Governors' Wind Energy Coalition

“It’s a seesaw,” said Michael W. Yackira, chairman of the Edison Electric Institute, the trade association of the investor-owned electric companies. Some of the factors are hard to quantify, making it uncertain whether, in the long term, natural gas’s net effect is positive for climate control.

The reduction is simple. When burned in a power plant, natural gas has a smaller carbon footprint than coal, and when it displaces coal, emissions decline.

If all the increase in gas-fired generation replaced coal, then the switch produced savings of 113.1 million tons of carbon a year.

But natural gas is starting to replace nuclear power, which can be seen as wiping out about 10 percent of the savings, because a reactor has a carbon footprint of nearly zero. Last year the owners of five reactors announced they would retire them. Some had mechanical problems or political opposition; some did not. But all were challenged by the drop in prices on the wholesale market, driven down by natural gas. And several other reactors are losing money and could close this year.

There are two other easy-to-see effects. The fracking for oil that has opened vast new supplies of gas is producing much of it in places where there is no pipeline. In those cases, the natural gas is burned off, or flared, because there is no way to ship it economically.

According to the Energy Information Administration, last year the producers flared enough gas to have produced 27 million megawatt-hours. That pushed emissions up by 16.5 million tons, about 15 percent as much as the reduction in coal burning saved.

And some of the natural gas escapes unburned. Its main component, methane, is a global warming gas and is far more powerful than carbon dioxide, although it does not persist quite as long in the atmosphere. Even before fracking became widespread, when natural gas was expensive to extract, there were emissions of methane. But from 2007 to 2013, the increase in gas consumption added methane with a carbon dioxide equivalent of about 19 million tons. That would wipe out another 17 percent of the savings from displacing coal. The number could be higher; some experts use a different formula to translate methane into a carbon dioxide equivalent.

There are other less apparent effects, including stunting the development of zero-carbon generating stations.

“Natural gas has also displaced some investment in renewables and nuclear,” according to a paper published in May in Environmental Science & Technology, a journal published by the American Chemical Society. The paper, written by researchers at Duke University, said that weighing all the factors, “whether the net effect is a slight decrease or increase depends on modeling assumptions.”

“Natural gas has made nuclear more challenging,” said Mr. Connaughton, who was also an environmental adviser to President George W. Bush.

It is probably also reducing the growth of wind energy, many analysts said. Wind contributes only slightly to generation capacity needs, but whenever it runs, it saves fuel, mostly natural gas, and that gas is now worth about half of what it was a decade ago.

“There is no question that depressed natural gas prices have had an adverse effect on the wind and solar industries,” said Kenneth Kimmell, the president of the Union of Concerned Scientists and the former commissioner of environmental protection in Massachusetts. “It’s stunting zero-carbon alternatives.”

He added, “Low natural gas prices decrease the benefit of energy-efficiency investments as well,” since the kilowatt-hours saved by installing a better air-conditioner or light bulb are worth less than they would have cost in a world with higher gas prices.

The problem, said Michael Greenstone, a professor of environmental economics at M.I.T., is that while zero-carbon technologies have advanced significantly in the last 10 years, “over the same period, there have been practically unimaginable advances in fossil fuels.”

Limiting carbon emissions will most likely mean resolving to leave cheap fossil fuels in the ground, he said. Almost nothing valuable is left undrilled or unmined, he said. “The history of leaving $100 bills buried in the ground is really a short one,” he said.

And while cheap natural gas helps spur economic development, it has some collateral benefits that are harder to measure, energy experts said. Paul Bledsoe, a White House aide who specialized in climate during the Clinton administration and now is a senior fellow at the German Marshall Fund, said that low natural gas prices enabled the “the suite of emissions reductions across the board” at old coal plants, including mercury, fine particles and sulfur dioxide.

Mr. Bledsoe said the low price was allowing the E.P.A. to push forward with rules limiting emissions of those conventional pollutants. Those rules have forced the retirement of many old coal plants, a step whose cost to the public is minimized by the availability of cheap natural gas as a substitute, he said; if natural gas cost today what it did in 2007, costs would be much higher and the E.P.A.’s ability to act would be much more limited.

Noting that natural gas was squeezing out some zero-carbon energy sources, Mr. Kimmell of the Union of Concerned Scientists said, “It would be a tremendous mistake to rely upon natural gas” to solve the climate problem.

“It’s not only just putting all your eggs in one basket, it’s potentially taking eggs already in the basket and allowing them to break.”