JOLIET, IL - MAY 07:  Transmission lines carry electricity from NRG Energy's Joliet Station power plant on May 7, 2015 in Joliet, Illinois. According to scientists, global carbon dioxide (CO2) concentrations have reached a new monthly record of 400 parts per million, levels that haven't been seen for about two million years. The Environmental Protection Agency (EPA) reports the combustion of fossil fuels to generate electricity is the largest single source of CO2 emissions in the United States, followed by the burning of fossil fuels for transportation.  (Photo by Scott Olson/Getty Images)