The EIA ascribes the majority of this decade-long drop—70 percent—to changes in fuel use in the electrical industry. Cheap natural gas, available due to fracking, has displaced significant amounts of coal from the energy landscape, resulting in a drop in coal production of more than 20 percent in the US (and the bankruptcy of a number of coal companies). Other contributing factors include more efficient energy use and a relatively mild winter, which lowered the energy devoted to heating.

Even though natural gas fracking is less than ideal, it’s still a lot better than coal-based energy production.

Source: US carbon emissions drop, now 12% below 2005 levels - Ars Technica