The U.S. has cut its carbon dioxide emissions four out of the last five years, largely by switching from coal to natural gas by power companies, according to the International Energy Agency (IEA).

As electric power plants incorporate using natural gas, which has been low priced in recent years, CO2 emissions have dropped. If natural gas producers are able to increase their profits by ganging together then coal might become just as economical or even more economical to the power companies, which could result in a switch back to more coal-fired electric generation.

The IEA has tracked natural gas prices as having risen 40 percent through the first quarter of 2013 compared to a year previous, while coal producers followed suit with a 14 percent price increase compared to a year previous.

Coal-powered electric generators have been under a lot of scrutiny for air pollution, and government environmental regulators don’t want to see a comeback in coal use.