"We need a dramatic shift off carbon-based fuel: coal, oil and also gas," Bill McKibbern, the founder of 350.org, wrote in an email to The Associated Press. "Natural gas provides at best a kind of fad diet, where a dangerously overweight patient loses a few pounds and then their weight stabilizes; instead, we need at this point a crash diet, difficult to do" but needed to limit the damage from climate change.
The EPA said it made the changes based on expert reviews and new data from several sources, including a report funded by the oil and gas industry. But the estimates aren't based on independent field tests of actual emissions, and some scientists said that's a problem.
Robert Howarth, a Cornell University professor of ecology who led a 2011 methane leak study that is widely cited by critics of fracking, wrote in an email that "time will tell where the truth lies in all this, but I think EPA is wrong."
Howarth said other federal climate scientists from the National Oceanic and Atmospheric Administration have published recent studies documenting massive methane leaks from natural gas operations in Colorado and other Western states.
Howarth wrote that the EPA seems "to be ignoring the published NOAA data in their latest efforts, and the bias on industry only pushing estimates downward - never up - is quite real. EPA badly needs a counter-acting force, such as outside independent review of their process."
The issue of methane leaks has caused a major split between environmental groups.
Since power plants that burn natural gas emit about half the amount of the greenhouse gases as coal-fired power, some say that the gas drilling boom has helped the U.S. become the only major industrialized country to significantly reduce greenhouse emissions. But others believe the methane leaks negate any benefits over coal, since methane is a highly potent greenhouse gas.
The new EPA figures still show natural gas operations as the leading source of methane emissions in the U.S., at about 145 million metric tons in 2011. The next biggest source was enteric fermentation, scientific jargon for belches from cows and other animals, at 137 million metric tons. Landfills were the third-biggest source, at 103 million metric tons.
But the EPA estimates that all the sources of methane combined still account for only 9 percent of greenhouse gases, even taking into account methane's more potent heat-trapping.
The EPA said it is still seeking more data and feedback on the issue of methane leaks, so the report may change again in the future.
The EPA revisions have international implications, too. The agency says the new report, Inventory of U.S. Greenhouse Gas Emissions and Sinks, was submitted to the U.N. Framework Convention on Climate Change by an April 15 deadline.