It's become something of a cliché in energy-policy discussions: The United States is making headway on global warming and slashing its carbon-dioxide emissions all because of a glut of cheap natural gas that's elbowing out dirtier coal power.

These guys also want credit. (AES)

But perhaps that natural-gas story is overly simplistic. A notable new analysis by Trevor Houser and Shashank Mohan of the Rhodium Group suggests that America's budding renewable-energy sector — particularly wind power and biomass — deserves a big chunk of the credit for driving down U.S. emissions. On the flip side, the report also suggests that coal could soon make a comeback.

Houser and Mohan take a novel approach to analyzing the recent drop in carbon pollution. They start by noting that at the end of 2012, U.S. carbon emissions were about 13 percent below 2005 levels. They then tried to tease out the causes of this drop by constructing a counter-factual — what would have happened if energy trends from the 1990s and early 2000s had continued apace?

That led them to this graph, which separates out the causes of the recent decline in emissions. (On the far left is what emissions were projected to be based on 1990-2005 trends. On the far right is what emissions actually were.)

The recession and financial crisis, obviously, made a big difference. A weaker economy has meant less demand for energy — that was responsible for more than half the drop compared with business as usual.

Meanwhile, Houser and Mohan find the U.S. economy actually hasn't become vastly less energy-intensive over time (the blue bar). Yes, overall efficiency has gone up — Americans are buying more fuel-efficient cars and trucks, etc. But the country is also no longer shedding manufacturing jobs as quickly as it was during the 1990s. So the amount of energy we use per unit of GDP has generally followed historical trends, improving only gradually.

The real change has come in the type of energy that the United States is using. The country is now relying more heavily cleaner forms of energy than it used to, and that explains about half of the fall in emissions. This is where natural gas proponents usually take credit. But according to Houser and Mohan, natural gas is only responsible for part of this shift:

Natural gas is indeed pushing out dirtier coal, and that makes a sizable difference (burning natural gas for electricity emits about half the carbon-dioxide that burning coal does). But wind farms are also sprouting up across the country, thanks to government subsidies. What's more, industrial sites are burning more biomass for heat and electricity, while biofuels like ethanol are nudging out oil. All of that has done a lot to cut emissions.

(Note that these calculations do not include fugitive methane emissions from natural gas or changes in land use from biofuel production. So that's one key caveat.)

The other side of this analysis, however, is that Houser and Mohan aren't particularly optimistic that U.S. carbon emissions will continue to decline. That's because natural gas prices are starting to rise again, and coal is likely to take back some of its market share in the years ahead:

That's one reason why many environmentalists are now warning that the United States will miss its climate target — keeping carbon emissions 17 percent below 2005 levels by 2020 — unless the Obama administration takes further steps, such as regulating coal plants through the EPA and tightening efficiency standards. The recent plunge in emissions has been dramatic. But it's not likely to last without further policy changes.

UpdateIt's worth pointing out that the Breakthrough Institute has published a response that disagrees with Rhodium's take. Their bottom line: "The coal-to-gas switch in the United States is responsible for at least 3-10 times the emissions reductions of non-hydro renewables."