By Bill Chameides
A recent paper argues that natural gas helps make the transition ... eventually. But is that good enough? Let's do some sleuthing.
The Case of the Fugitive Methane
There's been a whole lot of energy expended in the debate about the role of natural gas in the nation's and world's energy future. Much of this debate centers around the practice of horizontal drilling and hydraulic fracturing (fracking), techniques that have opened vast reserves of shale gas for exploitation but have also sounded alarm bells for many because of concerns about fracking's environmental impacts, especially on drinking water. (For concerns, see here, here, here, here and here.) Today's focus is on another one of those environmental concerns.
Chapter 1. Climate Change Comes Calling
Another pretty hot, ongoing debate around natural gas is whether its use provides a climate benefit or not. Natural gas emits less carbon dioxide (CO2) per unit of energy generated than other fossil fuels. So for many years, the consensus was that substituting it for coal or oil would lead to less global warming. (See here and here [pdf].) And so it's been argued that it would be a valuable "transition fuel," something that could be used in place of coal or oil while the world gets its collective act together and develops a true low-carbon economy fueled by renewable sources (e.g., wind and solar) as well as, depending upon who might be doing the talking, nuclear. (See here, here, here and here.)
Chapter 2. Climate Threatened by Sinister Methane Leakage
Interestingly enough, with the advent of fracking for shale gas and the consequent jump in estimates of natural gas resources (see here and here), the use of natural gas as a transition fuel actually seems to be feasible. Good news for the climate, one might conclude.
Not really, scientists like Bob Howarth of Cornell University, protested. Why? Before answering that, you need to know a couple of background facts. First, methane, the major component of natural gas, is itself a very potent greenhouse gas -- some 21 times more effective a warmer than CO2 on a 100-year basis. And second, when we use natural gas, there are inevitably fugitive emissions, leaks during mining, transport, and consumption that allow methane to escape into the atmosphere where it can do its global warming thing. What Howarth argued in a much-debated paper published last year is that the leakage rates are so high that, contrary to conventional wisdom, transitioning from coal to natural gas would actually lead to more global warming than just sticking with coal, even though coal is the most carbon-intensive of the fossil fuels.
Since the paper's publication, other investigators and studies have weighed in on the matter, including RealClimate's Gavin Schmidt; the Council on Foreign Relations' Michael Levi; Ramón Alvarez of Environmental Defense Fund and co-authors; and another Cornell scientist, Lawrence Cathles. But a definitive conclusion has been elusive because the actual magnitude of these fugitive emissions remains very poorly defined.
Chapter 3. Methane Leakage Exonerated?
The upshot of the debate about the importance of fugitive emissions has led to a general consensus that we need a very thorough investigation into the leakage issue. In short we need to first pin down the magnitude of fugitive emissions and then cut them down by locking the methane up. (See here and here.)
But now Cornell's Cathles argues in a new paper published last week in the journal Geochemistry Geophysics Geosystems that fugitive emissions may not be that sinister after all. Or at least not if natural gas is indeed used as a bridge fuel that is first phased in as coal and some oil are phased out and then eventually is itself phased out in favor of carbon-free energy sources.
Assuming periods of 50, 100, and 200 years to make the transition from coal to natural gas to renewables, Cathles's model calculations indicate that the long-term (i.e., multiple decades to century timescales) climate impacts of the fugitive methane emissions are relatively small. The reason is that methane has a relatively short lifetime in the atmosphere -- about 12 years. And so once natural gas is no longer used as a fuel, the methane in the atmosphere from fugitive emissions will be removed from the atmosphere and so the warming from those emissions will be essentially gone. CO2 on the other hand is long-lived and so, Cathles argues, over the long term using natural gas instead of coal or oil is preferable because less CO2 will have been emitted in that scenario. Well, it's preferable provided we use natural gas as a transition fuel that eventually gives way to even cleaner renewables and/or nuclear. And then there's the issue of the short-term climate effects from fugitive emissions.
Chapter 4. The Question of the Short Term
Cathles's point about the transient effects of methane fugitive emissions is well taken. But there is a potential catch and it relates to short-term climate effects. During the transition period, when fugitive methane from using natural gas would build up in the atmosphere, there is a possibility, depending upon the magnitude of the methane emissions, that we would experience more short-term warming than if we were to have stuck with coal and oil. We might think of this as the transient version of the Howarth argument.
Now, as long as the fugitive emissions are small or the Earth system is "reversible," the transient Howarth scenario does not seem all that worrisome. But what if the emissions are large? And what if the disturbances from global warming are not reversible? Then we would have a problem. The transition to natural gas would lead to more warming for a period of time until natural gas is phased out and the excess methane is removed from the atmosphere. With the exit of the excess methane, the extra warming would also go away. Cathles seems to argue that all would be well:
"Even when methane leakage is so large (L = 10% of consumption) that substituting gas for coal and oil increases global warming in the short term, the benefit of gas substitution returns in the long term."
But it is not all that obvious that the impacts from global warming are reversible. If fragile ecosystems like coral reefs are decimated by a decade or two of extra methane-induced warming, can we be sure that they will recover once the methane is flushed from the atmosphere? Probably not.
Now for this to be a concern, fugitive emissions would need to be large -- about 10 percent or more. That's' a very remote possibility. Even so, Cathles's interesting results notwithstanding, I don't think we can ignore fugitive emissions and just assume they're too small to care about. And in any event from an economic and environmental point of view, the less of that stuff the better.
Cathles would agree. In fact he concludes his paper with the policy recommendation that leakage of methane related to natural gas production, transport, and use be reduced to about 1 percent. Let's call it the one-percent solution.