In general, one would expect that depletion would ease the climate problem and this is, indeed, the opinion of several (possibly most) of the authors who have studied the question. Of course, depletion doesn't mean "running out" of a resource. Rather, the effect of depletion on production is best understood in terms of energy return on energy invested (EROEI). As a resource is progressively depleted, extraction must move to more expensive (lower EROEI) sources. That increases the price of the resource, lowers the demand and, eventually, causes production to decline. So, lower production means less greenhouse gases emitted and that is good for the atmosphere.
However, there is a problem: EROEI does not measure how "dirty" a fuel is, in particular in terms of greenhouse gas emissions. Hence, even depletion may not be a solution to the climate problem (this point was made, among others, by Zecca and Chiari.) A recent paper by Howarth et al. to be published in "Climatic Change" (see also a review on the BBC site) is adding - almost literally - fuel to the fire by examining the emission of shale gas extracted by the fracking process.
As well known, "shale gas" is the new resource that is promising to bring us nothing less than "a new era of abundance" ("unless politicians and environmentalists get their way.") Unfortunately, even if this new era were real (and there are serious doubts about this point) it comes with strings attached; and what strings! The study by Howarth and his coauthors arrives to the conclusion that shale gas might be a true disaster for the environment.
Natural gas is composed mainly of methane (CH4) which is a potent greenhouse gas. Because of the higher ratio of hydrogen to carbon, it is true that natural gas produces less CO2 than other fuels when burned. But the extraction, processing, and transportation of gas is never perfectly tight and a certain amount of gas always is lost into the atmosphere. In time, atmospheric CH4 is transformed into CO2 and its warming effect is reduced; but it never disappears.
After an examination of various sources of losses, Howarth and coauthors conclude that:
Summing all estimated losses, we calculate that during the life cycle of an average shale-gas well, 3.6 to 7.9% of the total production of the well is emitted to the atmosphere as methane (Table 2). This is at least 30% more and perhaps more than twice as great as the life-cycle methane emissions we estimate for conventional gas, 1.7% to 6%.
Here are the results in graphic form for a 20 year time horizon. For a 100 year case, emissions from shale gas look less dramatically higher than the others, but are still very important.
(Note that the scale of the figure says "Grams Carbon per MJ", but it seems clear from the text that the authors are not simply reporting the mass but the warming potential of the gases).
Of course, the data and the assumptions of the study can be criticized; as done in a recent paper on Scientific American. On the whole, however, its conclusions make sense: the complex operation of "fracking"is much more prone to gas losses than the simpler extraction of conventional gas. This is bound to generate more global warming.
What is striking in the paper by Howarth and the others is that shale gas does not seem to be so expensive in EROEI terms; that is in terms of the higher amount of energy needed for extraction. So, we are facing a fundamental shortcoming in the way we account for costs and benefits of what we are doing. In monetary terms, shale gas seems to be a good deal. In EROEI (energy) terms it is probably less good but it may still provide a positive return. It is in environmental terms - in the so called "external costs" that shale gas is a disaster. We don't have enough data to show that this is a general case - that is, if what is happening with natural gas is happening with all energy resources. But, if this is the case, our problem is not that the EROEIs of fossil fuels are too low; they might be too high!
So, it may be that society is reacting to scarcity in the wrong way by following a path that is perhaps easing the situation in the short term (getting more energy) but horribly worsening the problem in the medium/long term (global warming). If it were true that shale gas will bring a "new era of abundance" in terms of energy, then we risk to move toward a climate disaster: runaway global warming. But we just can't see it coming; we lack the necessary accounting tools, we don't have the appropriate decisional structures. So, we are blithely moving onwards (anyone said "lemmings?")
So, what do we do? Right now, with a wave of rampant climate denialism engulfing about everything in the debate, the idea that we can stop fracking by traditional method of international climate treaties seems to be unthinkable (and that should be clear also from this article on the Weekly Standard). Even more unthinkable is that we could do it by telling people to install high efficiency light bulbs. So, the only hope we have to avoid a climate disaster is to beat shale gas in its own terms: economic ones. Don't forget that shale gas may well be abundant, but it is also expensive in monetary and energy terms. So, if we can deploy renewable technologies that produce electricity at a lower price than gas, then there is a chance that shale gas will stay where it is: underground.
Ugo Bardi teaches physical chemistry at the University of Florence, in Italy. He is interested in resource depletion, system dynamics modeling, climate science and renewable energy. He works with ASPO (association for the study of peak oil) and he writes on "The Oil Drum" and in a number of Italian blogs. He has written two books on crude oil, in Italian, and his first book in English is "The Limits to Growth Revisited" which will be published by Springer in spring of 2011. He lives on the hills of Fiesole, near Florence, Italy.