This is the worst year in decades for US coal. During the first six months of 2016, US coal production was down a staggering 28 percent compared to 2015, and down 33 percent compared to 2014. For the first time ever, natural gas overtook coal as the top source of US electricity generation last year and remains that way. Over the past five years, Appalachian coal production has been cut in half and many coal-burning power plants have been retired.

This is a remarkable decline. From its peak in 2008, US coal production has declined by 500 million tons per year — that's 3,000 fewer pounds of coal per year for each man, woman and child in the United States. A typical 60-foot train car holds 100 tons of coal, so the decline is the equivalent of five million fewer train cars each year, enough to go twice around the earth.

This dramatic change has meant tens of thousands of lost coal jobs, raising many difficult social and policy questions for coal communities. But it's an unequivocal benefit for the local and global environment. The question now is whether the trend will continue in the US and, more importantly, in fast-growing economies around the world.

Health Benefits From Coal's Decline

Coal is 50 percent carbon, so burning less coal means lower carbon dioxide emissions. More than 90 percent of US coal is used in electricity generation, so as cheap natural gas and environmental regulations have pushed out coal, this has decreased the carbon intensity of US electricity generation and is the main reason why US carbon dioxide emissions are down 12 percent compared to 2005.

Perhaps even more important, burning less coal means less air pollution. Since 2010, US sulfur dioxide emissions have decreased 57 percent, and nitrogen oxide emissions have decreased 19 percent. These steep declines reflect less coal being burned, as well as upgraded pollution control equipment at about one-quarter of existing coal plants in response to new rules from the US Environmental Protection Agency.