As U.S. coal consumption has fallen, its exports of coal have risen. Pictured, Midwest Generation's Crawford Generating Station, a coal-fired power plant in Chicago. The city's two coal-fired plants are closing under a deal with city officials and environmental groups. M. Spencer Green/AP hide caption

itoggle caption M. Spencer Green/AP

An oil drilling rig is seen near Ray, N.D., last September. The well is being drilled into the Bakken Formation, one of the largest contiguous deposits of oil and natural gas in the United States. Karen Bleier/AFP/Getty Images hide caption

itoggle caption Karen Bleier/AFP/Getty Images