Recent congressional interest in U.S. energy policy has focused in part on ways through which the United States could secure more economical and reliable petroleum resources both domestically and internationally. Many forecasters identify petroleum products refined from Canadian oil sands as one possible solution. Canadian oil sands account for about 54% of Canada's total crude oil production, and that number is expected to rise from its current level of 1.9 million barrels per day (mbd) in 2013 to 4.8 mbd by 2030. Further, the infrastructure to produce, upgrade, refine, and transport the resource from Canadian oil sands reserves to the United States is in place, and additional infrastructure projects—such as the Keystone XL pipeline—have been proposed. Increased production from Canadian oil sands, however, is not without controversy, as many have expressed concern over the potential environmental impacts. These impacts may include increased water and natural gas use, disturbance of mined land, effects on wildlife and water quality, trans-boundary air pollution, and emissions of greenhouse gases (GHG) during extraction and processing. A number of key studies in recent literature have expressed findings that GHG emissions per unit of energy produced from Canadian oil sands crudes are higher than those of other crudes imported, refined, and consumed in the United States. While GHG emissions and other air quality issues originating in the upstream sectors of Canada's petroleum industry may not directly impact U.S. National Emissions Inventories or U.S. GHG reporting per se, many environmental stakeholders and policy makers have noted that the increased use of more emission-intensive resources in the United States may have negative consequences for both U.S. and global energy and environmental policy. The U.S. Department of State (DOS), in response to comments on the 2010 Draft Supplementary Environmental Impact Statement (2010 Draft EIS) for the Keystone XL pipeline project (which would connect oil sands production facilities in the Western Canadian Sedimentary Basin with refinery facilities in the United States), commissioned a contractor to analyze the life-cycle GHG emissions associated with these resources in comparison to other reference crudes. DOS presented this analysis in the 2011 Final Environmental Impact Statement (2011 Final EIS) released on August 26, 2011, as a "matter of policy," but noted that neither the National Environmental Policy Act (NEPA) nor DOS regulations (22 C.F.R. 161.12) nor Executive Orders 13337 and 12114 (Environmental Effects Abroad of Major Federal Activities) legally require that an EIS include an assessment of environmental activities outside the United States. The initial permit for the Keystone XL Project was denied due to insufficient time to prepare a rigorous, thorough, and transparent review of the pipeline's proposed routes through Nebraska. In May 2012, Keystone filed a new permit application for a revised route, implementing a new national interest determination. In accordance with this process, DOS released a revised Draft Supplementary EIS (Draft EIS) for the revised project on March 1, 2013, and a revised Final Supplementary EIS (Final EIS) on January 31, 2014, including an assessment of the indirect cumulative impacts and life-cycle GHG emissions of Canadian oil sands crudes. While DOS commissioned a different contractor to assist with the EIS, the data used to determine the GHG life-cycle emissions associated with the resource, as well as the market analysis used for supply and demand projections, remained largely unchanged. Hence, the 2014 Final EIS made similar findings to the 2011 Final EIS, including the following: 1. Canadian oil sands crudes "are more GHG-intensive than the other heavy crudes they would replace or displace in U.S. refineries, and emit an estimated 17% more GHGs on a life-cycle basis than the average barrel of crude oil refined in the United States in 2005," and 2. "Approval or denial of any one crude oil transport project, including the proposed Project, is unlikely to significantly impact the rate of extraction in the oil sands or the continued demand for heavy crude oil at refineries in the United States." Opponents of the pipeline, however, are critical of this impact assessment. They contend that the lack of transport infrastructure and the price discount it occasions has already affected production of the oil sands and, if continued, would further depress investment and development in the region. This report presents a summary of life-cycle emissions assessments of Canadian oil sands crudes and provides an analysis of their respective findings. The first section of the report, " Life-Cycle Assessment Methodology ," discusses the basic methodology of life-cycle assessments and examines the choice of boundaries, design features, and input assumptions. The second section of the report, " Results of Selected Life-Cycle Emissions Assessments ," compares several of the publicly available assessments of life-cycle GHG emissions data for Canadian oil sands crudes against each other, against those of other global reference crudes, and against those of other fossil fuel resources. The report concludes with a discussion of some tools for policy makers who are interested in using these assessments to investigate the potential impacts of U.S. energy policy choices on the environment. For a specific analysis of the GHG emissions attributable to the proposed Keystone XL pipeline, see CRS Report R43415, Keystone XL: Greenhouse Gas Emissions Assessments in the Final Environmental Impact Statement , by [author name scrubbed] . Life-cycle assessment (LCA) is an analytic method used for evaluating and comparing the environmental impacts of various products (in this case, the climate change implications of hydrocarbon resources). LCAs can be used in this way to identify, quantify, and track emissions of carbon dioxide and other GHG emissions arising from the development of these hydrocarbon resources, and to express them in a single, universal metric of carbon dioxide equivalent (CO 2 e) GHG emissions per unit of fuel or fuel use. This figure is commonly referred to as the "emissions intensity" of the fuel. The results of an LCA can be used to evaluate the GHG emissions intensity of various stages of the fuel's life cycle, as well as to compare the emissions intensity of one type of fuel or method of production to another. GHG emissions profiles modeled by most LCAs are based on a set of boundaries commonly referred to as "cradle-to-grave," or, in the case of transportation fuels such as petroleum, "Well-to-Wheels" (WTW). WTW assessments for petroleum-based transportation fuels focus on the emissions associated with the entire life cycle of the fuel, from extraction, transport, and refining of crude oil; to the distribution of refined product (e.g., gasoline, diesel, jet fuel) to retail markets; to the combustion of the fuel in end-use vehicles. Other LCAs (e.g., Well-to-Tank [WTT] or Well-to-Refinery Gate [WTR]) establish different (i.e., more specific) life-cycle boundaries to evaluate emissions (see Figure 1 ). Inclusion of the final combustion phase allows for the most complete picture of petroleum's impact on GHG emissions, as this phase can contribute up to 70%-80% of WTW emissions. However, other LCAs can be used to highlight the differences in emissions associated with particular stages as well as experiment with certain boundary assumptions. The choice of boundaries is an important component to any LCA and can lead to vastly differing reported results. Because of the complex life cycle of hydrocarbon fuels and the large number of analytical design features that are needed to model their emissions, LCAs must negotiate many variables and uncertainties in available data. Key factors that influence the results of an LCA include (1) composition of the resource that is modeled, (2) extraction process of the resource that is modeled, (3) design factors chosen for the assessment, and (4) assumptions made in the input data for the assessment. Some of these factors with respect to Canadian oil sands crudes are as follows: Crude Oil T ypes . Oil sands are a type of unconventional petroleum deposit. They are commonly formations of loose sand or consolidated sandstone containing naturally occurring mixtures of sand, clay, and water, as well as a dense and extremely viscous form of petroleum technically referred to as bitumen. Most LCAs do not include an assessment of raw bitumen, because it is near solid at ambient temperature and cannot be transported in pipelines or processed in conventional refineries. Thus, bitumen is often diluted with liquid hydrocarbons or converted into a synthetic light crude oil to produce the resource known as "oil sands-derived crude" or simply "oil sands crude." Several kinds of crude-like products can be generated from bitumen, and their properties differ in some respects from conventional light crude. They include the following: Upgraded Bitumen, or Synthetic Crude O il (SCO) . SCO is produced from bitumen through an upgrading process that turns the very heavy hydrocarbons into lighter fractions. Since the upgrading process begins at the production facility for SCO, the allocation of GHG emissions is weighted more heavily upstream than other crude types. Diluted B itumen ( D ilbit) . Dilbit is bitumen mixed with diluents—typically natural gas liquids such as condensate—to create a lighter, less viscous, and more easily transportable product. Mixing bitumen with less carbon-intensive diluents lessens the GHG emissions impact per barrel of dilbit in relation to bitumen or SCO. Some refineries need modifications to process large quantities of dilbit feedstock, since it requires more heavy oil conversion capacity than conventional crudes.