Annual energy-related emissions were 5,498 million metric tons in 2011, the EIA says in its report.

Energy intensity "” the energy measured in Btu per dollar of GDP "” fell by 5.1 percent, driving the decline in emissions even as the U.S. economy grew 2.8 in 2012. Half of the overall energy decline was from the residential sector, which needed less heating due to a warm first quarter of the year.

Typically, the first quarter of the year marks the high point for CO2 emissions.

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