When it comes to energy, Ernest Moniz seems to be an "all of the above" guy. President Obama's new nominee for the Energy Department has heaped praise on everything from solar panels and efficiency to nuclear power and shale-gas fracking.

Ernest Moniz is a physicist at the Massachusetts Institute of Technology. (Dennis Poroy)

It's that last bit that has triggered some grumbling from green groups. Moniz, a physicist at the Massachusetts Institute of Technology, has argued that natural gas can serve as a "bridge fuel" to a lower-carbon future. He outlined his views at length during a 2011 Senate hearing on a report he co-authored, "The Future of Natural Gas."

"In broad terms," Moniz testified, "we find that, given the large amounts of natural gas available in the U.S. at moderate cost ... natural gas can indeed play an important role over the next couple of decades (together with demand management) in economically advancing a clean energy system."

Here's how this "bridge" is supposed to work: In the near future, cheap natural gas will elbow aside coal in the U.S. electricity sector. Since burning natural gas for electricity emits about half the carbon-dioxide that burning coal does, this will curtail U.S. emissions a bit. (Indeed, that's already happening.) That, in turn, buys us some time to make the more arduous shift to even cleaner forms of energy, like solar or wind or even nuclear.

Indeed, in his testimony Moniz argued that natural gas would need to phase out by 2050, and that "we must continue to invest in research in carbon-free sources—renewables, nuclear and CCS [carbon capture and storage] for both coal and natural gas."

Some climate hawks have looked skeptically on this whole notion. They'll point out that natural gas is still a fossil fuel, capable of quickly heating the planet. What's more, cheap shale gas could thwart the development of solar and wind. And plus, our natural-gas infrastructure still leaks an unknown amount of heat-trapping methane, which means that the climate benefits of natural gas may well be oversold.

So what's the best way to sift through this debate? Perhaps some hard numbers can help. That's what energy expert Michael Levi tried to do in his recent paper, "Climate consequences of natural gas as a bridge fuel," published in Climatic Change.

As the title suggests, Levi tried to model the actual climate consequences of using natural gas as a bridge. And what he found was striking. Say we want to stabilize the amount of carbon in the atmosphere at about 450 parts per million — giving us a shot at limiting global warming below 2°C. If that's the goal, then the world can only use natural gas for a short while. Gas consumption would have to peak by 2020 or 2030. A very short bridge.

What sort of bridge is this?

By contrast, say the world can't or refuses to meet that ambitious climate goal and instead aims to stabilize atmospheric carbon concentrations at around 550 parts per million. This is risky: It would mean a very strong chance of breaching the 2°C threshold. Still, natural gas would be useful as a bridge in this scenario, with global gas use peaking somewhere between 2020 and 2060.

Meanwhile, some climate groups have argued that even 2°C is too dangerous and that we should try to stabilize at 350 ppm, which would mean taking carbon out of the atmosphere (the world is currently hovering around 393 ppm). In this scenario, we'd basically need to stop using all fossil fuels immediately. No room for natural gas.

So those are the basic scenarios. Natural gas only makes sense as a bridge if we're willing to chance a hefty dose of global warming — with all the risks that come with it, from sea-level rise to droughts to withering food production. By contrast, if we want to avoid a 2°C rise in temperatures, much of that natural gas will likely need to stay in the ground.

That's why Levi concludes that natural gas is better thought of as a "hedging tool" than a bridge. In the event that the world's policymakers won't do anything about climate change, then natural gas is at least less damaging climate-wise than coal. But that's about it.

Further reading:

— Can natural gas help tackle climate change? A primer.

— Here's a post by Levi explaining his "bridge" paper that's well worth reading.

— My colleague Steve Mufson has a mini-profile of Moniz.

— Note that there are plenty of other environmental risks from fracking, including air pollution and the possibility of groundwater contamination. Those are also worth discussing, although note that on the flip side, natural gas is cleaner than coal when it comes to a variety of air pollutants, such as soot and mercury.