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U.S. carbon emissions fell sharply in 2012. But don’t expect that to last.

This chart is inspiring plenty of headlines:

The big news: U.S. carbon-dioxide emissions from energy sources dropped 3.8 percent in 2012, and hit their lowest point since 1994.

Why? Cars and trucks kept getting more fuel-efficient. Electric utilities used less coal to generate power, turning to cleaner natural gas and wind instead. And a mild winter in 2012 meant that Americans needed less energy to heat their homes. (Note that the EIA isn't providing a complete accounting of U.S. greenhouse-gas emissions, just the carbon-dioxide from energy sources like coal, oil, and natural gas.)

That all seems like encouraging news on the climate-change front. But there's reason to think the emissions drop over the past few years is about to end.

For one, carbon-dioxide emissions for energy have already risen 2.6 percent in the first half of 2013, year over year, according to preliminary data. And the Energy Information Administration expects emissions to keep rising over the next few years:

Why the rebound? Over the past few years, natural gas prices have been abnormally low, dropping below $2 per million BTU, as a flurry of fracking operations and lack of storage facilities produced a glut of shale gas on the market. Coal got crushed. Now the market's finally settling down, gas prices have rebounded somewhat, and coal is clawing back some of its market share, though not all.

To be sure, the coal industry is still on pace to decline in the coming years — due to a flurry of new regulations from the Environmental Protection Agency and continued competition from shale gas and wind. Natural gas prices are expected to stay below $6 per million BTU for the foreseeable future, making new coal plants uneconomical. But the shift is likely to be more gradual.

That's why, barring any further policy changes, the EIA expects energy-related emissions to keep rising gently in the years ahead:

The trends above are noteworthy in their own right. Thanks to the gas boom and improved efficiency, U.S. carbon emissions from energy are expected to stay below their 2005 level until at least 2040. That's a real shift. But, as the chart shows, that won't be enough to meet the White House's climate goal of a 17 percent cut in emissions by 2020.

For that, something else will have to change. As energy analysts Trevor Houser and Shashank Mohan of the Rhodium Group put it earlier this year: "Further emission reductions will require new policy, whether in the form of EPA regulation, congressional legislation or state-level action." The EPA is working on carbon regulations for existing coal and natural gas plants, but we still have no idea what those rules will look like or whether they will hold up in court.

Here's another way to look at the situation. A recent study from the Stanford Energy Modeling Forum asked 10 different expert groups to forecast the effects of the shale-gas boom in the coming years. The modelers all had different assumptions, but on average, they predicted that emissions would rise between 2010 and 2035, whether shale-gas production was low (light blue) or high (dark blue). The green bars, by contrast, predict the effects of a price on carbon:

Under a business-as-usual scenario, the modelers, on average, expect emissions to rise about 0.15 percent each year between now and 2035 (in a "low shale" case). By contrast, if Congress put in place a $25/ton carbon tax that rose steadily, the modelers expect emissions would drop roughly 1 percent per year.

That seems to be the dynamic we're facing here. U.S. carbon emissions have seen a remarkable drop over the past few years, driven by a combination of a steep economic downturn, improved efficiency and cheap natural gas. That's a big deal and has transformed the U.S. energy economy — drastically and permanently.

But that fall in emissions has stopped in 2013, and barring either some unforeseen development (another recession, say, or a big technological breakthrough) or a sharp shift in policy, it's hard to see how emissions will keep dropping.

Further reading:

— Note that even though U.S. carbon emissions are falling, global emissions keep rising to record levels, as growth in China and India swamps any gains the United States might make.