According to the Environmental Protection Agency, landfills are among the nation's largest sources of methane, a greenhouse gas far more potent than carbon dioxide and a major contributor to global warming. Riley Duren, a research scientist at the University of Arizona, is chief executive officer of Carbon Mapper, a new nonprofit consortium. Carbon Mapper announced in April it was launching "a constellation" of methane-sensing satellites with partners that include NASA, the state of California and various universities and organizations.