First up, we have this report by three analysts at CO2 Scorecard, Shakeb Afsah, Kendyl Salcito, and Jonathan Koomey. They tried to dissect the causes of the drop in carbon emissions between 2011 and 2012 and found that the exceptionally warm winter was, by far, the biggest factor. Heating, after all, is a major source of energy use for homes:
Now, the warm winter only explains about 43 percent of the drop. Some credit also goes to important shifts in the U.S. energy system. Natural gas has been rapidly displacing coal for the electricity generation, and natural gas tends to be less carbon-intensive than coal. That helps. Americans are also becoming more efficient at using electricity and oil. (The drop in electricity doesn't count the decline in space heating due to the warmer winter.) And wind turbines are popping up around the country, pushing emissions down even further.
Trouble is, we might not be able to rely on all these factors to continue cutting emissions. Take the warm winter. True, if global warming continues apace, it's very likely that winters in the United States will keep getting warmer over time. But those energy savings will be partly offset by increased electricity use for air conditioning as summers get hotter, too. (There's also the possibility, as Jennifer Francis of Rutgers has found, that the Arctic melt could, periodically, lead to extremely cold winters in the United States as well.)
Then there's natural gas. Thanks to refined techniques for drilling in shale rock, the United States has recently become awash in cheap natural gas, which has now overtaken coal as the country's top source of electricity. So far, the gas boom has helped drive a drop in carbon emissions from the power sector (although that plunge is partly offset by a rise in methane leaks from gas drilling — see here for a longer discussion).
But there's a possibility that natural gas won't remain at its current cheap levels forever. Over at Climate Progress, Stephen Lacey points out that the Energy Information Administration now expects natural gas prices to rebound in 2013, thanks to a slowdown in drilling. That will enable coal to make a small comeback:
Because of the projected increase in natural gas prices relative to coal, EIA expects the recent trend of substituting coal‐fired electricity generation with natural gas generation
to slow and likely reverse over the next year. ... EIA expects that coal‐fired electricity generation will increase by 9 percent in 2013, while natural gas generation will fall by about 10 percent.EIA expects carbon dioxide emissions from fossil fuels, which fell by 2.3 percent in 2011, to further decline by 2.4 percent in 2012. However, projected emissions increase by 2.8 percent in 2013, as coal regains some of its electric‐power‐generation market share.
The EIA still expects natural gas to remain quite cheap for the foreseeable future — as Michael Levi points out, even in the agency's worst-case projections, natural gas prices will merely double from today's prices of $2.65 per MMBtu to $5 per MMBtu. So natural gas won't go away. But even moderately higher gas prices will allow coal to reclaim at least a fraction of its market share. (True, recent regulations will prevent any new coal facilities from being built, but existing plants can ramp up their output.)
That's why the analysts at CO2 Scorecard argue that the United States will likely need new policies — say, a price on carbon pollution or further support for clean energy sources—if it wants to keep driving emissions down. "One should not assume," they warn, "from the first quarter data that the emission reductions attributable to warm weather will contribute to continued emission reductions on an annual basis."