, and safety
advantages) the gas resource will be exhausted with comparatively little harm, perhaps giving clean
energy sources time to eliminate the desirability of utilizing coal while most of its resource base still
remains untapped. We should do everything we can to encourage developing countries to use natural
gas first and in preference to coal. It is not what we do, but what they do, that will count most for global
warming. Making natural gas scary will not help.
Elaboration of these points and the spreadsheet used to calculate Fig. 1 can be found at
http://blogs.cornell.edu/naturalgaswarming .
Lawrence M. Cathles August 16, 2013
Professor of Earth and Atmospheric Sciences
Cornell University
Ithaca, New York 14853
lmc19@cornell.edu