(a few very minor edits have been made since this post was written. -ed).
Michael Levi has a great and provocative new paper on natural gas as a bridge fuel – that is, shifting from coal to gas generation now, with a future shift to zero-carbon renewables. At the risk of oversimplification, the main finding is that natural gas doesn’t matter much if your long-term stabilization target is 450 ppm of atmospheric carbon, since the bridge can’t be very long. A very rapid shift to minimal emissions is needed to meet this target. But if your target is 550ppm or more, it looks much more useful. A secondary finding is that methane leakage is a mostly a sideshow—largely because dangerous warming happens over the long-term, while the high impact of methane relative to CO2 is only short term (it gets broken down eventually).
It’s an interesting set of results since it shows how the inertia of the climate system works against us and for us in different contexts.
Levi’s conclusions on methane leakage contrast with those of others, whose work is more pessimistic (much more so, in the case of Howarth et al.’s 2011 paper). Unlike the other papers, however, Levi looks at stabilization scenarios, under which natural gas is eventually phased out completely. I tend to think this is a better approach. But it’s worth reading Levi’s own explanation, both to capture the detail and to see whether you agree. The bridge scenarios also result in greater peak warming rates over the next century, exacerbating risk of triggering nonlinearities (tipping points) in the climate system, even though long-term temeperatures don't change much (Levi does note this risk in the paper).
Gas opponents may take heart from Levi’s general conclusions since they likely also view 450ppm (or less) as a critical target. But I look at it as an illustration of just how deep in the hole we already are, and therefore how hard it is to hit a long term target of 450 or less (absent geoengineering). If a big switch from coal to gas doesn’t help much, and short-term big shift to renewables is needed, that target is probably out of practical reach.
If that’s true, then the only basis for opposing gas as a bridge is if you think pursuing it undermines a realistic shot at near-term renewables by pushing them out of the market instead of coal. In other words, whatever you think the chances of a big, fast shift to zero carbon energy are, you might think they are smaller if the world dabbles with gas. See this RFF report for more on how and when this might occur. Some of Levi's scenarios include gas displacing renewables, but there are constraints, and they don't (at least directly) consider effects on innovation and learning-by-doing in renewable development.
But there are targeted policy solutions for the problem: direct support for renewable R&D, renewable portfolio standards, European-style feed-in tariffs, or, less optimally, US-style investment subsidies. We may need these tools even without a gas bridge – we certainly will if our target is 450 ppm.
Because those policies are available, I think the debate over gas as a bridge fuel is based on a false dichotomy between gas and renewables. Moving from coal to gas is cheap—the market is already doing it, even without a price on carbon. With a price, the trend will accelerate. That, according to Levi’s scenarios, makes a 550ppm goal easier to reach. But it doesn’t mean giving up on doing better than 550ppm. Renewable breakthroughs – supported by targeted policies where needed – could make that possible. If those breakthroughs don’t come, we’re still much better off than if we had discouraged gas expansion. Levi notes this, characterizing a coal-to-gas switch as a “hedge.” An all-in bet on renewables might be our best shot at 450ppm or less, but it’s a risky bet. If good policies are in place to promote renewables along with a carbon price that promotes gas in the short term, we have an option with similar (if not equal) chance of hitting that target, but much less risk.
It's also possible that a shift from coal to gas could make renewable deployment easier because gas may be better than coal at filling load gaps when the sun doesn't shine or wind doesn't blow. More research (and experience) is needed here.
An all-in bet on renewables is also politically impossible. That could change in the future, but by then we might have already exceeded 450ppm anyway. The real-world options are probably only business-as-usual or a lower-carbon path dependent on gas. Levi admits that his paper does not examine the political or economic plausibility of his various scenarios. Environmentalists are often accused of making the perfect the enemy of the good. I think the charge is often overstated, but here it seems apt. If the goal is minimizing atmospheric CO2, their efforts might be better focused on getting a carbon price (or at least flexible regulation), then preserving and extending policies to support renewables.
All this ignores local environmental impacts of gas development. That’s a tough (and, if you can believe it, more controversial) issue. Prudent regulation and effective enforcement are clearly necessary. But it’s worth remembering that these local impacts have to be compared to the alternative – the local impacts of mining and burning coal, and the global impacts of climate change.