WASHINGTON, March 28 (Reuters) - Over half of U.S. landfills observed by aerial surveys are super-emitting sources of methane, according to a new study in the journal Science published on Thursday. The study is the largest assessment to date of methane from landfills, the third-largest source of U.S. methane emissions, and suggests an opportunity to tackle climate change by targeting a prevalent and potent greenhouse gas. It was led by research group Carbon Mapper, with researchers from NASA Jet Propulsion Laboratory, Arizona State University, University of Arizona, Scientific Aviation, and the Environmental Protection Agency.