This winter, the United States reached a striking milestone. Carbon-dioxide emissions from the energy sector sank to their lowest levels in 20 years. At a glance, the country appears to be making major progress in tackling climate change. And many analysts give credit to the recent flood of cheap natural gas, which is shoving aside coal as America's top source of electricity.

Yet some environmentalists have argued that the accolades for natural gas are premature. True, the shale gas boom has led to lower carbon pollution from U.S. power plants. But the process to extract natural gas from shale rock — known as "fracking" — can release plenty of methane into the atmosphere, which also heats the planet. That muddies the climate picture a bit. What's more, natural gas is still a fossil fuel. Even if it produces less carbon than coal, it still produces a fair amount of carbon.

So how do all these factors shake out? Below is a primer, but here's the short version: It's still uncertain how big an improvement natural gas is over coal, because of those methane leaks. The good news is that those leaks can be plugged. The bad news is that even if the leaks are plugged, a flood of cheap natural gas isn't, by itself, enough to prevent the planet from heating up significantly. There's only so much more carbon the world can emit if it wants to avoid a 2°C rise in global temperatures. Natural gas can help avert drastic global warming, but only if paired with a broader set of efforts to curtail emissions.

1) Producing electricity from natural gas is less carbon-intensive than producing it from coal. If you burn natural gas to produce a certain amount of energy, you get, on average, about half as much carbon-dioxide as you get from burning coal. What's more, modern natural-gas power plants tend to be more efficient than coal plants. So natural gas beats coal from a carbon perspective. And carbon-dioxide is the main gas warming the planet. That's a key point.

2) But the production of natural gas also emits heat-trapping methane. Natural gas is itself mainly methane. And methane, when it escapes into the air, is a potent greenhouse gas — it lingers in the atmosphere for a shorter period than carbon dioxide, but it's far, far more effective at trapping heat. So every time some methane seeps out, during drilling or from pipeline leaks, natural gas's contribution to global warming goes up.

3) If these methane leaks are high enough, the climate benefit from switching to natural gas dwindles. A recent PNAS study found that if the leakage rate from natural-gas production rises to 6 percent, then a natural gas plant would contribute more to global warming than a coal plant would over the first 25 years of their lifespans. After that, however, the natural gas plant starts to win out. (That's because methane breaks down in the atmosphere more quickly, while carbon dioxide persists for hundreds of years.) But the leakage rates are one key question here.

4) Judging from existing research, natural gas appears to be an improvement over coal, though it's still not clear how much. Officially, the EPA estimates that those methane leakage rates are about 3 percent. That would make natural gas a clear winner. But the EPA number is only an estimate, and it's based on industry data that is hard to verify. One recent independent study sampled the air over a natural gas field in Colorado and found that the leakage rate might well be twice as high.

So there's a great deal of uncertainty over how much cleaner natural gas is than coal. As David McCabe, an atmospheric scientist with the Clean Air Task Force, explains, many of the half-dozen recent studies that have tried to compare coal with natural gas are plagued by questionable assumptions and flaws. "From the best of the collective work," McCabe notes, "we believe that burning natural gas for electricity produces about 30-50% less greenhouse gas than burning coal." But that's not a definitive number, and more research needs to be done here.

5) It's possible to plug those methane leaks and clean up natural gas further. The good news on natural gas is that those methane leaks can be reduced. Gas producers can employ a range of technologies, from better pipeline maintenance to dry seals on compressors, that can reduce the amount of methane escaping into the air. The Clean Air Task Force estimates that "more than half of [the leaked methane] could be eliminated, in just a few years, at little or no cost to the industry." That would make natural gas look a lot better from a climate perspective. Yet the industry will actually need to use these technologies. Currently, the EPA is proposing new rules on fracking wells that would help curb leaks from drilling.

6) Natural gas is still a fossil fuel and can't, on its own, avert significant global warming. The International Energy Agency has outlined some "golden rules" for natural-gas production that include plugging those pesky leaks at relatively low cost. If all those rules came to pass and natural gas use surged around the world, displacing coal in countries like China and India, then the IEA estimates that worldwide greenhouse gases would be about 1.3 percent lower in 2035. That's a real dent, though only a partial one. (See Andrew Revkin for a longer look at what would happen if China shifted from coal to gas.)

Now, if this was the only change made to our energy system, the IEA estimates 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." In other words, relying solely on natural gas to clean up emissions would put the world on pace for global warming that Tyndall Center director Kevin Anderson says is “likely to be beyond ‘adaptation.' "

This basically comes down to what Bill McKibben has called the "new math" of global warming. The best climate science suggests that world can only emit about 500 more gigatons of carbon by mid-century if we want a shot at staying below that 2°C threshold. Even if natural gas displaced coal entirely, we'd likely still reach that point soon (albeit at a slower pace).

7) At the moment, cheap natural gas appears to be hindering the development of even lower-carbon energy sources. In addition to pushing aside dirty coal, the flood of cheap shale gas in the United States has also undermined the advance of lower-carbon sources such as wind, solar, and nuclear power. And some analysts fear that could prove counterproductive in the long run. One study from MIT suggested that cheap natural gas could actually lead to higher greenhouse-gas emissions in the United States by 2050 if it stunts the growth of renewable energy.

"Shale gas is a great advantage to the U.S. in the short term, for the next few decades," MIT study author Henry Jacoby said in January. "But it is so attractive that it threatens other energy sources we ultimately will need."

8) Overall, natural gas can help tackle climate change if it's part of a larger, more comprehensive climate policy. As Michael Levi of the Council on Foreign Relations has pointed out, having a climate policy that explicitly reduces emissions (say, a carbon cap or tax) is likely to have a far bigger influence on the future course of climate change than how much cheap natural gas is  available. To see why, check out the graph from the U.S. Energy Information Administration below:

The top two lines predict the trajectory of U.S. power-sector emissions through 2035 if there's no climate policy at all. Whether or not there's lots and lots of natural gas doesn't make much difference. Emissions keep growing. The bottom two lines, meanwhile, assume that the United States puts a hard cap on carbon emissions. Emissions go down whether or not there's abundant natural gas. (Although cheap gas would make the transition much easier.)

"The lesson is simple," Levi notes. "It’s the policy, not the gas resource, that matters most."

So what more could be done on the policy front? As Jesse Ausebel explains here, there are ways to curb the emissions from natural gas even further. Natural gas plants might one day employ carbon-capture technology (CCS) to store their carbon underground, a technology that's still being developed. And natural gas could, in theory, help support a renewable energy system that eventually zeroes out emissions. Yet it's still not clear that either of these developments will automatically occur, unless there are changes in energy policy that actually push these steps forward.

Related: See also Levi's recent post on whether the United States should export its newly abundant natural gas. He argues that, on the margins, U.S. gas exports could help mitigate climate change if it allowed China and India to shutter some of their coal plants.