In his State of the Union address, President Obama heaped praise on the boom in natural-gas production across the United States. And he made a comment about fracking that triggered some grumbling from green groups.



What kind of bridge is this? (AP)

"If extracted safely," Obama said, natural gas is "the 'bridge fuel' that can power our economy with less of the carbon pollution that causes climate change."

Other administration officials have made this "bridge fuel" comment before. If fracking is done right, the idea goes, then all that natural gas can provide somewhat lower-carbon electricity as an interim step as we develop more carbon-free sources of energy. That's how we'll tackle global warming.

But does this idea really make sense? It's complicated, and there are reasons to be skeptical about the whole "bridge" concept. So here's a breakdown:

How the natural gas "bridge" is supposed to work: In the near future, the glut of cheap natural gas from shale fracking will push aside coal in the U.S. electricity sector. Because burning natural gas for electricity emits about half the carbon-dioxide that burning coal does, this will reduce U.S. carbon emissions. That, in turn, buys the country time to make the more arduous shift to even cleaner forms of energy, like solar or wind or nuclear.

To some extent, this is already happening. In recent years, increased use of natural gas has helped push down U.S. carbon emissions:

But there's a catch: The United States can't keep burning natural gas indefinitely if we want to make truly deep cuts in emissions. At some point, the nation will have to transition to cleaner energy if we want to avoid significant climate change. (In some cases, natural gas can help with that — by offering a backstop for renewable power.)

The exact timeline here is often left vague, though. Last year, Energy Secretary Ernest Moniz told the Senate that natural gas use would need to get phased out by mid-century or so, 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."

So, let's get more specific.

If the world wants to meet its climate goals, the "bridge" would have to be short. In a paper last year in Climatic Change, energy expert Michael Levi laid out some hard numbers and scenarios:

A short bridge. Say the world wants to stabilize the amount of carbon-dioxide 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 use natural gas for only a brief period before transitioning to carbon-free power. Global gas consumption would have to peak by 2020 or 2030. That's a short bridge.

A medium bridge. Say the world instead decides to risk more global warming and aims for a carbon target of 550 parts per million. This would pose a very strong chance of breaching the 2°C threshold. Still, natural gas would be useful as an intermediate bridge in this scenario, with global gas use peaking somewhere between 2020 and 2060.

A permanent bridge. And what if natural gas use continued indefinitely? The International Energy Agency has calculated what would happen if the world simply replaced a great deal of coal with natural gas and left it at that. The world would still be on track to increase atmospheric carbon emissions to about 650 parts per million, "a trajectory consistent with a probable temperature rise of more than 3.5°C in the long term, well above the widely accepted 2°C target."

(Key caveat: All of these scenarios would change if companies could figure out how to capture the emissions from natural gas and bury that carbon underground. Then natural gas could be a very low-carbon energy source. For now, however, that CCS technology is fairly rare and uncertain.)

Why environmentalists are skeptical of the bridge: Some climate hawks have looked askance at this whole "bridge fuel" notion. They'll point out that natural gas is still a fossil fuel, capable of heating the planet. And, they say, there are a few problems:

1) Energy companies involved in the shale boom certainly aren't planning to phase out natural gas anytime soon. Indeed, they're now building power plants and pipelines that are expected to last decades. That could make it harder to meet the 2°C goal.



(Ken James / Bloomberg)

2) Likewise, all this cheap shale gas could actually hinder the development of carbon-free sources like solar and wind and nuclear. One study from MIT suggested that the natural-gas boom could lead to higher U.S. greenhouse-gas emissions by mid-century if it stunts the growth of renewable energy.

3) The country's natural-gas infrastructure still leaks an unknown amount of heat-trapping methane, a potent greenhouse gas in its own right. That means that the climate benefits of natural gas may well be oversold. In his paper, Levi notes that methane isn't a huge problem if natural gas acts as a very short bridge toward the 2°C target. But methane leaks could be a bigger issue the longer we use natural gas. (The Obama administration is working on a plan to address that.)

4) Some climate groups have argued that even 2°C worth of global warming is too dangerous and that we should try to stabilize at 350 parts per million, which would mean taking carbon out of the atmosphere (the world is currently hovering around 397 parts per million ). In this scenario, we'd basically need to stop using all fossil fuels immediately.

So, there are a lot of different scenarios. Natural gas makes most 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 rising sea levels, to droughts to withering food production. But if we want to avoid a 2°C rise in temperatures, much of that natural gas may need to stay in the ground.

That's why, in his paper, Levi concluded that natural gas is better thought of as a "hedging tool" than as 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 to the climate than coal. But that's a very different idea.

Further reading:

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

— Here's a post by Michael Levi explaining his "bridge" paper.

— Methane leaks are undermining the shale-gas boom. Here’s how to fix that.

— There are plenty of other environmental risks from fracking, including air pollution and the possibility of groundwater contamination. Those are also worth discussing, but 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.