	Introduction

U.S. greenhouse gas (GHG) emission levels remain a topic of interest among policy makers and stakeholders. On June 25, 2013, President Obama affirmed his commitment to reduce U.S. GHG emissions by 17% below 2005 levels by 2020 if all other major economies agreed to limit their emissions as well. In addition, during a November 2014 trip to China, President Obama and President Xi of China made a bilateral announcement concerning GHG emissions. President Obama announced a new policy target to reduce U.S. net GHG emissions by 26%-28% by 2025. U.S. GHG Emissions

As Figure 1 illustrates, U.S. GHG emissions increased during most of the years between 1990 and 2007, and then decreased substantially in 2008 and 2009. Although emissions increased in 2010, levels decreased again in 2011 and 2012, eventually reaching levels comparable to those in 1995. In terms of the President's 2020 emissions target (17% below 2005 levels), U.S. GHG emissions in 2012—the most recent year with available GHG emission data—were approximately 10% below 2005 levels. GHG Emissions by Source

GHG emissions are generated throughout the United States by millions of discrete sources: smokestacks, vehicle exhaust pipes, households, commercial buildings, livestock, etc. The figure indicates that CO 2 from the combustion of fossil fuels—petroleum, coal, and natural gas—accounted for 78% of total U.S. GHG emissions in 2012. The largest contribution (40%) is from electricity generation. One approach often taken in climate change analysis is to examine several broad energy-related factors that influence GHG emission levels, including
population, income—measured here as per capita gross domestic product (GDP), energy intensity—measured here as energy use per gross domestic product, and carbon intensity—measured here as CO 2 emissions per energy use. Although decreases in population and/or per capita income would contribute to lowering U.S. GHG emissions, policies that would seek to directly limit these emissions drivers are essentially outside the bounds of U.S. public policy. The figure indicates that energy use increased from 1990 to 2000 at an annual average rate of 1.6%, and then remained relatively constant (excepting some annual fluctuations) through 2013. In contrast, U.S. GDP (in 2009$) has increased at an average annual rate of approximately 2.5% from 1990 through 2013. As the figure illustrates, U.S. energy intensity has been declining by about 2% each year for two decades: the U.S. energy intensity in 2011 was 31% lower than it was in 1990. Carbon Intensity

In this report, carbon intensity measures the amount of CO 2 emissions generated during energy use, which includes fuel combustion for electricity generation and transportation purposes. Energy sources (e.g., coal, natural gas, petroleum, nuclear, and renewables) vary dramatically in the amount of carbon released per unit of energy supplied. As indicated in Table 1 , coal generates approximately 80% more CO 2 emissions per unit of energy than natural gas, and approximately 28% more emissions per unit of energy than crude oil. Moreover, other energy sources, such as nuclear or specific renewable sources, do not directly generate any CO 2 emissions. Carbon Content of Energy Use

 Figure 7 illustrates the U.S. carbon content of energy use—CO 2 emissions per Btu—between 1990 and 2013. This includes energy used (i.e., consumed) in the electricity, industrial, transportation, commercial, and residential sectors. The figure indicates that this measure remained relatively constant from 1990 to 2005, when it began to decline. By 2013, the carbon content of energy use was approximately 8% lower than it was in 2005. Carbon Content of Electricity Generation

The recent changes in energy consumption are partially explained by changes in the energy sources used to generate electricity, because the electric power sector accounts for approximately 40% of total energy use. Moreover, the percentage share of renewable sources increased from 2% to 6%, and petroleum decreased from 3% to less than 1%.
On June 25, 2013, President Obama affirmed his commitment to reduce U.S. greenhouse gas (GHG) emissions by 17% below 2005 levels by 2020 if all other major economies agreed to limit their emissions as well. In addition, during a November 2014 trip to China, President Obama announced a new policy target to reduce U.S. net GHG emissions by 26%-28% by 2025. Whether these objectives will be met is uncertain, but emission levels and recent trends remain a topic of interest among policy makers.
U.S. GHG emissions increased during most of the years between 1990 and 2007, and then decreased substantially in 2008 and 2009. Although emissions increased in 2010, levels decreased again in 2011 and 2012, eventually reaching levels comparable to those from 1995. In terms of the President's 2020 emissions target, in 2012, U.S. GHG emissions were approximately 10% below 2005 levels—more than halfway toward the 2020 target.
In the United States, GHG emissions are generated by millions of discrete sources, including smokestacks, vehicle exhaust pipes, commercial buildings, and households. However, carbon dioxide (CO2) emissions from the combustion of fossil fuels—petroleum, coal, and natural gas—have received the most attention because they account for the vast majority of human-related GHG emissions: 78% of total U.S. GHG emissions in 2012.
In addition, (1) CO2 emissions from large stationary sources are easy to measure and have been tracked for almost 20 years, and (2) CO2 emissions from smaller sources can be estimated through relatively straightforward calculations. In 2012, the percentage contributions of CO2 emissions by sector were as follows:
40% from electricity, 35% from transportation, 15% from industrial, 6% from commercial, and 4% from residential.
Although multiple factors have some level of influence on U.S. GHG emission levels, it may be instructive to examine several broad energy-related factors including population, income, energy intensity (energy use per economic output such as gross domestic product, or GDP) and carbon intensity (CO2 emissions per unit of energy use). Although decreases in population and/or income would contribute to reducing U.S. GHG emissions, policies that would seek to directly limit these emissions drivers are essentially outside the bounds of U.S. public policy. Therefore, this report focuses on the impacts of energy intensity and carbon intensity on GHG emission levels.
As energy use has grown at a slower rate than the economy, U.S. energy intensity declined by about 2% each year for more than two decades. Between 1990 and 2013, U.S. GDP (in 2009$) increased at an average annual rate of approximately 2.5%. Energy use, in contrast, increased from 1990 to 2000 at an annual average rate of 1.6%, but then remained relatively constant (excepting some annual fluctuations) through 2013.
The U.S. carbon content of energy use remained relatively constant from 1990 to 2005, but by 2013, it was approximately 8% lower than in 2005. In this report, carbon intensity measures the amount of CO2 emissions generated per unit of energy used. Energy sources—coal, natural gas, petroleum, nuclear, renewables—vary dramatically in the amount of carbon released per unit of energy supplied. For example, coal combustion accounts for almost twice the carbon content per unit of energy than natural gas, and some energy sources, when consumed, do not directly generate any emissions.
This recent decrease in the carbon content of energy use is partially explained by changes in the energy sources used to generate electricity, because the electric power sector accounts for approximately 40% of total energy use. For example, between 2004 and 2013, the percentage of electricity from coal generation decreased from 50% to 39%, while the percentage of electricity generated using natural gas increased from 18% to 28%. In addition, renewable energy use increased by 100%, and the use of petroleum to generate electricity decreased by approximately 100%.