Over the past four years, drilling for shale gas has sent prices tumbling by almost 50 percent, down to roughly $3 per unit as opposed to $7 or $8. This has resulted in power plants making a rapid pivot from coal to natural gas. Just seven years ago, coal-fired power plants produced roughly half of the electricity in the country. Today, coal-fired plants account for 34 percent as of last March, the lowest number since the Energy Information Agency began keeping records in the 1970s.

The switch to a cleaner burning source of energy means that overall air pollution has been decreasing.

The International Energy Agency, an autonomous entity with 28 member countries, said that the United States had cut its emissions more than any other country over the previous six years, from 6 billion metric tons to 5.2 billion metric tons. In 1990, the United States emitted 5 billion metric tons.