Lead Author:
Peter J. Marcotullio, Hunter College, City University of New York
Contributing Authors:
Lori Bruhwiler, NOAA Earth System Research Laboratory
Steven Davis, University of California, Irvine
Jill Engel-Cox, National Renewable Energy Laboratory
John Field, Colorado State University
Conor Gately, Boston University
Kevin Robert Gurney, Northern Arizona University
Daniel M. Kammen, University of California, Berkeley
Emily McGlynn, University of California, Davis
James McMahon, Better Climate Research and Policy Analysis
William R. Morrow, III, Lawrence Berkeley National Laboratory
Ilissa B. Ocko, Environmental Defense Fund
Ralph Torrie, Canadian Energy Systems Analysis and Research Initiative
Expert Reviewer:
Sam Baldwin, DOE Office of Energy Efficiency and Renewable Energy
Science Lead:
Paty Romero-Lankao, National Center for Atmospheric Research (currently at National Renewable Energy Laboratory)
Review Editor:
Emily J. Pindilli, U.S. Geological Survey
Federal Liaison:
Nancy Cavallaro, USDA National Institute of Food and Agriculture

Energy Systems

This section presents a description of the state of the North American energy system by first identifying the size of the system in terms of population and economy, energy resources, and primary energy supply. End-use sectors of buildings, industry, and transportation, along with electricity generation, are then discussed and their regional contributions to the carbon cycle evaluated. Technologies for increasing efficiencies and lowering emissions levels are briefly described for each sector. The last subsection describes promising technologies for increasing carbon sinks.

The data compiled for this assessment come from a variety of sources, which have different methods of estimating and reporting energy use and emissions levels. For example, the International Energy Agency (IEA) of the Organisation for Economic Cooperation and Development (OECD) reports energy consumption on a net calorific value (or low heat value), while the U.S. Department of Energy’s (U.S. DOE) Energy Information Administration (EIA) and Canada report on a gross calorific value (or high heat value; IEA 2016c). (For a discussion of the different inventories and their sectoral scope and methodologies, see Appendix E: Fossil Fuel Emissions Estimates for North America.) This section presents data as consistently as possible, using ranges when there is significant disagreement between numbers. When possible, sources are combined using national data to present absolute values for energy and emissions from end-use sectors, and international sources are used in presenting shares of regional totals.

3.3.1 Size of the North American Energy System

By 2013, the North American energy system was serving around 491 million people, or about 6.7% of the global population (UN 2015). Of North America’s population, Canada contributed 7%, Mexico 26%, and the United States 67% (UN 2015). According to the World Bank (2016a), North America in 2013 had a combined GDP of more than $19.7 trillion (constant US$ 2010), almost 26% of world GDP. Within North America, the approximate 2013 GDP per capita was $49,200 for Canada, $49,900 for the United States, and $9,300 for Mexico (constant US$ 2010).

The World Energy Council (2016a) and BP (2017b) have identified massive fossil fuel energy reserves in North America (see Table 3.1). “Proven” or “proved” coal reserves exceed 7.2 zetajoules (ZJ), accounting for more than 27% of the world share in 2015 (for definitions of reserves and resources, see Box 3.1, Energy Resources and Reserves). Most North American coal is high quality: 46% is bituminous, 40.7% subbituminous, and only 13.2% lignite, which has the lowest heat content of the three types of coal (World Energy Council 2013). The majority of these coal reserves, almost 6.95 ZJ, are in the United States, which produced 23.8 EJ of coal in 2015. This production represents a 10.4% decline from 2014, as coal consumption has decreased by 20% from 2011 levels (Houser et al., 2017). Canada’s coal deposits, most of which are in the western provinces, are significant as well, reaching 193 EJ. Mexico’s coal reserves are small by comparison, totaling 37 EJ. At current production rates, North America has more than 270 years of proven coal reserves.

Table 3.1. North American Proven Energy Reserves (2015)a

Country or Region Coal Recoverable Reserves Oil Recoverable Reserves Gas Recoverable Reserves
Canada 193.0 EJb 1,163.9 EJ 74.9 EJ
Mexico 35.9 EJ 62.8 EJ 12.2 EJ
United States 6,950.1 EJ 276.7 EJ 393.6 EJ
North America 7,201.3 EJ 1,503.1 EJ 481.5 EJ
North America Share of Global 27.5% 14.0% 6.8%
R/Pb (Years) 276.0 33.1 13.0

a Sources: BP (2016); World Energy Council (2016a).
b EJ, exajoule; R/P, reserve-to-production ratio.

The continent’s proven oil reserves amounted to 1.5 ZJ in 2011, or more than 12% of the global total in 2015. Canada’s oil reserves, the largest in North America, are the third largest in the world after Saudi Arabia and Venezuela. Particularly significant to the carbon cycle are Alberta’s oil sands, which underlie 142,000 km2 of land in the Athabasca, Cold Lake, and Peace River areas in the northern part of the province. Mining and processing this unconventional source of oil currently account for approximately 8.5% of Canada’s total CO2e emissions (Government of Alberta [Canada] 2016). Oil sands also now represent about 98% of Canada’s growing oil reserves and about half the country’s production in 2011. Despite this large reserve, in 2015 the United States produced 23.7 EJ, more than twice as much as Canada’s production of 9.04 EJ. The United States also has developed unconventional technologies for extracting oil, including from shales. Proven oil reserves in the United States increased by 57% from 2005 to 2015 (EIA 2016k), and by 2012 shale oil accounted for about 22% of those reserves (EIA 2014a). Mexico’s oil reserves have decreased over the past decades. Although the country’s Cantarell oil field is one of the largest in the world, production has declined since 2003. In 2011, Mexico’s oil reserves were 62.8 EJ. According to BP (2016), oil reserves within the country have fallen from 285 EJ in 1995. Mexican oil production has been relatively stagnant since 2009 (World Energy Council 2016a). Overall, the North American share of total global proven oil reserves was 14% in 2016, with a projected use of more than 32 years of reserves under current conditions (BP 2017b).

In 2015, North America’s proven natural gas reserves reached 482 EJ. The United States has about 82% of the total proven natural gas reserves in North America, and the continent has approximately 6.8% of world reserves. As with oil, unconventional extraction techniques have expanded the region’s reserves dramatically. Over the last 10 years, shale gas reserves in the United States have increased ninefold. As of 2015, the United States produces 22% of the world’s natural gas and Canada produces almost 5%. Mexico also has increased gas production over the past decade, producing as of 2015 about 1.5% of the world’s natural gas (BP 2016). North American proven gas reserves are projected to last another 13 years under current production conditions. However, the United States estimates its national gas reserves will last another 86 years. These estimates disagree because of different definitions of reserves (see Box 3.1). While international analysis typically uses proven reserves to estimate how long an energy reserve will last, the United States uses both proven and unproven technically recoverable resources (EIA 2017e).

The concept of proven reserves is mainly for stock accounting that energy entities maintain to ensure adequate production in the near future. At a global scale, for example, proven oil reserves relative to current production have changed very little over decades. Resources have various definitions, but as a very broad generalization, technological advances have consistently overcome depletion of fossil fuel reserves. This outcome is likely to continue over the short to medium term. Using regional proven reserves, however, holds tremendous potential for increasing the atmosphere’s carbon concentration.

In 2013, the three economies of North America had a combined total energy use that exceeded 125.6 EJ (EIA 2016c), or approximately 22% of global primary energy use. Of the total, Canada was responsible for approximately 11.9% (14.9 EJ), Mexico 6.5% (8.2 EJ), and the United States 81.6% (102.6 EJ). The per capita energy-use levels are relatively similar between the United States and Canada but different for Mexico. For example, according to the World Bank (2016a), in 2015, energy use per capita in Canada and the United States was 318 gigajoules (GJ) and 284 GJ, respectively, while Mexico’s was about 62 GJ.

Although about 81% of North America’s total energy use is from fossil fuels, the continent also has significant renewable and low-carbon inputs to the electricity system (see Table 3.2). These include 1) the world’s leading installed hydropower capacity; 2) 13% of the world’s solar capacity; 3) 28% of the global geothermal capacity; 4) approximately 86.9 gigawatts (GW) of wind capacity, which is rapidly increasing (e.g., 8.6 GW of wind power installed by the United States in 2015, a 77% increase from 2016); 4) significant nuclear capacity at approximately 114 GW (i.e., 29% of global nuclear capacity and 36% of global nuclear generation in 2016; Nuclear Energy Institute 2017; IAEA 2017); and 5) uranium resources estimated at 0.82 Tg (World Energy Council 2016a). Changes in the regional renewable energy generation capacity, via increases in renewable resources, are having significant effects on the regional energy system’s contribution to the carbon cycle (for a discussion of the renewable resources in the region, see Section 3.4.3 and Section 3.6.4).

Table 3.2 North American Nonfossil Fuel Electricity Capacity (2015)a

Area Hydro-Installed Capacity (GW)b Solar-Installed Capacity (GW)b Geothermal-Installed Capacity (GW)b Wind-Installed Capacity (GW)b Nuclear-Installed Capacity (GW)b
Canada 79.2 2.2 1.5 11.2 13.5
Mexico 12.4 0.2 1.1 3.1 1.4
United States 102.0 27.3 3.6 72.6 99.2
North America 193.0 29.8 23.7 86.9 114.1

a Sources: BP (2016); World Energy Council (2016a).
b GW, gigawatts.

Fossil fuel combustion contributes considerably to the global carbon cycle. In 2013, North American CO2e emissions from fossil fuel combustion exceeded 6.45 Pg CO2e (1.76 Pg C). These emissions, down approximately 11% from 2007 levels, represent about 20% of the global total for energy-related activities (see Section 3.4.1 for details). Among North American CO2e emissions from fossil fuels, coal accounted for 28%, petroleum 44%, and natural gas 28%. Energy-related CO2e emissions exceeded 5.4 Pg (1.47 Pg C) for the United States and 0.56 Pg (153 Tg C) for Canada and were about 0.45 Pg (123 Tg C) for Mexico (EIA 2016f). For 2013, the World Bank (2016b) estimated that CO2e emissions per capita from energy use were 18.8 Mg (5.1 Mg C) for the United States; 15.3 Mg (4.17 Mg C) for Canada; and 6.5 Mg (1.77 Mg C) for Mexico, well below the averages for the two other countries.

3.3.2 North American Subsystem Contributions to Carbon Emissions

The North American subsystems include residential and commercial buildings, industry, and transportation end-use sectors along with the electricity-generation sector. Each subsystem is described in this section by identifying its major components, followed by a description of primary energy source contributions, the total energy use within the sector in 2013, and related carbon emissions during that year. Each energy sector description includes sector characteristics of each of the three nations defined as the “region,” concluding with a brief overview of new and emerging technologies that increase efficiencies and lower carbon emissions. The final part attempts to synthesize much of this information through the presentation and discussion of energy and CO2e emissions flow diagrams specific to the U.S. energy system.


The North American electric power system is integrated through more than 35 transmission interconnections between Canada and the United States and about nine between Mexico and the United States (CEA 2014). The U.S. electrical system is the largest within North America, including more than 7,700 power plants, 1.1 million km of high-voltage transmission lines, 10.5 million km of distribution lines, and almost 56,000 substations (U.S. DOE 2017d) with over 1 billion kilowatts (kW) of installed generating capacity (CIA 2018). The Canadian electrical system has more than 1,700 power plants (CGD 2016), over 160,000 km of transmission lines (IEA 2010), and about 148 million kW in installed generating capacity (CIA 2018). Mexico’s energy system is also large, expanding and integrating with the U.S. system and containing about 400 thermal power plants (CGD 2012) with over 65 million kW in installed generating capacity (CIA 2018). Mexico’s national transmission grid includes approximately 50,000 km of mostly high- and medium-voltage lines, and the country is constructing dozens of new natural gas–fired power plants to meet increasing electricity demand (EIA 2016j).

In 2013, North America generated 17.9 EJ of electricity, 18% of which was from nuclear power, 14% from hydropower, 6% from nonhydroelectric renewables, and 62% from fossil fuels, with about 7% of this total lost in transmission and distribution. Within North America, Mexico was responsible for 5.6% of the continent’s total electricity generation, Canada 12.8%, and the United States 81.5%. Together, the total electricity generated by these countries in 2013 was approximately 22.5% of the global total (EIA 2016c).

The U.S. electricity sector contributed about 34% of total national CO2e emissions, or 556 Tg C, in 2013 (U.S. EPA 2016). In Canada, electricity generation accounted for approximately 12% of national CO2e emissions, or 85 Tg CO2e (23 Tg C; ECCC 2016b). Canada’s lower share of national emissions from electricity generation is due to the high share of hydropower in electricity generation as well as the high-carbon intensity (see Section 3.6.3) of the country’s other sectors. According to ­SEMARNAT-INECC (2016), the Mexican electricity sector emitted approximately 127 Tg CO2e (34.6 Tg C) in 2013, or about 26% of net national CO2e emissions. Recently, however, the Mexican government ended its state-owned electricity monopoly and subsequently held the first power auction in 2016, awarding more than 1.7 GW to solar and wind generation (Meyers 2016), suggesting changes in the future.

Emerging trends have been stressing the North American electricity sector. This system was not designed for the distributed and often nondispatchable generation (electrical energy that cannot be turned on or off to meet demand fluctuations) that is dominating electricity supply growth, the electrification of the transportation and low-temperature heat markets, and the effects of climate change itself. Although challenging, this changing landscape provides opportunities for increased efficiencies and lower emissions levels achievable through a number of energy-sector advances. These improvements include 1) grid modernization, 2) applications of intelligent technologies and next-generation components with “built-in” cybersecurity protections, 3) advanced grid modeling and applications, 4) distribution generation and innovative control system architectures, and 5) improved storage capacity (U.S. DOE 2017d). New energy storage technologies, including batteries to overcome solar and wind intermittency challenges, can help make these technologies directly competitive with fossil-based electricity options (Kittner et al., 2017). Advances in nuclear power such as small- and medium-sized and modular technologies offer opportunities to increase the already large fleet of plants, although the future of this technology remains unclear (see Box 3.2, Potential for Nuclear Power in North America and Section 3.4.4).

Residential and Commercial Buildings

North America’s building stock varies in quantity and quality. In 2013, Canada had 14.8 million residential households occupying over 2 billion m2, plus 480,000 commercial buildings with 739 million m2 of floor space (Natural Resources Canada 2015; Natural Resources Canada 2018a). Mexico had an estimated 28 million residential households and 25.5 million m2 of commercial floor space (UNEP 2009). The U.S. had 114 million residential households occupying almost 18 billion m2 (EIA 2015b) and more than 5.5 million commercial buildings with a total floor space of over 8 billion m2 (EIA 2012c).

In 2013, the North American commercial sector used about 9.7 EJ of energy, mostly from electricity (58%), natural gas (37%), and oil products (7%). Residential buildings used about 13.3 EJ in 2013, supplied mostly by electricity (43%), natural gas (41%), heating oil (8.7%), and biofuels and waste (6.4%) (IEA 2016d). Given the large building stock in the region, the residential and commercial buildings sector accounts for a large share of energy use. In Canada, Mexico, and the United States, commercial and residential building operations account for about 20%, 30%, and 40%, respectively, of each country’s primary energy consumption.

Much of the energy use in buildings is from electricity and natural gas. In 2013, U.S. buildings consumed 73% of the country’s electricity and 52% of direct natural gas (60% of which was for electricity generation; EIA 2015b). In the residential sector, a significant fraction of overall energy consumption is for space heating and air conditioning, although in the United States the share of heating and cooling has dropped from 58% in 1993 to 48% in 2009 (EIA 2013a). The main U.S. sources of heating during the winter months are natural gas or electric furnaces and electric heat pumps, but the range of equipment and fuels varies across climate regions (EIA 2017h). Energy consumption for appliances and electronics continues to rise, signaling the importance of nonweather-related energy use in homes (EIA 2013a). In Canada, approximately 63% of residential energy use is for space heating, with another 24% for water heating (Natural Resources Canada 2016c; Natural Resources Canada 2018b).

Alternatively, removing electricity-related emissions from the buildings sector makes the sector’s share of CO2e emissions across the region the lowest among end-use sectors. For example, in 2013, the U.S. commercial and residential sectors together accounted for 10% of total national CO2e emissions (U.S. EPA 2016; see Figure 3.3). The U.S. commercial sector emitted approximately 59 Tg C, and the residential sector was responsible for about 89.5 Tg C. The Canadian buildings sector emitted 74 Tg CO2e (20.2 Tg C), or 10% of total national emissions (ECCC 2016b). In Mexico, the buildings sector emitted about 25.6 Tg CO2e (7.0 Tg C) in 2013, representing about 5% of total net national emissions for that year (SEMARNAT-INECC 2016).

Technological opportunities for improved energy efficiency and reduced carbon emissions from the building sector are extensive. By 2030, building energy use could be cut more than 20% using known cost-effective technologies. The United States identified potential technological improvements for the residential and commercial sectors, including high-efficiency heat pumps, thin insulating materials, windows and building surfaces with tunable optical properties, high-efficiency lighting devices, and low-cost energy-harvesting sensors and controls (U.S. DOE 2015a). Many of these technologies address thermal properties of buildings and technologies for space heating and cooling energy services, thus effectively reducing electricity and natural gas usage.


The extremely diverse North American industrial sector consists of mining, manufacturing, and construction. Mining enterprises extract raw materials from Earth’s crust that are used as inputs for manufacturing and construction. Construction enterprises create North America’s built environment, including buildings, industrial facilities, and infrastructure such as roads and the electric power grid. Manufacturing consists of a wide variety of small, medium, large, and very large facilities with subsectors including iron and steel, chemicals and petrochemicals, nonferrous metals, nonmetallic minerals, transport equipment, machinery, food and tobacco, paper, pulp and printing, wood and wood products, textile and leather, and nonspecified industry.

Manufacturing, in particular, represents a complex and diverse sector that both contributes to CO2e emissions and offers the potential for reductions over the lifetime of manufactured products and materials. Manufacturing involves global supply chains of raw materials, processed materials, components, and final products that are sourced and traded globally. Manufacturing’s complex supply and trade networks are exemplified in a case study by the Clean Energy Manufacturing Analysis Center (CEMAC) describing a typical solar crystalline silicon photovoltaic (PV) panel, a clean energy technology that reduces emissions from power production. This solar end product includes polysilicon made in the United States and exported to many other countries (US$1.8 billion in total exports in 2014). These countries then make PV cells and modules that are re-imported back to North America (US$3.9 billion; CEMAC 2017). Another example is the manufacture of turbine components (e.g., nacelles and blades) in the United States from steel and other materials from multiple sources; the parts are then installed in the United States and also exported (US$0.4 billion) to Canada, Brazil, and Mexico. Because these complex supply and trade networks are not comprehensively understood, further study could play an important role in supporting efforts to reduce emissions from industrial end uses.

In 2013, the total energy use for the North American industrial sector was about 14.7 EJ. The major energy sources for industry included natural gas (40%), electricity (29%), biomass and wastes (11%), oil and oil products (10%), coal (8%), and heat (2%; IEA 2016d). Additionally, about 6.11 EJ were consumed as industrial nonenergy use, or feedstock, major sources of which included oil and oil products (88%) and natural gas (12%; EIA 2016i). For the North American agriculture and forestry sectors, total energy use was approximately 1.3 EJ, supplied mostly by oil and oil products (76%), electricity (15%), natural gas (6%), and biomass and wastes (3%; EIA 2016i). The United States consumed 17.2 EJ, representing 78% of this sector’s total energy and feedstock consumption in North America in 2013.

In 2014, IEA reports that the total North American industrial sector emitted 1.65 Pg CO2e (450 Tg C), of which the United States contributed 1.24 Pg CO2e, or 338 Tg C (IEA 2016d). Based on a comparison of U.S. DOE datasets for U.S. industrial sector emissions and the World Resources Institute’s CAIT database for CO2e emissions, the industrial sectors in Canada, Mexico, and the United States in 2012 emitted approximately 0.19 Pg CO2e (51.8 Tg C), 0.17 Pg CO2e (46.4 Tg C), and 1.63 Pg CO2e (445 Tg C), respectively. These estimates represent 27%, 24%, and 26%, respectively, of each country’s total energy sector CO2 emissions in 2012. By comparison, U.S. DOE reported 1.5 Pg CO2e (410 Tg C) for the United States, Natural Resources Canada reported 0.179 Pg CO2e (48.8 Tg C) for Canada, and the National Institute of Ecology and Climate Change (INECC) reported 0.115 Pg CO2e (6.4 Tg C) for Mexico in 2013. If electricity-related emissions are excluded from the industrial sector, U.S. industrial emissions were approximately 264 Tg C and Canada’s industrial emissions were about 41 Tg C in 2013. Both sets of values have remained at these respective levels through 2015 (EIA 2018e; Natural Resources Canada 2018c). In Mexico, INECC separates electricity emissions from other sectors (SEMARNAT-INECC 2016).

State-of-the-art technologies available today could provide energy savings for the manufacturing sector, although many have not yet penetrated the market. Clean energy manufacturing includes the minimization of energy and environmental impacts from the production, use, and disposal of manufactured goods. These technologies exist for a broad range of services, such as operations to convert raw materials to finished products, effective management of the use and flows of energy and materials at manufacturing facilities, and innovative new materials and new manufacturing technologies for products that affect supply chains (U.S. DOE 2015b).


North America has a vast, extensive transportation infrastructure. The U.S. interstate highway system is about 77,000 km long (second in length only to China’s), and the country’s road system covers more than 6.5 million km and includes over 600,000 bridges. This infrastructure provides the nation’s nearly 11 million trucks and over 250 million passenger vehicles (WardsAuto 2015) with direct access to ports, rail terminals, and urban areas. In addition to its more than 600 smaller harbors, the United States has over 300 commercial harbors that support more than 46.4 million twenty-foot equivalent units (TEUs) of annual port container traffic (World Bank 2016c).7 There are 3,330 existing public-use airports in the United States composing the National Plan of Integrated Airport Systems, which supports more than 9.5 million registered annual carrier departures worldwide (World Bank 2016c). Finally, the U.S. rail network includes approximately 260,000 km of track, 76,000 rail bridges, and 800 tunnels that help move both passengers and freight around the country (ASCE 2013).

Canada’s transportation infrastructure includes more than 1.3 million km of public roads, 38,000 km of which are in the National Highway System used by about 1 million trucks and 20.1 million passenger vehicles (WardsAuto 2015). The country has more than 560 port facilities supporting over 5.5 million TEUs of annual port container traffic (World Bank 2016c), 900 fishing harbors, and 202 recreational harbors. Canada’s 26 major airports are part of the National Airport System, which supports more than 1.2 million registered carrier departures worldwide every year (World Bank 2016c). In addition, there are 71 regional and local airports; 31 small and satellite airports; and 13 remote airports, including 11 in the Arctic. The Canadian rail system includes 45,700 km of track (Transport Canada 2015).

Mexico has a road network of more than 365,000 km used by 8.8 million registered trucks and more than 22.9 million passenger cars (WardsAuto 2015). The country also has approximately 110 major airports that carry out more than 470,000 registered carrier departures worldwide yearly, and its 76 seaports and 10 river ports support over 5.2 million TEUs of port container traffic annually (World Bank 2016c). Railroads in Mexico’s estimated 26,700-km railroad network generally operate within cities, such as Mexico City and Guadalajara. A proposed high-speed rail link would connect these two cities with other locations across the country.

According to IEA (2017a), total North American energy use for transportation exceeded 30 EJ in 2013. The U.S. transportation sector consumed around 28.5 EJ of this energy, 91.6% of which was from petroleum, 3.3% from natural gas, and 5.0% from biofuels (EIA 2017b; IEA 2016d). Canada’s transportation sector consumed approximately 2.6 EJ (IEA 2017a), and about 94% of transportation fuels were petroleum products and 5.3% natural gas (CESAR 2018). Mexico’s transportation sector consumed about 2.1 EJ in 2013, equal to 48% of total national energy consumption, with almost all of it from motor vehicles (Secretaría de Energía de México 2016).

In 2013, North American transportation CO2e emissions exceeded 2.15 Pg CO2e (585 Tg C). The U.S. transportation sector alone contributed approximately 1.80 Pg CO2e (499 Tg C) in 2013, or more than 28% of the nation’s total greenhouse gas (GHG) emissions (U.S. EPA 2016). During the same year, Canadian emissions exceeded 0.2 Pg CO2e (54 Tg C), accounting for about 24% of the country’s total emissions (ECCC 2017b). In Mexico, emissions from road vehicles in 2013 dominated transportation emissions, with vehicles emitting 0.153 Pg CO2e (41.7 Tg C), equal to 31% of the net national total. Total Mexican transportation-sector emissions were 0.174 Pg CO2e (47.5 Tg C), equal to 34% of net national emissions for that year (SEMARNAT-INECC 2016). Mexican transportation energy use and emissions are expected to rise dramatically over the coming decades (IEA 2015b).

The North American transportation system is clearly large, complex, and highly integrated with regional economic and social development. Because of transportation’s importance as an energy sector and its significant effects—including economic costs, risks of dependence on oil, environmental impacts on air quality and health, and carbon emissions—advancing clean (i.e., low-emission) and efficient vehicle systems and technologies could have extensive impacts across societies. A range of technologies at various stages of research and development offer the potential to increase energy efficiency and mitigate impacts, including reducing contributions to the carbon cycle. Key technologies for light- and heavy-duty vehicles include 1) low-temperature combustion engines; 2) alternative fuels and lubricants; 3) advanced light-weight, high-strength materials for vehicle body systems; 4) improved batteries and electric drives; 5) lower-cost and more durable fuel cells; and 6) more efficient onboard hydrogen storage. Beyond vehicle improvements, a variety of existing or developing technologies can be leveraged to meet projected increases in North American air, water, off-highway, and rail transportation. Improved technologies could reduce the energy intensity of the entire transportation system, resulting in significant reductions in carbon emissions (U.S. DOE 2015b).


Given the complexity of the energy system, comprehending the size of relative energy flows from primary supply to end use is difficult. Sankey diagrams, developed by Matthew Henry Sankey in 1898, demonstrate flows to and from individual system components via the width of the bands, which, in this case, are directly proportional to energy production, usage, and losses. This visual account helps to summarize not only how the system works, but where efforts to change operations may be most effective. Figure 3.3 presents Sankey diagrams for U.S. energy use and CO2e emissions in 2013. On the left side of the diagrams are the primary energy supply sources, and on the right side are the energy end uses with electricity generation in the middle. A few immediately notable points are reviewed in this chapter: 1) renewables make up a small share of energy flows (although that share is growing); 2) most coal fuel is used for electricity generation (although the band width is decreasing); 3) natural gas fuel is split largely between electricity generation and residential, commercial, and industrial energy uses (all of which are increasing); 4) most petroleum fuel is used for transportation with some for industry; 5) values for rejected or unused energy are larger than those for energy services (suggesting a potential for enhanced efficiency); and 6) the electricity generation and transportation sectors are the largest sources of CO2e emissions, followed by industry.


Figure 3.3: Flows of U.S. Energy Use and Carbon Emissions, 2013

Figure 3.3: Key: Tg C, teragrams of carbon. [Figure source: Adapted from Lawrence Livermore National Laboratory (2018), flowcharts.llnl.gov/commodities/energy.]


3.3.3 Carbon Sink Technologies

Carbon sequestration, the process of capturing and storing atmospheric carbon, has been proposed as a way to slow the atmospheric and marine accumulation of GHGs that are released by burning fossil fuels. One set of increasingly popular sequestration technologies comprises carbon capture and storage (CCS) and carbon dioxide utilization (CDU). CCS captures CO2 emissions produced from the use of fossil fuels in electricity generation and industrial processes, thus preventing them from entering the atmosphere after their subsequent storage in deep geological formations. The CCS process also can be used to take carbon directly out of the atmosphere, typically including CO2 capture, transport, and storage in depleted oil and gas fields or saline aquifer formations.

North American CCS achieved an important milestone in 2014, with Canada’s Boundary Dam Unit 3, with a net capacity of 120 megawatts (MW) becoming the first commercial power plant to come online with CO2 capture. The 38 large-scale CCS projects either in operation or under construction have a collective CO2 capture capacity of about 60 Tg per year, while the 21 in operation now capture 40 Tg CO2 per year (Global CCS Institute 2016). The present pace of progress in CCS deployment, however, falls short of that needed to achieve average global warming of 2°C (IEA 2015a). Constraints include financial and technological challenges to overcome low efficiency and energy losses, as well as a lack of public acceptance (Haszeldine 2009; Smit et al., 2014). Regardless, CCS technologies often are included in scenarios as an increasingly effective way to remove CO2 from the atmosphere (see Section 3.8). One particularly important application is bioenergy with carbon capture and storage (BECCS), which has been indicated as a key technology for reaching low-CO2e atmospheric targets (Fischer et al., 2007).

Carbon dioxide usage includes direct and indirect aspects. The most successful direct use has been in enhanced oil recovery (EOR) and enhanced coalbed methane (ECBM; CH4) recovery, in which CO2 is injected into oil or natural gas fields to enhance the resource recovery rate (NETL 2010, 2017). Indirect CDU technologies involve the reuse of CO2 emissions from power plants or industrial processes to produce value-added products. Indirect CDU includes using chemical, biochemical, and biotechnological means to create energy fuel, polymers, and carbonates from the CO2. Overcoming technical, economic, and strategic challenges remains an issue before this option becomes viable (Al-Mamoori et al., 2017; Song 2006).

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