A New Paper on Natural Gas as a Bridge Fuel

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.