As the Obama administration inches toward a major expansion of natural gas exports, one of the thorniest questions is how that growth will affect greenhouse gas emissions, possibly worsening the problem of global warming.

Although gas contains less carbon than other fossil fuels, it emits more methane, a much more potent greenhouse gas than CO2 in the short term. Methane leaks into the atmosphere from gas production wells, and from the pipelines that deliver the gas to export terminals. Then you have to count CO2 emissions from the significant amount of energy needed to liquefy the gas so it can be shipped abroad. Finally, exports would likely boost natural gas prices—and that could encourage burning dirtier coal instead.

Quantifying all this pollution is enormously complicated, and attempts to do so can lead to some surprising results, as shown by a new study from the Department of Energy's National Energy Technology Laboratory. It reached the startling finding that in terms of global greenhouse gas emissions, for China to buy liquefied natural gas (LNG) from the United States might be no cleaner than for China to keep on burning its own coal.

The study's conclusions are discussed in detail by Joe Romm of Climate Progress here, and by Steve Mufson of the Washington Post here.

The national lab's finding is important, because as China struggles to shift away from coal in the face of soaring pollution, imported LNG is seen as "an important part of the solution," as the authoritative International Energy Agency put it this week. The IEA predicts China will nearly double its use of gas in the next five years. That would make it a major market for LNG from the United States, assuming that the Energy Department approves a significant number of the new export terminals it currently has under consideration.

Environmental groups who oppose that expansion say the greenhouse gas picture is even worse than the national lab's study suggests.

"We believe that the implementation of a massive LNG export plan would lock in place infrastructure and economic dynamics that will make it almost impossible for the world to avoid catastrophic climate change," said a coalition of 16 environmental groups opposing one proposed export terminal, the $3.8 billion Cove Point facility in Lusby, Maryland.

In a March 18 letter to President Obama, they urged him to order the Federal Energy Regulatory Commission (FERC) to conduct a full environmental impact statement for the project, rather than accept the less comprehensive assessment that the agency has deemed sufficient.

According to Daniel Weiss of the Center for American Progress, the Obama administration has already conditionally approved enough new export terminals to handle nearly one fifth of the nation's projected natural gas production in 2020. By then, exports are expected to have grown 14-fold, with another quadrupling expected by 2030.

"Ignoring the potential increase in methane pollution from future LNG exports won't make climate change go away—it will only make its impacts more deadly, destructive and expensive," he said at a recent Congressional hearing.

But when FERC has conducted full-blown environmental impact statements for export terminals, it has refused to look comprehensively at the greenhouse gas emissions the projects would entail. It typically considers only the carbon dioxide emissions at the actual site of the export projects—a fraction of the life-cycle emissions that begin the moment the drills puncture the earth and end when the gas is consumed, perhaps halfway around the world.

The Environmental Protection Agency, which is charged with overseeing the work of other agencies when they do impact studies, has repeatedly found FERC's studies wanting and urged the agency to take a broader view of greenhouse gas (GHG) emissions.

In a recent letter reviewing FERC's draft impact statement for a plant in Texas, the EPA asked FERC to consider whether the proposed project would increase the demand for domestic natural gas and the environmental impacts associated with any additional production. "We recommend that the final EIS quantify all GHG emissions associated with the project, including those emissions associated with production, transportation and combustion of the natural gas," the EPA said in the letter.

FERC has responded in similar instances that while it's reasonable to assume that gas exports would drive production up, it's not possible to say where the extra gas would come from, or how it would be produced or transported.

"The environmental impacts from induced production and pipeline transportation which may result from additional gas development are not 'reasonably foreseeable,'" it argued in another case.

The Sierra Club, in extensive comments to FERC, has challenged that stance, saying that not only is a comprehensive analysis feasible, but that it is mandated under the National Environmental Policy Act. Not only should this kind of analysis be done for each proposed export facility, the club's lawyers argue, but the cumulative effects of all such projects must be calculated to assess the overall climate effects of the project.

The Energy Department's new study takes a step in that direction—the department has filed it as part of the rulemaking docket in every LNG application case it has under review. But the study didn't look at the real-world effects of any actual project; rather, it was an attempt to think broadly about how growing exports of natural gas might compare to the status quo under a variety of assumptions about energy markets, technology, and so forth. It informs the debate, but does not settle the question.

A more comprehensive approach would be to conduct a broad life-cycle analysis looking at natural gas exports through all the projects that are being contemplated—a so-called "programmatic" environmental impact statement. But it's hard to know at this stage how many of the permit applications will actually ever be built, so even that big-picture approach would leave questions unanswered.

One problem in figuring out the greenhouse gas footprint of expanding natural gas production is that estimates of leakage have been all over the map.

One of the first scientists to warn that pollution from methane leaks could eliminate the advantage of natural gas as a low-carbon fuel, Robert W. Howarth of Cornell University, has just published a new paper based on a great deal of research conducted in the past two or three years.

"Using these new, best available data and a 20-year time period for comparing the warming potential of methane to carbon dioxide, the conclusion stands that both shale gas and conventional natural gas have a larger greenhouse gas footprint than do coal or oil," he wrote.

"Am I recommending that we continue to use coal and oil, rather than replace these with natural gas? Not at all. Society needs to wean itself from the addiction to fossil fuels as quickly as possible. But to replace some fossil fuels (coal, oil) with another (natural gas) will not suffice as an approach to take on global warming."