Total U.S. carbon dioxide emissions in 2007 increased by 75.9 million metric tons (1.3 percent) compared with 2006 emissions (see Figure 5 on right), to 6,022 million metric tons (MMT). The increase offset a 1.4-percent drop in 2006 (to 5,946 MMT), raising the total back close to the 2005 level (6,032 MMT). 

The important factors that contributed to the increase in carbon dioxide emissions in 2007 included: unfavorable weather, with both heating and cooling degree-days above 2006 levels (see discussion on "Weather Effects on energy-Related Carbon Dioxide Emissions, 2006 and 2007"); and the combination of a 2.5-percent increase in electricity demand and a 14.2-percent decline in hydropower generation that resulted in a 2.9-percent increase in emissions from the electric power sector.

Energy-related carbon dioxide emissions account for 98 percent of U.S. carbon dioxide emissions (see Table 5 below). The vast majority of carbon dioxide emissions come from fossil fuel combustion, with smaller amounts from the nonfuel use of energy inputs, and the total adjusted for emissions from U.S. Territories and international bunker fuels. Other sources include emissions from industrial processes, such as cement and limestone production.

Energy-related carbon dioxide emissions account for more than 80 percent of U.S. greenhouse gas emissions. EIA breaks energy use into four end-use sectors (see Table 6 below), and emissions from the electric power sector are attributed to the end-use sectors. 

Growth in energy-related carbon dioxide emissions has resulted largely from increases associated with electric power generation and transportation fuel use. All other energy-related carbon dioxide emissions (from direct fuel use in the residential, commercial, and industrial sectors) have been either flat or declining in recent years (see Figure 6 on right).

Reasons for the growth in electric power and transportation sector emissions include: increased demand for electricity for computers and electronics in homes and offices; strong growth in demand for commercial lighting and cooling; substitution of new electricity-intensive technologies, such as electric arc furnaces for steelmaking, in the industrial sector; and increased demand for transportation services as a result of relatively low fuel prices and robust economic growth in the 1990s and early 2000s.⁶ 

Commercial sector emissions (see Table 8 below) are largely the result of energy use for lighting, space heating, and space cooling in commercial structures, such as office buildings, shopping malls, schools, hospitals, and restaurants. 

Lighting accounts for a larger component of energy demand in the commercial sector (approximately 20 percent of total demand in 2006) than in the residential sector (approximately 11 percent of the total).

Commercial sector emissions are affected less by weather than are residential sector emissions: heating and cooling accounted for approximately 37 percent of energy demand in the residential sector in 2006 but only about 19 percent in the commercial sector.¹⁰

Although weather has a smaller effect on emissions in the commercial sector than in the residential sector, it contributed, along with 2-percent economic growth, to the 4.3-percent increase in 2007—well above the average annual increase from 1990 to 2006.

In the longer run, trends in emissions from the commercial sector parallel economic trends. Commercial sector emissions grew at an average annual rate of 2.0 percent from 1990 to 2007—about the same rate as growth in real per capita income (see Figure 8 on right).

Emissions from direct fuel consumption in the commercial sector declined from 1990 to 2007, while the sector’s electricity-related emissions increased by an average of 2.6 percent per year.

Unlike commercial sector emissions, trends in U.S. industrial sector emissions (see Table 9 below) have not followed economic growth trends. In 2007, industrial carbon dioxide emissions fell by 0.8 percent from 2006 and were 2.8 percent below their 1990 level. 

Decreases in industrial sector carbon dioxide emissions have resulted largely from erosion of the older energy-intensive (specifically, coal-intensive) U.S. industrial base.

Coke plants consumed 22.7 million short tons of coal in 2007, down from 38.9 million short tons in 1990. Other industrial coal consumption declined from 76.3 million short tons in 1990 to 56.5 million short tons in 2007, as reflected by the drop in emissions from coal shown in Figure 9 on right.

The share of manufacturing activity represented by less energy-intensive industries, such as computer chip and electronic component manufacturing, has increased, while the share represented by the more energy-intensive industries has fallen.

By fuel, only petroleum and purchased electricity use in 2007 were above 1990 levels for the commercial sector. Coal use has fallen since 1990, and natural gas use, which rose in the 1990s, has fallen since 2000.

The transportation sector has led all U.S. end-use sectors in emissions of carbon dioxide since 1999; however, with higher fuel prices and slower economic growth in 2007, emissions from the transportation sector were essentially unchanged from their 2006 level.

Petroleum combustion is the largest source of carbon dioxide emissions in the transportation sector, as opposed to electricity-related emissions in the other end-use sectors.

Increases in ethanol fuel consumption in recent years have mitigated the growth in transportation sector emissions somewhat (emissions from energy inputs to ethanol production plants are counted in the industrial sector).

Transportation sector emissions from gasoline and diesel fuel combustion generally parallel total vehicle miles traveled (see Figure 10 on right).

The electric power sector transforms primary energy inputs into electricity. The sector consists of companies whose primary business is the generation of electricity. 

Carbon dioxide emissions from electric power generation rose by 2.9 percent in 2007 (see Table 11 below). The increase resulted from growth in total electricity generation (2.5 percent) and an increase in the carbon intensity of the electricity supply (0.4 percent).

Higher overall carbon intensity of power generation in 2007 was the result of a large drop in generation from hydropower resources (down by 40 billion kilowatthours), which more than offset increases in generation from wind and nuclear power plants (up by 6 and 19 billion kilowatthours, respectively). While nuclear generation rose by 2.4 percent from 2006 to 2007, generation from renewable fuels—including hydropower—fell by 9.6 percent, and generation from fossil fuels increased by 4.1 percent.

From 2000 to 2007, as the overall efficiency of U.S. electricity generation has increased, there has been a decline in electric power sector energy losses¹¹ relative to total sales, which has helped to mitigate the sector’s carbon dioxide emissions (see Figure 11 on right). For example, generation from natural gas rose by 57 percent from 2000 to 2007, but emissions from natural-gas-fired generation rose by only 33 percent over the same period.

“Other emissions sources” in total accounted for 1.7 percent (105.1 MMT) of all U.S. carbon dioxide emissions in 2007 (see Figure 12 on right). 

The largest source of U.S. carbon dioxide emissions other than fossil fuel consumption is cement manufacture (see Table 15 below), where most emissions result from the production of clinker (consisting of calcium carbonate sintered with silica in a cement kiln to produce calcium silicate).

Limestone consumption, especially for lime manufacture, is the source of 15 to 20 MMT of carbon dioxide emissions per year.

In addition, “other sources” include: soda ash manufacture and consumption; carbon dioxide manufacture; aluminum manufacture; flaring of natural gas at the wellhead; carbon dioxide scrubbed from natural gas; and waste combustion in the commercial and industrial sectors.