At first glance, the shale-gas boom in the United States looks like good news for efforts to tackle global warming. Cheap natural gas is pushing out dirtier coal in the power sector, which is one reason U.S. carbon-dioxide emissions have fallen 12 percent since 2005.

But there's always been a massive caveat to this story — methane. If too much methane is leaking out of our natural-gas infrastructure, then the shale boom might be worsening climate change. And we don't quite know how much methane is seeping out, exactly.

That's why a new study this week on methane leaks by the University of Texas is such a big deal. By taking detailed measurements from select wells around the country, the study found that leaks from shale-gas fracking appear to be quite low — which implies that swapping out coal for shale gas is indeed beneficial from a climate perspective. But the "ifs" and "buts" around this study are important. So let's take a look:

The methane conundrum

As natural gas gets extracted from the ground and processed and transported, some of it can leak out into the atmosphere as methane. And methane is a powerful greenhouse-gas in its own right, trapping more than 20 times as much heat as carbon-dioxide over a 100-year period.

That complicates the climate picture. On the one hand, burning natural gas for electricity emits just half the carbon-dioxide that you get from burning coal. But if the methane "leakage rate" from all that natural-gas infrastructure gets above 3.2 percent, one recent study found, then natural gas starts to lose its climate advantage.

And no one knows what the leakage rate actually is. The Environmental Protection Agency estimated the rate at around 1.5 percent in 2013. But that's essentially an educated guess, based on engineering assumptions rather than direct measurements. Other studies have estimated even higher leak rates, with numbers varying wildly:

A new, optimistic study for natural gas

So that brings us to a new peer-reviewed paper published this week in the Proceedings of the National Academy of SciencesThis study took direct measurements of 489 shale-gas wells being drilled around the United States — a far more detailed look than had been done before. (The study was conducted by the University of Texas and funded by both the Environmental Defense Fund and nine oil and gas firms, including Shell and Chevron.)

The study concluded that the methane leakage rates from these wells were fairly low — lower, in fact, than the EPA's estimates. The EPA had estimated that about 1.2 million tons of methane were probably seeping out of these wells. But the researchers found that only around 957,000 tons of methane were coming out.

What's more, the study found that new techniques required by the EPA to capture methane after wells have been drilled were very effective at minimizing certain leaks. (These techniques are known as "green completions.") By contrast, other parts of the production process not regulated by the EPA, such as valves and chemical pumps, were leaking more methane than previously thought.

All in all, this seems like positive news for natural gas. It implies that the shale-gas boom, by displacing coal, really can reduce America's overall greenhouse-gas emissions when you take everything into account. But the story's not done yet...

Now the caveats

This study, though significant, isn't the final word. For one, the paper only looked at natural gas production — the drilling and setting up and operation of the wells. These activities accounted for about half the methane leaks from natural gas in 2011. But that still leaves the other half.

There's also a lot of methane that seeps out when that natural gas gets processed and moved across the country in pipelines. That's why Environmental Defense Fund is financing 16 different studies to get a complete look at America's natural-gas infrastructure. This study is just one piece.

Another crucial caveat: The University of Texas researchers only looked at 489 wells — or about 2 percent of the amount drilled in a given year. What's more, the wells weren't selected entirely at random. They were mainly looking at the newest wells, the ones being drilled today.

So this study gives us a decent idea of what the best practices for fracking look like. If companies are using state-of-the-art production methods, they can minimize leaks. But the study isn't necessarily representative of what's going on everywhere in the country, particularly at thousands of older wells. Case in point: The National Oceanic and Atmospheric Administration recently sampled the air over a gas field in Utah and found methane emissions 10 times as high as from the wells studied by the University of Texas.

Those older wells may still need to do a lot more to minimize leaks. "In order for the thousands of other wells in the United States to achieve a comparably low level of emissions to those observed by this study, all production companies would need to conduct more frequent leak-detection and repair and replace their aging high-bleed pneumatics with lower-bleed devices," notes James Bradbury of the World Resources Institute, who recently published a policy paper on how best to minimize methane leaks.

The University of Texas researchers also pointed out that they will need to conduct still more measurements to get a more accurate overall picture. In particular, there was one aspect of the production process that stood out: Gas operators periodically clear out liquid blockages from their wells, and this process (known as "liquids unloading") sometimes releases huge amounts of methane. It's still not clear how often this actually occurs.

What does this mean for climate change?

This new study suggests that it's certainly possible to minimize methane leaks from fracking wells — which would mean shale gas has clear climate benefits over coal in the electricity sector. But we still don't know if all the fracking wells around the country are following these best practices, and we still need a better sense of how much natural gas might be leaking out of pipelines and distribution networks.

What's more, there are still other variables at play here. If natural gas is elbowing aside coal in the electricity sector, that can help reduce emissions. But if cheap natural gas is undercutting wind or solar or low-carbon nuclear power, then the climate benefits don't materialize. (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.)

In other words, natural gas is still a fossil fuel in its own right. It may lead to fewer global-warming emissions than coal, but it still produces emissions. A recent paper from energy expert Michael Levi found that the world's natural-gas use would likely need to peak by 2020 or 2030 in order to stabilize carbon concentrations in the atmosphere at around 450 parts per million. The methane question doesn't alter that big picture overly much.

Further reading: 

-- Climate Central has a useful calculator that lets you plug in different assumptions about methane leaks and see how much climate benefit switching from coal to natural gas actually has.

-- Environmentalists are already criticizing the University of Texas study for its ties to the oil and gas industry. Other outside experts, however, say "the research and the reputations of the researchers appear solid."

-- Note also 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.

-- Can natural gas help tackle global warming? A primer.

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