The Department of Energy has run the numbers on US carbon emissions in 2009, and found they dropped by about seven percent compared to the previous year, the largest decline the agency has ever seen. Although the economic downturn had been expected to lower emissions, the slowdown in GDP only accounted for about a third of the change. Greater efficiencies, less-energy-intensive activities, and a shift away from coal use accounted for the rest. The numbers haven't been this low since the mid-1990s and, even adjusting for the economy, take us a substantial distance towards President Obama's goal of cutting emissions to 17 percent below 2005 levels by the end of the decade.

The DOE report, compiled by its Energy Information Administration, provides both a retrospective analysis of the past few decades, and a breakdown of the factors contributing to the most recent year's decline. But both of them point in roughly the same direction: the relationship between economic growth and carbon emissions that held in the 1990s no longer exists. In that decade, carbon emissions grew at about half the rate of the US GDP. In the more recent decade, GDP growth dropped by half, but carbon emissions actually declined over the course of the decade—by an average of nearly one percent annually.

For 2009, the report ascribes the large decline in emissions to three factors that had roughly equivalent impacts: the decline in the GDP, a less energy-intensive economy, and a less carbon-intensive energy supply. The first of those factors isn't especially interesting, so we'll look at the latter two.

Residential and commercial energy use have remained pretty flat for the last three years, but transportation started a gradual decline in energy consumption in 2007. Although 2008 saw a huge decline in driving due to fuel prices, the cost of fuel dropped in 2009. As one might expect, total miles traveled rose, although only by a small fraction of a percent. Nevertheless, total fuel consumption was down from 2008 for every month of the year, spurred in part by an increase of 1.5mpg in the average fleet fuel economy. Given that the fuel economy is set to rise rapidly through 2016, this sector is likely to continue to improve.

But the big change in the energy intensity (energy burned per unit of GDP) came from the industrial sector, which has been declining since 2004. The report ascribes this to the rapid growth of the electronics industry, which is less energy intensive than industries like the production of primary metals, and therefore has a lower carbon intensity. A growth in the service economy also contributed. The industrial sector is still the largest consumer of energy in the US, with transportation now a close second.

The electric power sector became greener still, seeing a drop in carbon intensity of four percent (compared to about three percent for industrial). In fact, the drop in carbon emissions from the electric sector accounted for over half of the total national decline. The biggest contributor here was the fall in coal use, which went down by several percent. That was offset by increases in natural gas, renewables, and nuclear power.

The fall in natural gas prices that has helped drive the shift away from coal has also ensured that the plants that are now coming online are much more efficient. As a result, the amount of power generated per unit of carbon emissions has been climbing steadily over the last decade. The rise in nuclear power came despite the fact that no new plants have been built in decades; we're simply better at extracting more power from existing facilities. But the biggest change came from the boom in wind power. In 2009, wind produced 71 billion kWhrs of electricity, up from only 6 billion at the start of the decade.

Again, these are trends that seem likely to continue. The current administration is pushing for licensing and approval of both renewable and nuclear generating capacity, and many states have renewable power mandates on the books. So, for the next few years at least, it's likely that the carbon intensity of the electrical power sector will continue to drop.

Obviously, the DOE report recognizes that, if any economic recovery occurs that's heavily weighted towards energy-intensive activities, these trends may not hold. But the report suggests that long-term trends are likely to look a lot like the current ones for the reasons mentioned above. That's good news for the US' goals—Obama's end-of-decade target is certainly looking much more realistic. But, if the energy-intensive industries end up booming in nations with a greater reliance on coal (yes, we're thinking China, too), then this may not be as meaningful on the global scale.