The important factors that contributed to a drop in carbon dioxide emissions in 2006 (see Figure 5 on right) included: total energy consumption in 2006 that was 0.5 percent below the 2005 total—due in part to favorable weather conditions (both heating and cooling degree-days were below 2005 levels) and in part to higher energy prices that helped to dampen energy demand.

A decline in the carbon intensity of electric power generation that resulted from increased use of natural gas, the least carbon-intensive fossil fuel, and greater reliance on non-fossil energy sources also contributed to the decrease.

Relatively small increases in emissions from other sources of carbon dioxide, such as industrial processes, and from the U.S. Territories, which in total represent only a minor share of U.S. emissions, were not enough to offset the declines from major energy sources.

Energy-related carbon dioxide, including emissions resulting from nonfuel uses of energy fuels (primarily petroleum) and adjustments for U.S. Territories and international bunker fuels, account for 98 percent of carbon dioxide emissions (see Table 4 below)

Emissions from other sources, such as industrial processes, account for 2 percent of carbon dioxide emissions.

Residential sector carbon dioxide emissions consist of:
- Direct fuel consumption (principally natural gas) for heating and cooking
- Electricity for cooling (and some heating), for lighting, and increasingly for televisions, computers, and other
  household electronic devices (see Table 5 below).

Weather dominates the year-to-year changes for residential energy demand and related emissions:
- In 2006, heating degree-days declined by 7.4 percent (see Figure 6 on right), and cooling degree-days declined by
  almost 2 percent, from 2005 levels.

In the longer run, residential emissions are affected by population growth and income. From 1990 to 2006:
- U.S. population grew by an average of 1.2 percent per year
- Residential sector carbon dioxide emissions grew by an average of 1.4 percent per year
- Per capita income (measured in constant dollars) grew by an average of 1.8 percent per year.

Unlike commercial sector emissions, trends in U.S. industrial sector emissions (see Table 7 below) have not followed the trends in economic growth.

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 23.0 million short tons of coal in 2006, down from 38.9 million short tons in 1990.

Other industrial coal consumption declined from 76.3 million short tons in 1990 to 60.5 million short tons in 2006.

Total industrial coal use dropped by more than 27 percent from 1990 to 2006 (see Figure 8 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.

Since 1999, the transportation sector has led all U.S. end-use sectors in emissions of carbon dioxide.

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 9 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.

From 2005 to 2006, electricity demand was essentially flat and the carbon intensity of the electricity supply fell, leading to a drop in electric power sector emissions (see Table 9 below).

From 2000 to 2006, as the overall efficiency of U.S. electricity generation has increased, there has been a decline in electric power sector energy losses¹⁹ relative to total sales, which has helped to mitigate the sector’s carbon dioxide emissions (see Figure 10 on right).

From year to year, demand for electricity can be affected by the number of cooling degree-days.

All the end-use sectors except transportation have increasingly relied on electricity as an energy source, displacing other fuels.

Carbon dioxide emissions from electricity generation in the commercial and industrial sectors are counted in those sectors.

“Other emissions sources” in total accounted for 1.8 percent (108.8 MMT) of all U.S. carbon dioxide emissions in 2006 (see Figure 11 on right).

The largest source of U.S. carbon dioxide emissions other than fossil fuel consumption is cement manufacture (see Table 13 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 million metric tons 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.