Energy Information Administration

Energy-related carbon dioxide emissions in the United States from January through March were the lowest of any recorded for the first quarter of the year since 1992, the federal Energy Information Administration reports.

The agency attributed the decline to a combination of three factors: a mild winter, reduced demand for gasoline and, most significant, a drop in coal-fired electricity generation because of historically low natural gas prices. Whether emissions will continue to drop or begin to rise again, however, remains to be seen, experts said Friday.

“While this is a positive step, we shouldn’t just say, ‘Oh, we’ve got plenty of natural gas, we can just switch to that, problem solved,’ and move on,” said Jay Apt, the director of the Carnegie Mellon Electricity Industry Center, who was not involved in compiling the study.

Carbon dioxide emissions from energy consumption totaled 1.34 billion metric tons in the first quarter, down nearly 8 percent from a year earlier, the Energy Information Administration said.

Although natural gas is a more efficient fossil fuel than coal, burning it still produces carbon dioxide emissions. One of its strengths is that it produces more kilowatts of power than the equivalent amount of coal and it provides more energy for each carbon dioxide molecule emitted into the atmosphere. This so-called carbon efficiency is a crucial factor that allows scientists to project carbon dioxide emissions, with more efficient energy sources contributing less to climate change than the more inefficient sources.

Coal-fired electric power generation puts out about twice the amount of carbon dioxide — around 2,000 pounds for every megawatt hour generated — than electricity generated by burning natural gas. But that is still about 1,100 pounds per megawatt hour for electricity from natural gas. Scientists suggest the United States needs to reduce emissions to around 350 to 400 pounds per megawatt hour to stabilize atmospheric concentrations.

The extraction of large natural gas deposits in the Marcellus Shale has contributed to the rise of inexpensive natural gas, causing prices to decline in the last four years and making it a far cheaper option than burning coal. In 2005, coal accounted for half of all electricity generated in the country. But the embrace of natural gas, which now accounts for about 30 percent of electricity generation, has caused coal’s share to retreat to 34 percent, a 40-year low.

Yet Michael Mann, a climate scientist who directs the Earth System Science Center at Penn State University, emphasized on Friday that, in addition to carbon dioxide emissions, natural gas wells contribute to other ills. When shale gas is taken from the earth, researchers suggest, “fugitive methane” – a far more potent greenhouse gas than carbon dioxide — can escape into the atmosphere through fissures in the ground.

“We may be reducing our CO2 emissions, but it is possible that we’re actually increasing the greenhouse gas problem with methane emissions,” he said.

Hydraulic fracturing, or fracking, the means by which much of the shale gas is being acquired, also raises questions about potential environmental effects like groundwater contamination, critics say.

Of course, wind and solar energy greatly outperform any fossil fuel when it comes to carbon efficiency. But last year those sectors supplied less than 5 percent of the nation’s electricity .

Dr. Apt is among those who believe government intervention would be needed to cut emissions to acceptable levels.

“If we see more and more variability in the climate, not just droughts but also more storms,” he said, “there may very well emerge a consensus that we need to finally do something to stop this very dangerous unprecedented experiment that we’re doing on the planet.”

He continued: “My fear is that if the U.S. is so laggard in greenhouse gas regulation that we will be buying technologies from abroad rather than selling them, as we did with clean air and water.”