Is it because the economy is struggling? Because we're decarbonizing our economy?
Neither. It's largely because of energy efficiency. Here's why.
Carbon emissions can be expressed by the Kaya Identity, which Roger Pielke Jr discusses in his book (which is very good, and I should have read it earlier):
total emissions = POP * (GDP/POP) * EI * CI
POP = population
GDP/POP = per capita GDP
EI = energy intensity = total energy consumption/GDP
CI = carbon intensity = carbon emissions/total energy consumption
You can gather up all this data: population from FRED, real GDP from FRED, energy consumption from the EIA, and carbon emissions from the EIA. [Actually the EIA shows total carbon emissions increasing by 11% over the last 20 years (1990-2010), but then the claim that they're at a 20-year low is based on the first four months of 2012.]
What you find is that, over the 20 years 1990-2010, population has increased by 24%, per capita GDP by 31%, energy intensity has decreased by 29%, and carbon intensity has decreased by 4%. Energy intensity is what's kept emissions down (relatively), not decarbonization of energy.
And, in fact, most of that decarbonization came in the last 5 years, due to fracking of natural gas -- carbon intensity decreased 4% over the five years 2005-2010. Population increased 5%, per capita GDP decreased 1%, and energy intensity decreased 6%.
I'll look for data past 2010. I have all of it except total energy consumption, which I just haven't looked for.
So the story is, our flat emissions over the last two decades are almost all due to using energy more efficiently, with only about 1/8th of the decrease due to decarbonizing energy. This has changed only in the last half-decade, where decarbonization has been responsible for about 40% of the emissions decline (of 6%).
It seems to me the lessons from this are:
1. Using energy more efficiently is very important, but something we all do anyway because we like to save money.Update: After I did these calculations I read a little further in Roger's book, where he does the same calculations for the globe and comes to the same conclusion: energy efficiency gains have dominated the last 20 years.
2. The economic crisis has somewhat exaggerated the emissions decline, but since it (hopefully) won't continue, further emissions reductions will be trickier.
3. We're doing very little to decarbonize the economy per se. But then, why would we, when we can dump carbon waste into the atmosphere for free? You can't beat free.
Update: Here's a plot of the evolution of each factor, relative to its 1990 value: