U.S. energy-related carbon dioxide (CO2) emissions increased in 2013 by 129 million metric tons (2.5%), the largest increase since 2010 and the fourth-largest increase since 1990. Emissions trends reflect a combination of economic factors (population multiplied by per capita output [GDP/population]), energy intensity (energy use per dollar of GDP), and carbon intensity (carbon emissions per unit of energy consumed).
In the decade prior to 2013, energy intensity decreased on average by 2.0% per year; given that it increased by 0.5% in 2013, this meant there was a 2.5% swing compared to trend. Energy intensity changes can reflect weather variations that directly affect energy use for heating and cooling as well as changes in the composition of economic activity. Heating degree days, a measure of heating requirements, increased about 19% between 2012 and 2013. As compared to the 2003-12 trend, the increase in energy intensity added about 134 million metric tons.
Carbon intensity declined by 0.3% in 2013, but as this decline was less than the previous decade, it led to an increase of about 29 million metric tons of emissions as compared to trend. One factor driving carbon intensity lower has been the changing fuel mix in the electric power sector. The share of electricity generated from natural gas and renewables generally increased while the share from coal decreased through 2012, when natural gas prices fell to their lowest level in more than a decade following a mild winter. With higher natural gas prices in 2013, coal's generation share rose from 39% in 2012 to 40% in 2013, slowing the rate of carbon intensity reduction.
Growth in per capita output in 2013 contributed 38 million metric tons over trend as it was greater than the average rate over the previous decade. Slower population growth (0.7%) put slight downward pressure on emissions growth as compared to the previous decade. Without slower population growth, 2013 emissions would have been about 8 million metric tons higher.