And they were down by a pretty significant amount. In fact, as the Energy Information Administration says, "in 2009, energy-related carbon dioxide emissions in the United States saw their largest absolute and percentage decline (405 million metric tons or 7.0 percent) since the start of EIA’s comprehensive record of annual energy data that begins in 1949."
So why did this happen? The three major factors were a big drop in per capita GDP, a drop in the energy intensity of the economy (how much energy we use per unit of GDP), and a drop in the carbon intensity of the energy we use (how much carbon we emit per unit of energy used).
The one-line explanation is that the economic collapse, combined with relatively high prices for coal and petroleum, made people cut back on energy consumption and switch away from the dirtiest sources. Emissions from coal were down 12 percent and emissions from petroleum were down 5.3 percent.
Renewables, especially wind power, helped a little bit. The explosion in wind power since 2000 "was responsible for about 39 million metric tons of avoided emissions in 2009." That's nearly 10 percent of the total drop.
And, you know, your decision to bike to work instead of driving certainly contributed too.
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