The EIA explains that this drop is largely the result of changes in our electricity mix. Over the past decade, shifts in sources of electricity, from coal to natural gas for example, have accounted for 68% of the total decrease in energy-related CO2 emissions.
For decades, coal was the main source of power in the U.S, but last year it was tied with natural gas, each accounting for around 33% of generation. The EIA predicts that this year, for the first time ever, we will get more of our electricity from natural gas than from any other source.
Utilities are making the switch, in part, because natural gas is cheap right now and it emits about half as much CO2 as coal when burned for electricity.
The EIA points out that when it comes year-to-year fluctuations in energy-related CO2 emissions, the economy and the weather are both big factors.