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# U.S. carbon dioxide emissions from energy use fell in 2011

August 17, 2012, 2:48 pm
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After an increase in 2010 of 3.3 percent, energy-related carbon dioxide emissions declined in 2011 by 2.4 percent and were 526 million metric tons (9 percent) below the 2005 level. Energy-related carbon dioxide emissions have declined in the United States in four out of the last six years. Main Image Credit: Vitaly Krivosheev-Fotolia.

## U.S. Energy-Related Carbon Dioxide Emissions, 2011

Release Date: August 14, 2012   |  Next Release Date: August 2013

### U.S. carbon dioxide emissions from energy use fell in 2011

After two years of declining carbon dioxide emissions (2008 and 2009) and one year of increasing emissions (2010), carbon dioxide emissions in 2011 fell, but at a less dramatic rate than in 2009. Unlike 2009, the 2011 decline occurred during a year of positive growth in the Gross Domestic Product (GDP).1

1For a full definition of Gross Domestic Product, see the end of this analysis.
Note: this analysis examines the level and sources by sector and fuel of energy-related carbon dioxide emissions in 2011 as presented in Section 12: Environment of the Monthly Energy Review (MER).

### Part of the 2011 emissions decrease is due to slower economic growth

In 2011, GDP grew by 1.8 percent, but emissions decreased by 2.4 percent (136 million metric tons). This indicates that the carbon intensity of the economy declined by about 4.2 percent.2 The 2011 decrease is only the fourth year since 1990 to experience a decline in carbon intensity of greater than 3.5 percent for the economy as a whole and only the sixth year since 1990 to experience an emissions decline. Since 1990, energy-related carbon dioxide emissions in the United States have grown much more slowly than GDP – in 2007 emissions were 19 percent greater than their 1990 level, but by 2011 were only about 9 percent above the 1990 level. GDP has increased by 66 percent over that same time period.

2The carbon intensity of the economy is carbon dioxide per unit of economic output (CO2/GDP). The carbon intensity of the energy supply is carbon dioxide per unit of energy consumption (CO2/Btu).

### Various factors combine to produce changes in energy-related carbon dioxide emissions

Using the “Kaya Identity,” changes in energy-related carbon dioxide can be understood in terms of economic output (the total change in output as measured by the GDP is the growth in per capita output multiplied by the growth in population), the energy intensity of the economy and the carbon intensity of the fuel mix to meet the demand for energy.3 Since 2000 the population growth rate has declined slowly and annual swings in GDP growth are largely attributed to changes in per capita output.4 Since 2000, only three years have achieved growth in per capita output above 2 percent (2000, 2004, and 2005). Energy-related carbon dioxide emissions increased in all those years. In 2011, growth in population (0.7 percent) and output per capita (1.1 percent), combined to produce GDP growth of 1.8 percent. This was more than offset by a decline in carbon intensity of the energy supply (CO2/Btu) of 1.9 percent. This meant that the decrease in energy intensity of 2.3 percent yielded a decrease in energy-related carbon dioxide of about 2.4 percent.

3For a full definition of Kaya Identity, energy intensity, carbon intensity of the energy supply and carbon intensity of the economy see the end of this analysis.
4Population source: Table 1. Annual Estimates of the Population for the United States, Regions, States, and Puerto Rico: April 1, 2010 to July 1, 2011 (NST-EST2011-01), U.S. Census Bureau, Population Division, Release Date: December 2011 http://www.census.gov/popest/data/national/totals/2011/index.html.

### Energy use changes in main sectors in 2011

The industrial sector experienced energy consumption growth of 0.7 percent in 2011.5 The commercial sector fell slightly (0.3 percent). Energy consumption in the residential sector fell by 1.1 percent and in the transportation sector by 1.4 percent. The sum of these sector changes meant that total energy consumption fell by 0.5 percent – this coupled with economic growth of 1.8 percent meant that the energy intensity of the economy fell by 2.3 percent.

5End-use sector energy consumption allocates primary energy used in the electric power sector to the end-use sectors based on their share of electricity use.

### Special factors drove the consumption decreases in the residential and transportation sectors

Weather is an important factor in residential energy consumption variations from one year to the next. In 2011, cooling degree-days (CDD) were slightly higher than in 2010 (0.7 percent). This would tend to put upward pressure on electricity demand and related emissions. On the other hand, heating degree-days (HDD) fell by 3.2 percent and residential sector energy consumption declined by 1.1 percent.6 In the figure below, the monthly change in heating degree-days and cooling degree-days is weighted by the proportion of degree-days for that month in the prior year (2010). In 2011, transportation-related carbon dioxide emissions fell primarily due to higher fuel costs, improvements in fuel efficiency, and a reduction in miles traveled. In 2010, the price of regular gasoline averaged $2.78 per gallon. In 2011, the average price rose to$3.53 per gallon – an increase of 27 percent. It is estimated that the miles per gallon (mpg) of light duty vehicles improved by 1.0 percent (20.4 to 20.6 mpg) from 2010 to 2011.7 Vehicle miles traveled fell from an average of 8,127 million miles per day in 2010 to 8,029 million miles per day in 2011 (1.2 percent). This contributed to a decline in gasoline consumption of 2.9 percent which, in conjunction with changes in other transportation fuels, resulted in a decline in total energy consumption in the transportation sector of 1.4 percent.

6For a full definition of cooling and heating degree-days see the end of this analysis.
7Estimate from the National Energy Modeling System AEO2012.

### Role of electricity in the carbon intensity of the energy supply decreases in 2011

The carbon intensity of the energy consumed declined in every sector of the economy. A carbon intensity decline in the electric power sector (-4.0 percent) which accounted for 40 percent of total U.S. primary energy use in 2011, helped achieve the lower carbon intensity of the energy supply. Because primary energy use in the electric power sector is allocated to the end-use sectors based on their share of electricity use, the drop in reported carbon intensity for the end-use sectors was greatest in the residential and commercial sectors (3.1 percent and 3.2 percent, respectively) as these sectors rely heavily on electricity to meet their energy needs.

### Impact of fuel supply mix in the electric power sector on the carbon intensity of energy supply in 2011

As mentioned above, the carbon intensity of the energy supply (CO2/Btu) declined in every sector in 2011. With the exception of the transportation sector, this decline was influenced by the decline in the carbon intensity of the electric power sector. The share of non-carbon emitting generation in the electric power sector grew from 30 percent in 2010 to 31 percent in 2011. It was a particularly good year for hydropower as generation increased by 25 percent from 2010. Wind power generation increased by 26 percent and solar energy from both thermal and photovoltaic systems increased by 49 percent, but from a small base. Geothermal generation rose about 10 percent. Natural gas generation (the lowest carbon intensity per Btu of the fossil fuels) increased 3 percent and coal (almost twice as carbon intensive as natural gas) declined by 6 percent.

### Unusual decline in coal generation in 2011

Since 1949, the 2011 decline in coal generation of over 6 percent is second only to the decline in 2009 of almost 12 percent. As recently as 2005, coal's share of electric power sector generation was over 51 percent. By 2011 that share had declined to just over 43 percent. Petroleum generation, which was small to begin with, has also lost share. Natural gas, on the other hand, has steadily grown in market share. The introduction of new, efficient gas-fired capacity and a recent decline in the price of natural gas has helped boost natural gas' share from 14 percent in 2000 to 24 percent in 2011.

### Energy-related carbon dioxide emissions in 2011 compared to the last several years on a month-by-month basis

Total monthly emissions show some seasonality with peaks at the beginning and end of each year. There is also a summer peak at a lower level than the winter peak. While all three years (2009-2011) show the same peaks and valleys, the economic slowdown is evident throughout 2009. The decline in heating degree-days in December 2011 is also evident in the data as the December emissions are well below the prior two years.

### Emissions by major fuels compared to their consumption

While coal provides 20 percent of U.S. primary energy consumption, it contributes to 34 percent of energy-related carbon dioxide emissions. Petroleum provides 36 percent of the energy consumption, but 42 percent of the emissions. Natural gas, on the other hand, provides 26 percent of the energy consumed but 24 percent of the emissions. About 18 percent of total U.S. energy consumption was from sources that either do not emit carbon dioxide such as nuclear, hydropower, wind and solar or emit carbon dioxide as part of the natural carbon cycle (biomass). Other energy sources, e.g., plastics in municipal solid waste facilities that convert waste to energy, emit small amounts of carbon dioxide but they are less than one percent of the total.

### Implications of the carbon dioxide emissions decrease in 2011 for future emissions

It is difficult to draw conclusions from one year of data. Just as 2009 was an atypical year in terms of the magnitude of the emissions decline, and 2010 did not signal a new trend in emissions growth, there are specific circumstances (for example, the large increase in hydropower generation) that contributed to the decline in emissions in 2011. Other factors, such as improvements in vehicle fuel efficiency, abundant supplies of natural gas, and increased use of non-hydro renewable generation, however, could play a continuing role in 2012 and subsequent years.

For the Energy Information Administration's (EIA) projections on emissions and the factors that contribute to their underlying trends, see either our short-term forecast through 2013 that is updated monthly at www.eia.gov/forecasts/steo, or longer-term projections through 2035 that are updated annually at www.eia.gov/forecasts/aeo. EIA's projections of international energy consumption and emissions to 2035 can be found at http://www.eia.gov/forecasts/ieo/.

Starting in the fall of 2010, EIA expanded its reporting of energy-related carbon dioxide emissions in both the Monthly Energy Review (MER) and the Short-Term Energy Outlook (STEO). The MER reports monthly energy-related carbon dioxide emissions derived from our monthly energy data in Chapter 12, while the STEO forecasts these emissions to accompany its traditional forecasts of energy use. For the full range of EIA's emissions products see: http://www.eia.gov/environment/.

Terms used in this analysis:

British thermal unit (Btu): The quantity of heat required to raise the temperature of 1 pound of liquid water by 1 degree Fahrenheit at the temperature at which water has its greatest density (approximately 39 degrees Fahrenheit).

Carbon intensity (economy): The amount of carbon by weight emitted per unit of economic activity. It is most commonly applied to the economy as a whole, where output is measured as the gross domestic product (GDP). The carbon intensity of the economy is the product of the energy intensity of the economy and the carbon intensity of the energy supply. Note: this value is currently measured in the full weight of the carbon dioxide emitted.

Carbon intensity (energy supply): The amount of carbon by weight emitted per unit of energy consumed. A common measure of carbon intensity is weight of carbon per Btu of energy. When there is only one fossil fuel under consideration, the carbon intensity and the emissions coefficient are identical. When there are several fuels, carbon intensity is based on their combined emissions coefficients weighted by their energy consumption levels. Note: this value is currently measured in the full weight of the carbon dioxide emitted.

Cooling degree-days (CDD): A measure of how warm a location is over a period of time relative to a base temperature, most commonly specified as 65 degrees Fahrenheit. The measure is computed for each day by subtracting the base temperature (65 degrees) from the average of the day's high and low temperatures, with negative values set equal to zero. Each day's cooling degree-days are summed to create a cooling degree-day measure for a specified reference period. Cooling degree-days are used in energy analysis as an indicator of air conditioning energy requirements or use.

Energy intensity: A measure relating the output of an activity to the energy input to that activity. It is most commonly applied to the economy as a whole, where output is measured as the gross domestic product (GDP) and energy is measured in Btu that allow for the summing of all energy forms. On an economy-wide level, it is reflective of both energy efficiency as well as the structure of the economy. Economies in the process of industrializing tend to have higher energy intensities than economies that are in their post-industrial phase. The term energy intensity can also be used on a smaller scale to relate, for example, the amount of energy consumed in buildings to the amount of residential or commercial floor space.

Gross domestic product (GDP): The total value of goods and services produced by labor and property located in the United States. As long as the labor and property are located in the United States, the supplier (that is, the workers and, for property, the owners) may be either U.S. residents or residents of foreign countries.

Heating degree-days (HDD): A measure of how cold a location is over a period of time relative to a base temperature, most commonly specified as 65 degrees Fahrenheit. The measure is computed for each day by subtracting the average of the day's high and low temperatures from the base temperature (65 degrees), with negative values set equal to zero. Each day's heating degree-days are summed to create a heating degree-day measure for a specified reference period. Heating degree-days are used in energy analysis as an indicator of space heating energy requirements or use.

Kaya Identity: An equation stating that total energy-related carbon dioxide emissions can be expressed as the product of four inputs: 1) population, 2) GDP (output) per capita, 3) energy use per unit of GDP, and 4) carbon emissions per unit of energy consumed.  The change in the four inputs can approximate the change in energy-related carbon dioxide emissions.

Primary energy: Energy in the form that it is first accounted for in a statistical energy balance, before any transformation to secondary or tertiary forms of energy. For example, coal can be converted to synthetic gas, which can be converted to electricity; in this example, coal is primary energy, synthetic gas is secondary energy, and electricity is tertiary energy. In the context of this analysis it would mean energy consumed directly by a home, business or industrial operation as opposed to electricity generated elsewhere and supplied to the end-user.

For other definitions see the EIA glossary: http://www.eia.gov/tools/glossary/.

Glossary

### Citation

Draggan, S. (2012). U.S. carbon dioxide emissions from energy use fell in 2011. Retrieved from http://www.eoearth.org/view/article/51cbf4307896bb431f6af13c