The American Petroleum Institute said hydraulic fracturing of shale reserves helped lead to the gas production increase credited with lowering U.S. emissions. The Environmental Protection Agency said the amount for greenhouse gasses emitted in 2012 -- the last full year for which data are available -- decreased 3.4 percent from the previous year. API spokesman Zachary Cikanek said an increase in the production of cleaner-burning natural gas was in part responsible for lower reported greenhouse gas emissions. "Innovations in hydraulic fracturing and horizontal drilling have helped make the U.S. the largest producer of natural gas in the world," he said in a statement Tuesday. "America is leading the world in reducing greenhouse gasses thanks, in part, to the revolution in natural gas production." Hydraulic fracturing, known also as fracking, is highly controversial because some of the chemicals used in the process are viewed as an environmental threat. The EPA said greenhouse gas emissions in 2012 were 10 percent lower than levels reported in 2005. The agency attributed the decline to "multiple factors," including improved fuel efficiency in vehicles, lower emissions from the electricity sector, and changes in weather patterns. Carbon dioxide accounted for the bulk of reported greenhouse gas emissions and the bulk of the overall emissions came from the power sector.