The US Environmental Protection Agency (EPA) has released the national greenhouse gas inventory, which finds that overall emissions during 2007 increased by 1.4% from the previous year. The report, Inventory of US Greenhouse Gas Emissions and Sinks: 1990-2007, is the latest annual report that the United States has submitted to the Secretariat of the United Nations Framework Convention on Climate Change.
Total emissions of the six main greenhouse gases in 2007 were equivalent to 7,150 Tg of carbon dioxide. These gases include carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride. The report indicates that overall emissions have grown by 17.2 percent from 1990 to 2007.
The transportation end-use sector accounted for 1,924.6 Tg of CO2 in 2007, which represented 33% of CO2 emissions from fossil fuel combustion, 26% of CH4 emissions from fossil fuel combustion, and 67% of N2O emissions from fossil fuel combustion, respectively. Fuel purchased in the US for international aircraft and marine travel accounted for an additional 108.8 Tg CO2 in 2007; these emissions are recorded as international bunkers and are not included in US totals according to UNFCCC reporting protocols.
Among domestic transportation sources, light duty vehicles represented 61% of CO2 emissions, medium- and heavy-duty trucks 22%, commercial aircraft 8%, and other sources 10%. Other findings:
From 1990 to 2007, transportation emissions rose by 29% due, in large part, to increased demand for travel and the stagnation of fuel efficiency across the US vehicle fleet.
The number of vehicle miles traveled by light duty motor vehicles (passenger cars and light-duty trucks) increased 40% from 1990 to 2007, as a result of a confluence of factors including population growth, economic growth, urban sprawl, and low fuel prices over much of this period.
The primary driver of transportation-related emissions was CO2 from fossil fuel combustion, which increased by 29% from 1990 to 2007. This rise in CO2 emissions, combined with an increase in HFCs from virtually no emissions in 1990 to 67.0 Tg CO2 Eq. in 2007, led to an increase in overall emissions from transportation activities of 28%.
Domestic transportation CO2 emissions increased by 27% (404.7 Tg) between 1990 and 2007, an annualized increase of 1.5%.
Since 2005, the growth rate of emissions has slowed considerably; transportation CO2 emissions increased by just 0.3% in total between 2005 and 2007. Almost all of the energy consumed by the transportation sector is petroleum-based, including motor gasoline, diesel fuel, jet fuel, and residual oil.
Carbon dioxide emissions from passenger cars and light-duty trucks totaled 1,147.0 Tg in 2007, an increase of 21% percent (197.5 Tg) from 1990. CO2 emissions from passenger cars and light-duty trucks peaked at 1,181.3 Tg in 2004, and since then have declined about 3%. Over the 1990s through early this decade, growth in vehicle travel substantially outweighed improvements in vehicle fuel economy; however, the rate of Vehicle Miles Traveled (VMT) growth slowed considerably starting in 2005 while average vehicle fuel economy increased.
Among new vehicles sold annually, average fuel economy gradually declined from 1990 to 2004, reflecting substantial growth in sales of light-duty trucks relative to passenger cars. New vehicle fuel economy improved beginning in 2005, largely due to higher light-duty truck fuel economy standards, which have risen each year since 2005. The overall increase in fuel economy is also due to a slightly lower light-duty truck market share, which peaked in 2004 at 52% and declined to 48% in 2007.
The increase in overall emissions in 2007 was due primarily to an increase in carbon dioxide emissions associated with fuel and electricity consumption. The following factors were primary contributors to this increase:
Increased demand for heating fuels and electricity due to cooler winter and warmer summer conditions in 2007 than in 2006;
Increased consumption of fossil fuels to generate electricity; and
A significant decrease (14.2%) in hydropower generation used to meet this demand.
The inventory tracks annual greenhouse gas emissions at the national level and presents historical emissions from 1990 to 2007. The inventory also calculates carbon dioxide emissions that are removed from the atmosphere by “sinks,” e.g., through the uptake of carbon by forests, vegetation and soils.