The new rules add a further complication for the coal industry, which once produced more than half the nation's electricity, but which is suffering through layoffs and bankruptcies.
Alpha Natural Resources, the fourth-largest coal producer in the country, filed for Chapter 11 protection on Monday, just hours before Obama's announcement of the new rules. Since January, at least six other U.S. coal producers have filed for bankruptcy protection, according to The New York Times.
Electricity that's generated from natural gas, which is cheaper to produce and burns cleaner than coal, grew more than 51 percent over a decade to more than a trillion kilowatt-hours in 2014, according to the Energy Information Administration. Renewable sources have also gained ground in recent years.
As of July, the First Trust Nasdaq Clean Edge Energy Index Fund (QCLN), which tracks green companies like SolarCity and First Solar, is up 26 percent over three years. In contrast, FactSet's U.S. coal industry index fund lost 26 percent over that time period.