Conventional wisdom, strongly promoted by the natural gas industry, is that natural gas drives down American emissions of carbon dioxide by substituting for carbon-rich coal. The climate stabilization plan announced by the Obama administration last week relies on that. But in other ways, cheap natural gas drives up emissions.
"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.
That part is easy to understand.
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 — including Duke Energy, which is mothballing the broken Crystal River 3 nuclear plant — announced they would retire those plants. Nuclear generation is being challenged by the drop in natural gas prices. 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 that gas 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.
The Energy Information Administration says that 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, there were emissions of methane when natural gas was expensive to extract. But from 2007-13, 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.
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," said 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 is suppressing the development of new nuclear plants, experts say, leaving the country with an aging fleet of reactors. James L. Connaughton, an adviser to Constellation Energy when it was trying to build a third reactor at the Calvert Cliffs plant south of the District of Columbia, cites the collapse of natural gas prices in 2008 as part of the reason the company abandoned that project and four that were to follow.
"Natural gas has made nuclear more challenging," said 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.