EIA said emissions developments in 2009 reflect a combination of factors, including some particular to the economic downturn, other special circumstances during the year, and other factors that may reflect persistent trends in our economy and our energy use. In contrast to the 0.9 percent average annual emissions decline from 2000 to 2009, the prior 1990-to-1999 time period saw U.S. energy-related carbon dioxide emissions grow on average by 1.4 percent per year. Robust GDP (Gross Domestic Product) growth (3.3 percent annually) from 1990 to 1999 on average dropped by half (to 1.6 percent) from 2000 to 2009, particularly due to the recent economic downturn. However, even with the reduction in economic growth since 2000, emissions would nonetheless have grown by 0.6 to 0.7 percent annually had the proportional relationship between economic and emissions growth remained the same as during the 1990s.
EIA indicates that changes in carbon dioxide emissions can be decomposed into changes in four major contributing factors: (1) population, (2) per capita GDP, (3) energy intensity of the economy, and (4) carbon intensity of the energy supply. All of these fell in 2009 except for population. Population grew 0.9 percent. The downturn of the economy caused per capita GDP to fall (3.3 percent) resulting in a total GDP decline of 2.4 percent. Energy intensity and the carbon intensity of the energy supply also both fell more than 2 percent. These three factors (GDP, energy intensity, and carbon intensity) combined in roughly equal proportions to cause emissions to fall by 7.0 percent.
EIA also reports that total energy consumption fell across all end-use sectors by 4.8 percent, attributable to a decline in energy intensity of around 2.4 percent, plus a decline in GDP of 2.4 percent. While this drop in energy intensity was large, it is not unprecedented. The average decline in energy intensity from 2000 to 2008 was 2.0 percent. The economic decline did not affect the other sectors as much as the industrial sector, which saw the greatest drop in energy consumption – 9.9 percent.
EIA provides much more analysis of each of the contributing factors and summarizes that, "Total emissions of energy-related carbon dioxide fell across all end-use sectors in 2009, with the drop especially pronounced in the industrial sector. As the economy recovers, the structure of that recovery will be important to the future emissions profile of the United States. If energy-intensive industries lead the economic recovery, emissions would increase faster than if service industries or light manufacturing play the leading role. If coal, which was more heavily impacted by the recent economic downturn than other energy sources, rebounds disproportionately, the carbon intensity of the energy supply could rise above the 2009 level. However, longer-term trends continue to suggest decline in both the amount of energy used per unit of economic output and the carbon intensity of our energy supply, which both work to restrain emissions."
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