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. President Xi agreed to "peak" Chinese carbon dioxide (CO 2 ) emissions around 2030, perhaps earlier, and to increase the non-fossil share of China's energy to around 20% by 2030 compared to 2005 levels. A question for policy makers is whether U.S. GHG emissions will remain at current levels, decrease to meet the President's 2020 and 2025 goals, or increase to former (or even higher) levels. Multiple factors, including socioeconomics, technology, and climate policies, may impact GHG emission levels. The human-related GHG emission and related data in this report are from publicly available sources, particularly reports and tables produced regularly by the Environmental Protection Agency (EPA) and the Energy Information Administration (EIA). This first section of this report provides current levels and recent trends in U.S. GHG emissions. The second section takes a closer look at some of the key factors that influence emission levels. The third section discusses the challenges in making GHG emission projections by comparing observed emissions with preobserved emission estimates. 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 are generated throughout the United States by millions of discrete sources: smokestacks, vehicle exhaust pipes, households, commercial buildings, livestock, etc. Figure 2 illustrates the breakdown of U.S. GHG emissions by gas and type of source. 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 next section examines CO 2 emissions from fossil fuel combustion. In the context of GHG emission reduction programs and legislative proposals, CO 2 emissions from the combustion of fossil fuels have received the most attention. CO 2 emissions are fairly easy to verify from large stationary sources, like power plants. For almost 20 years, measurement devices have been installed in smokestacks of large facilities, reporting electronic information to EPA and the appropriate state. For smaller sources, CO 2 emissions are a relatively straightforward and accurate calculation based on the carbon content of fossil fuels consumed. In addition, according to EPA, "changes in emissions from fossil fuel combustion have been the dominant factor affecting U.S. emission trends." Figure 3 illustrates the CO 2 emission contributions by sector from the combustion of fossil fuels. The largest contribution (40%) is from electricity generation. The generated electricity is distributed almost equally to the residential, industrial, and commercial sectors. Multiple variables impact U.S. GHG emission levels. 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. As illustrated in the equation below, GHG emission levels can be approximated by multiplying together these four factors. 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. Thus the most relevant factors in terms of climate change policy are energy intensity and carbon intensity, which are discussed below. Energy intensity is the amount of energy consumed—often measured in metric tons of oil equivalent (toe) or British thermal units (Btus)—per a level of economic output such as GDP. Figure 4 illustrates the U.S. total energy consumption and GDP between 1990 and 2013. 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. Thus, U.S. energy intensity declined between 1990 and 2013. Figure 5 compares the energy intensities of the United States, the European Union (28 nations), and China between 1990 and 2011. 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. However, the energy intensity in the United States was 42% higher in 2011 than in the European Union. The degree to which the U.S. economy is composed of industries with relatively high energy intensities likely plays a role in determining the energy intensity of United States. Figure 6 indicates that the percentage contribution to the U.S. GDP from selected energy-intensive industries decreased from 4.2% in 1998 to 3.2% in 2011. Figure 6 includes 44 industries in the six-digit North American Industry Classification System (NAICS). These industries have an energy intensity of at least 5%, measured by dividing energy expenditures by the value of an industry's shipments. 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. 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. This decline is largely related to the recent change in the portfolio of fuels consumed for energy purposes. Figure 8 compares the consumption percentage of different energy sources in the United States. The figure indicates that the trajectories of coal and natural gas have diverged in recent years. Since 2008, coal's percentage contribution has decreased from 23% to 19%. In contrast, natural gas's percentage contribution has increased from 24% to 27% during that time frame. In addition, renewable energy's share of total energy consumption has increased substantially over the past decade. 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. As an illustration of recent changes in the electricity sector, Figure 9 compares electricity generation by energy source between 2004 and 2013. The figure indicates a substantial decrease in coal use with a simultaneous increase in natural gas. Moreover, the percentage share of renewable sources increased from 2% to 6%, and petroleum decreased from 3% to less than 1%. Accurately forecasting future GHG emission levels is a complex and challenging, if not impossible, endeavor. Consequently, analysts often provide a range of emissions based on different scenarios or assumptions. As discussed above, several broad energy-related factors influence emission levels. In addition, other variables have impacts in ways that cannot be accurately predicted. Such variables may include technological developments, energy price fluctuations, availability of less carbon-intensive energy sources (e.g., hydroelectric, other renewables, and nuclear power), seasonal weather and temperature patterns, and policy changes in the United States and abroad. EIA provides annual forecasts of CO 2 emissions in its Annual Energy Outlook (AEO) publications. Regarding its various estimates, EIA states the following: The projections in the AEO are not statements of what will happen but of what might happen, given assumptions and methodologies. The AEO Reference case projection assumes trends that are consistent with historical and current market behavior, technological and demographic changes, and current laws and regulations. The potential impacts of pending or proposed legislation, regulations, and standards are not reflected in the Reference case projections. Figure 10 compares actual CO 2 emissions between 1990 and 2012 with selected EIA emission projections made in past years. In general, actual emissions have remained well below projections, particularly the projections made in 2008 or earlier. For example, the AEO from 2000 projected CO 2 emissions would be almost 6.7 billion metric tons in 2012, about 20% higher than has been observed. By comparison, the more recent projections (AEO 2012 and 2014) indicate that CO 2 emissions will remain relatively flat over the next decade.