The EPA denied a petition to regulate greenhouse gas emissions from motor vehicles for three reasons: 1) the EPA lacks authority under the Clean Air Act to regulate CO2 and other greenhouse gases for climate change purposes; 2) regulating fuel economy, which is the only practical way to reduce vehicle emissions of CO2, is assigned by Congress to the Department of Transportation; and 3) regulating greenhouse gas emissions from motor vehicles would be inappropriate at this time. The EPA took this action in response to a petition filed in 1999 by environmental groups seeking to compel the EPA to regulate greenhouse gases from motor vehicles under the Clean Air Act.