There is a new force as well: high prices underpinned the widely trumpeted investment in hydraulic fracturing, or fracking, of shale rock rich in oil and natural gas, which pushed the price of gas to some $2 per thousand cubic feet last April, down from $9 four years ago. Cheap gas, in turn, has encouraged power companies to switch to the cleaner fuel, replacing the most heavily polluting source of energy that we know, coal.

Since 2007 the share of the nation’s electricity produced by gas-powered generators has jumped to 30 percent from 21 percent; CO2 emissions from electricity generation have tumbled more than 15 percent. This new fuel brings potential problems of its own. Environmental groups have sounded the alarm about chemicals and methane leaking from wells, potentially contaminating local water supplies and releasing additional carbon into the air.

But fracking also appears, against all odds, to have brought Mr. Obama’s early, hopeful promise to cut CO2 emissions by 17 percent between 2005 and 2020 within reach.

Will our carbon footprint continue to shrink? The Energy Department forecasts that CO2 emissions will tick up nearly 2 percent this year and 0.7 percent in 2014, as the economy recovers. Coal use in power plants is also expected to rebound as gas prices rise from their 2012 trough.

Historical precedent is not promising. The drive for energy efficiency that started in the 1970s did not continue once oil prices fell in the 1980s; among other things, American drivers fell in love with S.U.V.’s and trucks. In 1981, the Ford F-series pickup truck became the nation’s best-selling light vehicle. In 1986, Ronald Reagan had the White House solar panels taken down.

Nonetheless, there are some encouraging signs that this time may be different. The shift from coal to gas-fired power plants should be sped up by new rules requiring old coal generators to install expensive environmental equipment. Oil prices are supported by fast rising demand from the developing world and are unlikely to plunge despite new sources found in Canadian tar sands and American shale.

The United States’ experience with new fuels also offers some options for countries intent on pursuing economic growth while restraining carbon emissions.