Annual US greenhouse gas emissions relative to 1990 (1990=0). Source: EPA. Click to enlarge.

The major contributors to the decrease in emissions from 2011-2012 were the decrease in energy consumption across all sectors in the US economy, and the decrease in carbon intensity for electricity generation due to fuel switching from coal to natural gas. Other factors included a decrease in transportation sector emissions attributed to an increase in fuel efficiency across different transportation modes and limited new demand for passenger transportation.

CO₂ from fossil fuel combustion is the largest source of anthropogenic greenhouse gas emissions in the US and has accounted for approximately 78% of global warming potential (GWP)-weighted emissions since 1990; it remained approximately 78% of total GWP-weighted emissions in 2012.

Emissions of CO₂ from fossil fuel combustion increased at an average annual rate of 0.3% from 1990 to 2012. The fundamental factors influencing this trend include:

1. a generally growing domestic economy over the last 23 years;

2. an overall growth in emissions from electricity generation and transportation activities, along with

3. a general decline in the carbon intensity of fuels combusted for energy in recent years by most sectors of the economy.

Between 1990 and 2012, CO₂ emissions from fossil fuel combustion increased from 4,745.1 Tg CO₂ Eq. to 5,072.3 Tg CO₂ Eq.—a 6.9% total increase over the 23-year period. From 2011 to 2012, these emissions decreased by 198.8 Tg CO₂ Eq. (3.8%).

The five major fuel consuming sectors contributing to CO₂ emissions from fossil fuel combustion are electricity generation, transportation, industrial, residential, and commercial. CO₂ emissions are produced by the electricity generation sector as they consume fossil fuel to provide electricity to one of the other four end-use sectors. In the report, EPA distributes electricity generation emissions to each end-use sector on the basis of each sector’s share of aggregate electricity consumption.

2012 CO2 emissions from fossil fuel combustion by sector and fuel type. Source: EPA. Click to enlarge.

Transportation End-Use Sector. The primary driver of transportation-related emissions was CO₂ from fossil fuel combustion, which increased by 16% from 1990 to 2012. This rise in CO₂ emissions, combined with an increase in HFCs from close to zero emissions in 1990 to 72.9 Tg CO₂ Eq. in 2012, led to an increase in overall emissions from transportation activities of 18%. The increase was due, in large part, to increased demand for travel with limited gains in fuel efficiency over the same time period.

The number of vehicle miles traveled by light-duty motor vehicles (passenger cars and light-duty trucks) increased 35% from 1990 to 2012, as a result of a confluence of factors including population growth, economic growth, urban sprawl, and low fuel prices during the beginning of this period, EPA said.

When electricity-related emissions are distributed to economic end-use sectors, transportation activities accounted for 34.4% of US CO₂ emissions from fossil fuel combustion in 2012.

The largest sources of transportation greenhouse gases in 2012 were:

• passenger cars (43.1%);
• light duty trucks, which include sport utility vehicles, pickup trucks, and minivans (18.4%);
• freight trucks (21.9%);
• commercial aircraft (6.2%);
• rail (2.5%); and
• ships and boats (2.2%).

From 2011 to 2012, CO₂ emissions from the transportation end-use sector decreased by 0.5%. EPA attributed the decrease largely to a small increase in fuel efficiency across different transportation modes and limited new demand for passenger transportation.

Carbon dioxide emissions from passenger cars and light-duty trucks totaled 1,061.0 Tg CO₂ Eq. in 2012, an increase of 12% (110.6 Tg CO₂ Eq.) from 1990. CO₂ emissions from passenger cars and light-duty trucks peaked at 1,184.3 Tg CO₂ Eq. in 2004, and since then have declined about 10%.

• Passenger car CO₂ emissions increased by 21% from 1990 to 2012;
• light-duty truck CO₂ emissions decreased by 6%; and
• medium- and heavy-duty trucks increased by 70%.

Carbon dioxide from the domestic operation of commercial aircraft increased by 3% (3.4 Tg CO₂ Eq.) from 1990 to 2012. Across all categories of aviation, CO₂ emissions decreased by 22.5% (42.2 Tg CO₂ Eq.) between 1990 and 2012.71 This includes a 65% (22.9 Tg CO₂ Eq.) decrease in emissions from domestic military operations.

Commercial aircraft emissions have decreased 19% since 2007. Decreases in jet fuel emissions (excluding bunkers) are due in part to improved operational efficiency that results in more direct flight routing, improvements in aircraft and engine technologies to reduce fuel burn and emissions, and the accelerated retirement of older, less fuel efficient aircraft.

Almost all of the energy consumed for transportation was supplied by petroleum-based products, with more than half being related to gasoline consumption in automobiles and other highway vehicles. Other fuel uses, especially diesel fuel for freight trucks and jet fuel for aircraft, accounted for the remainder.