U.S. Transportation Sector Emissions Down 4.7%
According to the US Energy Information Administration, which is part of the US Department of Energy, greenhouse gas emissions in 2008 were smaller in the US than in 2007. The decline was estimated to be of 157.2 million metric tons carbon dioxide equivalent (MMTCO2e), or about 2.2% of total. In the transportation sector, the drop in 2008 was was significantly bigger at 4.7%. At first glance, this seems like a cause for celebration, but not so fast...
The EIA writes that the decline in GHG emissions was mostly due to 3 factors, and the first two of those three factors probably won't have the same effect in years to come:
The decrease in U.S. CO2 emissions in 2008 resulted primarily from three factors: higher energy prices-- especially during the summer driving season--that led to a drop in petroleum consumption; economic contraction in three out of four quarters of the year that resulted in lower energy demand for the year as a whole in all sectors except the commercial sector; and lower demand for electricity along with lower carbon intensity of electricity supply.
The third one is interesting. Demand will probably pick up along with economic activity, but the lower carbon intensity (energy producers emit less carbon per unit of electricity on average) is a good sign. Of course, the U.S. is still far from having a clean grid, but at least it's moving in the right direction.
As you can see in the second graph, petroleum is the largest fossil fuel source for energy-related CO2 emissions, contributing 42% of the total. Coal is second, 37%; but "coal produces more CO2 per unit of energy, petroleum consumption--in terms of British thermal units (Btu)--made up 44.6% of total fossil fuel energy consumption in 2008, as compared with coal's 26.8%." So moving away from coal can make a big difference simply because it produces so much CO2 per unit of energy released.
Via EIA (pdf), Green Car Congress, WSJ
More on Energy and Climate
Global Warming Could Reduce California's Hydropower Output
Crap. Global Warming Could Reduce "Catch Potential" in Tropical Fisheries by 40%
Oh No! Global Warming Is Affecting Beer Production