From 2007 to 2009, when emissions declined the most, the study finds that 83 percent of the decrease was due to economic factors including consumption and production changes, and just 17 percent of the decline related to changes in the fuel mix.
Feng K, Davis SJ, Sun L, Hubacek K. 2015. Drivers of the US CO2 emissions 1997-2013. Nature Communications. doi: 10.1038/NCOMMS8714