The American energy grid increasingly relies on clean natural gas instead of coal, a development made possible by hydraulic fracturing, National Public Radio reports today.

Bill Pentak stands in the middle of a construction site, looking up at his company's latest project towering overhead — a new natural gas power plant.

"This plant was sited precisely where it is because of its access to the abundant, high-quality natural gas that's found a mile to two miles beneath our feet," he says.

He's referring to the Marcellus Shale underfoot. In recent years, this part of north-central Pennsylvania has become home to one of the most productive gas fields in the world.

Pentak works for Panda Power Funds, a Dallas, Texas-based company that has two more gas plants planned for Pennsylvania.

The projects are part of a bigger story.

"What's taking place here is taking place across the United States," Pentak says.

Just a decade ago, NPR notes, around half of America’s energy derived from coal. Today, it’s down to 40 percent. Natural gas now supplies around one-third of American energy, whereas 10 years ago, it supplied one-fifth or less.

As we’ve written before, that’s excellent news for the environment. The United States today enjoys its lowest level of carbon emissions in around 20 years, a development the federal government attributes to our increased reliance on natural gas, which about half as carbon-intensive as coal.

The environmental gains stretch out over the long-term; the Energy Information Administration’s annual energy outlook, published in April, found that even as consumption grows, carbon emissions will stay relatively stable over the next 25 years; largely because of the shift toward natural gas, emissions will remain below 2005 levels, even though fossil fuel will continue to meet four-fifths of American energy demand.

But NPR’s report also highlights how natural gas—abundant, relatively easy to transport, and scalable—has helped ensure the energy grid remains reliable, even factoring in the use of less predictable green sources, such as solar and wind energy.

So when the sun isn't shining, or the wind isn't blowing, [Pentak] says gas can fill in the gaps.

"You have to have something that can be quickly dispatched to make up for that lost power, or it threatens the reliability of the grid," he says.

Fracking, though much maligned, has made a truly significant difference on emissions reductions. It’s worth noting that, despite decades of environmental regulation and green subsidy schemes, the only real progress has come from the private sector.