The U.S.’s energy-related CO2 emissions fell by 205 million metric tons in 2012, and by 12% between 2005 and 2012. Why? The conventional explanation is that electricity producers took advantage of lower gas prices, and started using less coal, which has a higher carbon footprint. And that idea has taken hold. A recent Wall Street Journal piece put it boldly in the lede: “U.S. carbon-dioxide emissions have fallen dramatically in recent years, in large part because the country is making more electricity with natural gas instead of coal.”
But is this true? A new study by research group CO2 Scorecard challenges it. Instead of a switch to gas, authors Shakeb Afsah and Kendyl Salcito say, the biggest contributor was energy efficiency:
CO2 Scorecard breaks it down and shows that nearly 75% of the  decline is accounted for by demand reduction primarily due to the economy-wide energy efficiency and conservation measures in the transportation, residential and commercial sectors; the mild winter in the first quarter of 2012 also gave a helping hand.
Afsah and Salcito looked at eight regions around the country. They found that, in some regions, gas also replaced sources like hydro and nuclear, leading to more emissions, not less. And, overall, there was a big drop in energy use. The coal wasn’t replaced at all.
By far, demand reduction through energy efficiency and conservation measures in the transportation, residential and commercial sectors had the largest impact on CO2 savings (~100 million metric tons)—nearly two times more than the effect of natural gas.
The question may seem arcane. But where emissions reductions come from is important to debates over climate and energy policy. And Afsah and Salcito’s conclusion could bolster the case for more efficiency investment–something the administration has been very focused on.
In one respect, if the U.S. is increasing energy efficiency, that’s not very surprising. Among industrial countries, it has a comparatively low rate of energy productivity (the amount of GDP per unit of energy), and there’s plenty of room for improvement. Maybe we’re beginning to make it?