A May report by the U.S. Energy Information Administration gave preliminary evidence that total energy-related CO2 emissions in the U.S. fell by 1.7 percent from 2005 to 2006. If the numbers hold up to subsequent revisions, that's very good news.
There are short-term and long-term explanations behind the numbers. The biggest short-term explanation is that the weather in 2006 was milder than in 2005. The winter was warmer, while the summer was cooler. This led to fewer heating and cooling days. Accordingly, the biggest decrease in energy-related CO2 emissions in both absolute and percentage terms occurred in the residential sector, where emissions dropped by 3.7 percent.
While year-to-year temperatures can go up and down, the long-term trends give some reason for optimism. Overall, the U.S. is continuing to become more energy- and emissions-efficient. Energy-intensity (units of energy per dollar of total economic ouput) fell by 4.0 percent from 2005 to 2006, and carbon-intensity (CO2 emissions per dollar of output) fell by 4.5 percent. Since 1990, the EIA reports that carbon-intensity dropped by more than a quarter, or just under 2 percent per year. At least some of this can be explained by restructuring in the economy and the steady shift from industrial activity to services and other types of commercial activity.
Another positive trend is that the long-run overall growth in emissions appears to be slowing. Since 1990, overall emissions have grown by about 1 percent per year. However, since 2000, there has only been one year (2004) with emissions growth above 1 percent. Emissions declined in 2001, partly due to the recession and partly due to the brief transportation shut-down following the 9/11 attacks. After 2001 though, growth in emissions has been very modest; this occurred despite relatively strong growth in the economy.
The short- and long-term pictures would be brighter still were it not for the transportation sector. Transportation has overtaken the industrial sector as the largest contributor to emissions. Since 2000, the growth rate in transportation emissions has been nearly triple the growth rate of residential emissions and 50 percent higher than the growth rate of commercial emissions. Energy-related CO2 emissions continue to be dominated by the use of petroleum.
Both sides of the climate change debate should be cheered by these numbers. For the stay-the-course crowd, the numbers show that regular market and technological forces, and perhaps voluntary action, can slow the production of greenhouse gases. For the policy-change crowd, the numbers show that zero growth and even reductions in energy-related CO2 emissions may be more easily achievable than we might have thought, and that reductions can occur without driving the economy into recession. The numbers also seem to justify a special focus on improving the efficiency of the transportation sector.