WASHINGTON -- As natural gas production in the United States reaches an all-time high, a major unanswered question looms: What does growing hydraulic fracturing mean for climate change?
The Obama administration lists natural gas as one of the "clean energy sources" it wants to expand. When burned, natural gas emits about half the heat-trapping carbon dioxide as does coal. Yet production of natural gas can result in releases of methane into the air.
Methane, the primary component of natural gas, is 25 times more potent as a greenhouse gas than carbon dioxide. Methane can enter the atmosphere when gas is stored or transported, but it's particularly a concern with shale gas production during flowback -- when fracking fluids, water and gases flow out of a well after drilling but before the gas is put into pipelines.
Companies often burn or capture the methane during flowback. How extensively or effectively that's done overall, however, isn't clear.
The oil and gas industry is the biggest source of U.S. methane emissions, accounting for about 40 percent, according to the Environmental Protection Agency. The industry says that figure is inflated because equipment is widely used to keep methane from entering the air.
It's generally agreed, however, that there isn't good data on how much methane is entering the atmosphere from natural gas operations.
"Even small leaks can wind up undoing most of the global warming benefit we think we're getting when we substitute natural gas for coal," said Mark Brownstein, who leads the natural gas and oil team at the Environmental Defense Fund.
"We can continue to debate what the leak rates are. Or let's get the data and let's fix the leaks and move on," he said.
The environmental group plans to work with Southwestern Energy, Shell and other energy companies involved in natural gas production to measure methane emissions in shale gas fields nationwide and in processing and distribution. It doesn't oppose fracking but wants to reduce methane releases to fight climate change.
While industry groups say the EPA's estimate of methane emissions is too high, a study by Robert Howarth and colleagues at Cornell University released in April, which has stirred up much controversy, said the agency's estimate is far too low. Howarth said shale gas has higher greenhouse-gas emissions than coal and isn't suitable as a "bridge fuel" to cleaner energy.
Later studies have disagreed with that assessment. But Howarth and the other scientists have agreed that better data on emissions are needed.
A shale gas committee set up to advise the Energy Department last year made the same case.
And the White House has agreed that not enough is known about the effects of fracking on climate change.
"Many factors affect the greenhouse-gas footprint of natural gas production, including gas well productivity and whether methane produced during well completion is captured, flared or vented," said Steve Fetter, principal assistant director for energy at the White House Office of Science and Technology Policy.
"Although our understanding and measurement of these factors is improving, more research is needed to further shrink the uncertainty surrounding natural gas production's greenhouse-gas footprint, and the administration is supporting a number of such studies right now."
One way to greatly reduce methane emissions during flowback is to burn it. But when the flow of gas is sporadic or low, it can be difficult to keep the flare going and the gas is vented to the atmosphere instead, IHS Cambridge Energy Research Associates said in a report last year.
The energy consulting company's report said flowback water also contains some dissolved methane. When this water is pumped into open pits, the methane can evaporate into the atmosphere. Companies are increasingly using enclosed tanks instead of pits, and venting is sometimes done to release pressure in storage vessels, the report said.
The report said companies increasingly use "reduced emissions completions" to prevent methane leaks. Equipment used for this procedure separates gas and liquid hydrocarbons from the flowback fluid for sale. The industry group America's Natural Gas Alliance says this equipment is routinely used.
The EPA is under court order to release a rule April 3 to require this and other equipment that would keep air pollutants out of the atmosphere. The agency estimates that these measures would reduce methane emissions by 26 percent.
The American Petroleum Institute, the industry lobbying group, opposes the rule and says the cost to industry would be much higher than the EPA estimates. Howard Feldman, the institute's director of scientific and regulatory affairs, said the rules are burdensome and "a waste of time and resources."
Some companies say they're already using the equipment the EPA has been ordered to require.
"What we found is in the last couple years the technology has evolved quite a bit," said Matt Pitzarella, a spokesman for Range Resources of Fort Worth, which operates mainly in Pennsylvania.
Range has equipment that either captures or burns methane. It also uses leak detectors.
Pitzarella said the company's efforts to avoid venting methane reduce the risk of fire and costly shutdowns. He said the technological advances have lowered emissions below the EPA's estimate but added that the company supports additional research.
"We've seen very little emissions," he said.
"From our economic analysis, there are some additional expenses, but from a safety and cost perspective we think it makes sense," he said.