Yale Environment 360: The continuing expansion of renewable energy technologies, advances in energy efficiency, and the rapid shift from coal to natural gas for generating electricity combined to bring down U.S. carbon dioxide emissions last year to their lowest levels since 1994, according to a report by Bloomberg New Energy Finance. The report said that CO2 emissions fell 13 percent in the last five years alone, which means that the U.S. is now more than halfway toward reaching President Obamas goal of cutting emissions...