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A New Paper on Natural Gas as a Bridge Fuel

by Michael Levi
January 4, 2013

I have a new paper (PDF) in Climatic Change that explores the climate consequences of natural gas as a bridge fuel. [Update: The article is now behind a paywall. If you don’t have access, you can download an unformatted pre-print version here.]

Here’s the abstract (followed by a discussion):

Many have recently speculated that natural gas might become a “bridge fuel”, smoothing a transition of the global energy system from fossil fuels to zero carbon energy by temporarily offsetting the decline in coal use. Others have contended that such a bridge is incompatible with oft-discussed climate objectives and that methane leakage from natural gas system may eliminate any advantage that natural gas has over coal. Yet global climate stabilization scenarios where natural gas provides a substantial bridge are generally absent from the literature, making study of gas as a bridge fuel difficult. Here we construct a family of such scenarios and study some of their properties. In the context of the most ambitious stabilization objectives (450 ppm CO2), and absent carbon capture and sequestration, a natural gas bridge is of limited direct emissions-reducing value, since that bridge must be short. Natural gas can, however, play a more important role in the context of more modest but still stringent objectives (550 ppm CO2), which are compatible with longer natural gas bridges. Further, contrary to recent claims, methane leakage from natural gas operations is unlikely to strongly undermine the climate benefits of substituting gas for coal in the context of bridge fuel scenarios.

I’m not going to go through the details of the paper, but I want to discuss some of the physical intuition that underlies it, and add some explicit comparisons with a couple other papers that have garnered a lot of attention (and that motivated this work).

The underlying explanation for the results on methane is intuitively straightforward. When one models mitigation scenarios, peak temperatures are typically realized many decades after greenhouse gas emissions (and intensive natural gas use) have fallen deeply. That’s because the climate system has a lot of inertia. This means that it’s the long-term impact of methane — known to be much smaller than its short-term impact — that really influences peak temperatures. That weakens the ultimate impact of methane.

In particular, gas is never worse than coal for peak temperatures, even with 5 percent leakage, regardless of the choice of emissions target. I explore a wide range of scenario pairs that differ only in their relative use of coal and gas. In every pair, peak temperatures are higher in the cases that feature coal than in those that feature gas. This is a consequence of the phenomenon that I just mentioned: because peak temperatures lag the decline of conventional fossil fuel combustion by several decades, the effect of methane leakage largely dies out (loosely speaking) before it can influence peak temperatures much.

All of this is compounded by the fact that, if one wants to keep to an aggressive emissions target, a natural gas bridge can’t last long. A short bridge means relatively little in the way of methane leakage, and a relatively small impact on peak temperatures as a result. This corollary of this result, though, is that using gas as a bridge instead of keeping coal around a bit longer (assuming the same path for zero-carbon energy in both cases) doesn’t make much of a difference to carbon dioxide emissions if you’re trying to stabilize concentrations near 450 ppm. The bridge is simply too short for the distinction to be large.

Some of these results change a bit when you’re looking at scenarios that stabilize carbon dioxide concentrations around 550 parts per million. Extreme methane leakage can now be more consequential for peak temperatures, because the natural gas bridge is longer, allowing for more methane to be emitted. (Lower leakage rates of 1-2 percent, consistent with mainstream estimates, are still of only minor consequence.) At least as important is that substituting gas for coal in the context of such targets can be far more consequential (because fossil fuels without CCS can stick around longer). The upshot is that, even with an aspiration to keep carbon dioxide concentrations below 450 parts per million, transitioning from coal to gas may be valuable as hedge in case an ultimate transition to zero-carbon energy occurs late.

Comparisons with Howarth et al. and Wigley

These results differ from those in two papers on natural gas and methane that have garnered particularly widespread attention for their alarming results. Robert Howarth and colleagues combined high estimates of methane leakage with a focus on 20-year warming potentials to conclude that natural gas is worse for climate change than coal. The new Climatic Change paper shows that the 20-year horizon is completely inappropriate for discerning the impact of methane leaks on peak temperatures.

Tom Wigley raised a similar concern about Howarth et al. in a paper published in 2011. (He kindly helped me replicate the results in his paper.) To avoid Howarth’s reliance on global warming potentials, he constructed a scenario in which natural gas use rises strongly through 2100 and then declines through 2200, ultimately ending at approximately present levels. He then estimated the impact of methane emissions on temperature profiles over the course of his scenario, rather than on a particular time horizon, finding that methane negated any warming benefits for many decades. But there is an important limitation to that paper: natural gas use is never phased out in its scenarios. (They are not stabilization scenarios.) That makes it impossible for that paper to discern the impact of methane leakage on peak temperatures. (Temperatures never peak in the paper’s scenarios.) My new paper was originally motivated by a desire to address this issue. The result should cool down some of the alarm that the earlier paper generated.

Limits and Directions for Future Work

My new paper looks strictly at the climate consequences of bridge fuel scenarios. It does not dive into two other critical questions: Are such scenarios technologically, economically, or politically plausible? And what are their economic, security, and environmental costs and benefits? Both questions are massive and are essential to address. The paper says nothing about whether pushing into natural gas in the short run would make it more or less likely for the world to make a timely transition to zero-carbon energy after that; in-depth study of the plausibility of different pathways is essential to addressing that. Moreover, peak temperatures are only one criterion by which scenarios should be judged. Comprehensive assessments need to take issues like economic cost and local environmental consequences into account.

I can’t stress this strongly enough: My paper does not say that any particular pathway is “better” or “worse” or “preferable”. It explores some important properties of theoretical paths that have been widely discussed but poorly investigated. In doing that, it shows that recent studies have tended to overestimate the importance of methane, but that, at the same time, some commentators have given too much credit to the potential value of natural gas as a bridge fuel for achieving stringent climate goals. Taking things to the next level, and understanding how a near-term shift to gas might affect long-term trends and outcomes, will require considerably more in-depth work on how gas fits into economic and political systems.

Post a Comment 8 Comments

  • Posted by MarkB

    I have to wonder how difficult it could be to measure methane leakage. In fact, I seem to recall now that it has already been done.

    Personally, I’m far more concerned about natural gas being used as a road – to get to jobs and economic health today – than as a bridge to any hypothetical future. The immediate benefit from burning gas rather than coal is known, and in fact serves as the basis for much environmental regulation. Coal kills – today. Natural gas? Not so much.

  • Posted by Andy Kerr

    The analysis should have also considered the goal of 350 ppm. While even more “ambitious,” it is certainly a more desirable, if not necessary, target.

    “If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm, but likely less than that…. If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.”

    Hanson, James, et al. 2008. Target Atmospheric CO2: Where Should Humanity Aim?. The Open Atmospheric Science Journal. Vol. 2: 217-231. http://pubs.giss.nasa.gov/docs/2008/2008_Hansen_etal.pdf/

  • Posted by Seth Kaplan

    Well done paper that advances the conversation. One next step would be to consider (like the European Roadmap 2050 did) how this fits together when you use an active power model for the electric sector that “builds” and “dispatchs” plants. That can capture the ability of quick start gas (like the new flexible systems that involve two gas turbines paired with a heat exchanger to create plant that can be either peak or de-facto combined cycle “baseload” plant) to assist in renewable integration.

    What the Roadmap 2050 model showed was that as you moved to very high levels of renewables (wind and solar) on the system there was high value to such ability to ramp the remaining fossil generation up and down – it showed gas units going on and off over 200 times a year in 2030.

    Eventually, as the Roadmap 2050 modeling showed that to get to 80% reductions economy-wide you need 95%-100% electric reductions, you need to get rid of all gas unless CCS is commercial and even then gas with CCS would be a niche. This means that eventually storage, tons of transmission to move power around big areas to deal with wind and sun patterns and highly flexible load will be needed to balance all the wind and solar. And all those resources need to start getting built now so they can build up and be there as we ease off of methadone (gas) having kicked the heroin (coal).

  • Posted by richtfan

    michael, we don’t need to transition to a non carbon based fuel society. there is nothing wrong in any way with using carbon fuels. this is foolish.

  • Posted by Viel Blauthaven

    Yes, James Hansen would like to starve the world’s plants of their food, CO2. But that is his problem. More rational humans understand the need to expand the planet’s biomass output. And that will require a lot of CO2.

  • Posted by David Meiser

    Nature recently published a study which states that methane leakage is around 9% from some natural gas operations, how does that leakage rate figure in this study?

    http://www.nature.com/news/methane-leaks-erode-green-credentials-of-natural-gas-1.12123

    [ML: That isn't a Nature-published study -- it's a news report on a conference presentation. The team's last study was severely flawed, so I'll wait to see an actual paper.]

  • Posted by William Blackley, MD

    I don’t think some of the persons posting on this understand the science of CO2/ozone, etc. beyond an eighth grade text book.

    Writing in the journal Science, researchers concluded that elevated atmospheric CO2 actually reduces plant growth when combined with other likely consequences of climate change – namely, higher temperatures, increased precipitation or increased nitrogen deposits in the soil.

    Other coauthors of the Science study are former Stanford doctoral student Erika S. Zavaleta, now a Nature Conservancy post-doctoral fellow at the University of California-Berkeley; Nona R. Chiariello, research coordinator of Stanford’s Jasper Ridge Biological Preserve; and Elsa E. Cleland, a graduate student in the Stanford Department of Biological Sciences.
    The study was supported by the National Science Foundation, the Morgan Family Foundation, the David and Lucile Packard Foundation, the Jasper Ridge Biological Preserve, the Carnegie Institution of Washington, the U.S. Department of Energy, the U.S. Environmental Protection Agency, the Switzer Foundation and the A.W. Mellon Foundation.

    Increases in CO2 may lead to reduced plant growth, less oxygen and more CO2.

  • Posted by James Saunders

    Michael, a highly thought provoking article and paper; thanks for taking the time to break down the issues.

    I’m very interested in the recent Climate Change paper that you mentioned and I can’t immediately identify which one it is. Is there any chance you could provide a link or more complete reference?

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