The carbon dioxide intensity of U.S. energy supply (CO2 per unit of energy) in 2009 also dropped as a result of lower natural gas prices relative to coal.

FORBES: U.S. Carbon Emissions Plunge

There was also a slight increase in the CO2 intensity of U.S. energy supply (CO2 per unit of energy) in 2010, which is in contrast to a drop of 2.4% in 2009.

FORBES: U.S. Carbon Emissions Exploded in 2010, Largest Single Year Rise in 22 Years