The U.S. Environmental Protection Agency (EPA) released long-awaited greenhouse gas rules for new power plants this week. Using the Clean Air Act, the agency standard would set the first national limits on the amount of carbon dioxide (CO2) emissions new power plants can emit. The EPA proposed the rule after delaying it several times since July 2011.

Power plants are the largest source of  CO2 in the nation, accounting for approximately 40 percent of these emissions, according to the Energy Information Administration. The rule basically requires new coal plants to emit the same amount of CO2 as an average plant fueled by natural gas—causing U.S. coal shares to slip following the announcement. While some in Congress already are threatening to nullify the rule, plummeting natural gas prices had much of