The Center for Climate and Energy Solutions just reported that CO2 pollution in America is at rates last seen in the mid 1990s, partly due to power plants switching from coal to cheaper natural gas.

While natural gas is less polluting than coal, it still pollutes, and its main ingredient, methane, often leaks from wells and pipelines. Eileen Claussen, former assistant secretary of state and now president of the C2ES, says switching away from coal to natural gas is unquestionably a benefit — but it's not some solve-all.

"Natural gas is a fossil fuel and burning it does result in greenhouse gas emissions," she said.

And when it comes to methane, it's often leaked out of pipelines and wells — not in huge amounts, but at rates concerning, she says, because methane is an especially potent greenhouse gas.

"One of the challenges is to make sure there are virtually no leaks of methane as we move natural gas to the power plant, to the home, to the manufacturing facility," Claussen added.

To date, though, there's been only limited research on where methane leaks come from — fracking, for example, may lead to methane leaks, but it's not proven — but Claussen said researchers are finally started to look into it.

"Then we can apply the technological solutions that we know work to wherever those leaks are or could be," she added.

As it stands, its estimated just 1.5 percent of of methane leaks, but that's still a problem, in part because methane is released a number of other ways as well. For example, methane is released by landfills and even by cows.

"Methane is a very potent greenhouse gas with a relatively short lifetime," Claussen said. "It’s more potent than carbon dioxide, but carbon dioxide lives in the atmosphere much longer. So if you want to make short term gains in addressing greenhouse gas emissions, dealing with methane is really important."

Claussen said there are a number of technologies that can be used to help tamp down on methane leaks: zero-bleed pneumatic controllers, improved valves, corrosion resistant coatings and dry seal compressors, among them.

If there's anything to be especially optimistic about, it's that the natural gas industry is as interested as the environmentalists in eliminating leaks. Every molecule that's leaked out is lost profit.

"All the efforts to date, let’s say over the last ten years or so, have been undertaken voluntarily by the industry. EPA estimates — and these are estimates — that leaks from natural gas systems declined by about 10 percent between 1990 and 2011 even with the expansion of natural gas infrastructure," Claussen said.

Still, even with better leak controls, natural gas isn't a permanent solution.

Claussen calls it a bridge to a clean energy solution.

"It’s a pretty long bridge. We have a lot of natural gas. If we use it to substitute for other fossil fuels, greenhouse gas emissions will continue to go down," she said. "But again, we have to make sure that we have better solutions, whether it’s wind or solar or nuclear, by the time we get to 2030, 2040, 2050."