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APPENDIX B Contribution of U.S. Transportation Sector to Greenhouse Gas Emissions and Assessment of Mitigation Strategies T he body of this report describes how climate change is expected to impact the U.S. transportation sector and identifies ways in which this impact might be ameliorated. The committeeâs charge also directed it to review what is known about the contribution of transportation to greenhouse gas (GHG) emissions: Drawing heavily upon analyses already prepared, the study will summarize current and projected contributions of transportation to climate change and the potential effects, costs, and benefits of strate- gies to reduce transportationâs impact. This would include strategies, for example, that affect land use patterns, influence mode choice, and involve alternatively fueled or more efficient motor vehicles. This appendix addresses this aspect of the committeeâs charge. HOW THE TRANSPORTATION SECTOR INFLUENCES CLIMATE CHANGE Transportation vehicles emit GHGs when fuel undergoes combustion in their engines. The vast majority of these combustion-related emissions consist of carbon dioxide (CO2).1 But road transport vehicles also emit 1 The U.S. Energy Information Agencyâs annual publication Emissions of Greenhouse Gases in the United States provides estimates of transport sector emissions of CO2, CH4, and N2O. In 2003, CO2 accounted for 97 percent of the total, when each gas is converted into its global warming 210
Contribution to Emissions and Assessment of Strategies 211 small amounts of nitrous oxide (N2O) and methane (CH4). Aircraft oper- ating at high altitudes emit not only nitrogen oxides (NOx) [which increases the rate of ozone production by speeding the oxidation of car- bon monoxide (CO) and CH4] but also water vapor [which generates contrails that, depending on the time of day they are produced, either reflect solar radiation back into space (daytime) or trap it (nighttime)] (IPCC 1999; see also Stuber et al. 2006). Transport activity is also associated with two additional categories of emissions: (a) those produced in the extraction, production, and distribu- tion of transport fuels and (b) those produced in the manufacture, distribution, and disposal of transport vehicles.2 A rough idea of the rela- tive significance of these additional categories of emissions can be obtained from life-cycle studies that attempt to track all emissions related to a vehi- cle and its fuel. One of the best known of these studies estimates that the life-cycle CO2 emissions generated by a 1996-vintage midsize U.S. passen- ger car using gasoline as its fuel total 263 g/km, of which the vehicle manufacturing cycle (including disposal) accounts for 18 g/km (6.8 per- cent); the fuel cycle, 49 g/km (18.7 percent); and fuel combustion, 196 g/km (74.5 percent) (Weiss et al. 2000, 5â8).3 In this appendix, the committee attempts to provide as comprehen- sive a picture as possible of transport-related GHG emissions. It was not feasible to include emissions from each life-cycle stage or emissions of each GHG gas; we do, however, take care to identify which emissions are included in the data presented. CURRENT AND PROJECTED TRANSPORT-RELATED GREENHOUSE GAS EMISSIONS According to the 2005 edition of the International Energy Agency (IEA) publication CO2 Emissions from Fuel Combustion, worldwide CO2 emis- potential. Nearly all the remainder was accounted for by N2O (U.S. Energy Information Administration 2004, 31, 49, 62). This publication provides no information on aerosols produced by transport activity, but these are believed to be relatively insignificant. 2 The second of these categories is of concern only with respect to road vehicles. The number of nonroad vehicles (locomotives, ships, and aircraft) is so small that the GHG emissions related to their manufacture, distribution, and disposal are minimal. 3 The report assumes 95 percent recycling of metals and 50 percent recycling of plastics. In the report, the emissions figures are stated in grams of carbon per kilometer. For consistency with the other emissions data in this appendix, the figures have been converted here to grams of CO2 per kilometer.
212 Potential Impacts of Climate Change on U.S. Transportation sions from fuel combustion in 2003 totaled 25.0 billion tonnes (IEA 2005). The transport sector accounted for 5.9 billion tonnes, or 23.6 percent of this total (IEA 2005).4 Another IEA publication (IEA 2006) provides âref- erence caseâ projections of emissions for 2050. According to that report, total CO2 emissions from fuel consumption in 2050 will be 57.6 billion tonnes (IEA 2006). Transport sector emissions will be 11.7 billion tonnes, or 20.3 percent of this total. The two IEA publications just cited do not provide a high level of modal detail. However, the World Business Council for Sustainable Developmentâs Sustainable Mobility Project (SMP) has published detailed modal estimates of emissions from fuel combustion for 2000 and projec- tions at 5-year intervals to 2050 (World Business Council for Sustainable Development 2004).5 The SMP also published estimates and projections of CO2, N2O, and CH4 emissions from the production and distribution of transport fuels. The SMPâs figures were generated by a model that was benchmarked to the IEA transport sector totals. Table B-1 shows the esti- mates and projections generated by this model. Light-duty passenger vehicles (LDVs), consisting of passenger cars, pickup trucks, sport utility vehicles (SUVs), and minivans, account for the largest share of transport-related emissions. This will continue to be the case even in 2050 if present trends continue. However, emissions from other modes, notably air transport and trucks used to haul freight, are extremely significant and are projected to grow faster than emissions from LDVs. The SMP report also provides estimates and projections of transport- related emissions by country/region. These are shown in Figure B-1. The United States is included in the region âOECD [Organisation for Economic Co-operation and Development] North Americaâ along with Canada and Mexico. One notable feature of Figure B-1 is the differences in relative growth rates of emissions within different countries/regions. Generally speaking, 4These are emissions from fuel combustion. 5The documentation for this model, as well as the model itself, can be found at www. sustainablemobility.org. The SMP characterizes its projections as what might occur âif present trends continue.â For a description of what is meant by the phrase âif present trends continue,â see Box 2.1 in the SMP report (p. 27). The report also can be found at the web address just cited. The report draws on 2003 data, the most recent available at the time the report was published. This appendix, which draws heavily on the SMP report, uses 2003 data because it would have been impractical to update the data contained in the SMP report.
Contribution to Emissions and Assessment of Strategies 213 TABLE B-1 World Transport Well-to-Wheels (Vehicle + Upstream) CO2-Equivalent Emissions by Mode (megatonnes) Year AAGR (%) Mode 2000 2025 2050 2000â2025 2025â2050 Freight + passenger rail 207 341 503 2.0 1.6 Bus 396 436 480 0.4 0.4 Air 733 1,487 2,583 2.9 2.2 Freight truck 1,446 2,423 3,582 2.1 1.6 Light-duty passenger vehicle 2,798 4,152 5,901 1.6 1.4 Two- and three-wheeler 110 209 313 2.6 1.6 Water 638 826 1,015 1.0 0.8 Total 6,328 9,874 14,378 1.8 1.5 Note: AAGR = annual average growth rate. Source: Data generated by the International Energy Agency/Sustainable Mobility Project (IEA/SMP) Spreadsheet Model. 14000.0 12000.0 Africa Latin America 10000.0 Middle East India Megatonnes 8000.0 Other Asia China 6000.0 Eastern Europe FSU 4000.0 OECD Pacific OECD Europe OECD North America 2000.0 0.0 2000 2005 2010 2015 2020 2025 2030 2035 2040 2045 2050 Year FIGURE B-1 Transport greenhouse gas emissions by region (all modes). (Source: Data generated by IEA/SMP Spreadsheet Model.)
214 Potential Impacts of Climate Change on U.S. Transportation emissions from countries not presently members of the OECD are pro- jected to grow much more rapidly than emissions from countries that are members of the three OECD regions. The factors responsible for this faster growth are discussed in more detail below. IEA estimates that U.S. transport-sector emissions from fuel combus- tion in 2003 totaled 1.8 billion tonnes, of which road transport (LDVs, motorized two- and three-wheelers, buses, medium- and heavy-duty trucks) accounted for 85 percent. The U.S. Environmental Protection Agency (EPA) provides estimates having a greater level of modal detail (see Table B-2). Comparison of the figures from Table B-2 with those from Table B-1 implies that in 2003, U.S. transport emissions of CO2 from fuel combustion accounted for 30 to 31 percent of total world transport CO2 emissions from fuel combustion, depending on whether international avi- ation and marine bunkers are included.6 The emissions factors developed by the SMP for the production and distribution of each type of transport fuel suggest that including fuel cycle emissions would add another 17.5 percent to the U.S. total (and 15.0 percent to the world total). EPA does not publish projections of future emissions at a similar level of detail. And, as already noted, the SMPâs projections are for OECD North America (i.e., the United States, Canada, and Mexico). According to the SMP data in Figure B-1, OECD North American transport-related emissions will have fallen from 37 percent of the world transport-related total to 26 percent by 2050. This decline in share is accounted for not by any absolute reduction in North American emissions but by the much more rapid rate of growth projected for emissions in regions (other than the other two OECD regions) outside North America. STRATEGIES FOR REDUCING TRANSPORT-RELATED GREENHOUSE GAS EMISSIONS The charge to the committee quoted above recognizes that a range of possible approaches exist by which transport-related GHG emissions 6 âInternational aviation and marine bunkersâ denotes fuel loaded on transport vehicles in the United States but consumed in international operations. Generally speaking, âinternational bunkersâ are not included in national totals, though they are included in the world totals cited above. The IEA estimates that in 2003, the combustion of international aviation bunkers accounted for 359 million tonnes of CO2 emissions. The combustion of international marine bunkers is estimated to have accounted for 459 million tonnes of CO2 emissions.
TABLE B-2 U.S. CO2 Emissions from Fossil Fuel Combustion in Transportation End-Use Sector, 2003 (Tg CO2 Eq.) Fuel Type Distillate Mode Fuel Oil Aviation Residual Natural Sharea Vehicle Type Gasoline (Diesel) Jet Fuel Gasoline Fuel Oil Gas LPG Electricity Mode Totala (%) Road vehicles 1,464.1 78.9 Automobiles 630.2 3.4 0.0 633.6 34.2 Light-duty trucks 460.9 17.6 0.0 0.3 478.8 25.8 Other trucks 39.6 301.1 0.5 341.2 18.4 Buses 0.3 8.0 0.6 0.0 8.9 0.5 Motorcycles 1.6 1.6 0.1 Rail 39.6 3.2 42.8 2.3 Waterborne 82.1 4.4 Ships and boats 17.0 29.5 46.5 2.5 Ships (bunkers) 6.0 18.6 24.6 1.3 Boats (recreational) 11.0 11.0 0.6 Aircraft 230.8 12.4 Commercial aircraft 122.8 122.8 6.6 Military aircraft 20.5 20.5 1.1 General aviation 9.4 2.2 11.6 0.6 Other aircraft 16.3 16.3 0.9 Aircraft (bunkers) 59.6 59.6 3.2 Pipeline 34.8 34.8 1.9 Fuel totala 1,166.6 369.7 228.6 2.2 48.1 35.4 0.8 3.2 1,854.6 100.0 Fuel sharea (%) 62.9 19.9 12.3 0.1 2.6 1.9 0.0 0.2 Note: Totals may not sum because of independent rounding. LPG = liquefied petroleum gas. a Includes aircraft and waterborne international bunkers. Source: USEPA 2005, Table 3-7.
216 Potential Impacts of Climate Change on U.S. Transportation might be reduced. The committee believes that the best way of organiz- ing the present discussion of this range of approaches is through the use of the âASIF decomposition.â The CO2 emissions from fuel combustion by transport vehicles can be characterized by the following equation: G = AâSiâIiâFi,j where G = CO2 emissions from fuel combustion by transport; A = total transport activity; Si = modal structure of transport activity; Ii = energy consumption (fuel intensity) of each transport mode; and Fi,j = GHG emissions characteristics of each transport fuel (i = trans- port mode, j = fuel type). The product of the first two variables on the right-hand side of this equation, A and Si, is the demand for transport services provided by transport mode i. The product of the last two variables, Ii and Fi,j, is the GHG generated by each unit of transportation service provided by mode i using fuel type j.7 Historically, the primary driver of transport-related GHG emissions has been the growth of total transport activity (A). The primary offsetting factor has been a reduction in the energy required to produce each unit of transport services (I). However, improvements in transport energy effi- ciency have been overwhelmed by the increase in transport activity. Changes in the modal structure of transport activity (S) have tended to boost GHG emissions in two ways. First, activity has tended to shift from less energy-intensive transport modes (e.g., rail) to more energy-intensive modes (e.g., truck). Second, in some modes (e.g., LDVs), the load factor (the percentage of vehicle capacity actually utilized) has fallen sharply.8 Changes in the emissions characteristics of transport fuels (F) have had little impact one way or another. 7 This formulation was originally popularized by Lee Schipper. This particular version is taken from IEA (see IEA 2000, 22). 8 In both trucking and air transport, improvements in average load factors have tended to offset some of the impact of the inherently higher energy intensiveness of the mode. The energy intensiveness of the modes (I) has increased somewhat as a result. (It takes more energy to move heavier average loads.) But the increase in energy required is considerably less than proportional to the increase in load.
Contribution to Emissions and Assessment of Strategies 217 80.0 70.0 Africa Trillions of Passenger-Kilometers Latin America 60.0 Middle East 50.0 India Other Asia 40.0 China Eastern Europe 30.0 FSU 20.0 OECD Pacific OECD Europe 10.0 OECD North America 0.0 2000 2005 2010 2015 2020 2025 2030 2035 2040 2045 2050 Year FIGURE B-2 Passenger transport activity by region. (Source: World Business Council for Sustainable Development 2004, Figure 2-2, p. 30.) Reducing the Volume of Transport Activity (A) or Altering the Modal Structure of Transport Activity (S) The SMP report projects that worldwide personal transport activity, which totaled 32.3 trillion passenger-kilometers (pkm) in 2000, will grow to 74.0 trillion pkm by 20509 (see Figure B-2). Worldwide goods transport activity (excluding waterborne),10 which in 2000 totaled 14.4 trillion tonne-kilometers (tkm),11 is projected to grow to 45.9 tkm (see Figure B-3). These projections imply average annual rates of growth of 1.7 percent for personal transport activity and 2.3 percent for goods transport activity (again excluding waterborne). Figures B-2 and B-3 also indicate that rates of growth of both personal and goods transport activity are likely to vary widely across countries/ 9Passenger-kilometer is defined as the transportation of one passenger a distance of 1 kilometer. 10 The SMP report does not project waterborne freight activity. According to the United Nations Conference on Trade and Development (UNCTAD 2005), in 2000, world seaborne trade totaled 23.7 trillion tonne-miles, or 43.9 tonne-kilometers (tkm). Of this total, 41 percent was oil and oil products, 29 percent was the five main dry bulk commodities (including iron ore, coal, and grain), and 30 percent was other dry cargoes (including containerized cargoes). We assume that the âmilesâ reported by UNCTAD (2005) are nautical miles. If so, this means that in 2000, ocean shipping accounted for 75 percent of all tkm of freight carried. 11 Tonne-kilometer is defined as the transportation of 1 metric ton (tonne) of freight a distance of 1 kilometer.
218 Potential Impacts of Climate Change on U.S. Transportation 50.0 45.0 Africa 40.0 Latin America Trillions of Tonne-Kilometers 35.0 Middle East India 30.0 Other Asia 25.0 China EasternEurope 20.0 FSU 15.0 OECD Pacific OECD Europe 10.0 OECD North America 5.0 0.0 2000 2005 2010 2015 2020 2025 2030 2035 2040 2045 2050 Year FIGURE B-3 Goods transport activity (excluding waterborne). (Note: Waterborne activity not available by country/region. According to the United Nations Conference on Trade and the Environment, in 2000, worldwide waterborne transport activity totaled 43.9 trillion tkm.) (Source: World Business Council for Sustainable Development 2004, Figure 2-5, p. 32.) regions, reflecting basic economic and demographic changes discussed below. At present, the majority of both personal and goods transport activity occurs within or between countries that are members of the OECD.12 Over the next half-century, however, transport activity is pro- jected to grow much more rapidly in those countries that are not presently OECD members. These higher growth rates, if achieved, imply that non- OECD personal transport activity will exceed OECD personal transport activity by about 2025. The crossover point for goods transport activity is likely to be even soonerâperhaps as early as 2015. Drivers of the Volume of Personal and Goods Transport Activity Numerous factors influence the rate of growth of personal and goods transport activity, but the following are especially important: (a) the level and rate of growth of real per capita income, (b) the rate of population 12 The OECD was formed in 1961 by the following countries: Austria, Belgium, Canada, Denmark, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, the Netherlands, Norway, Portugal, Spain Sweden, Switzerland, Turkey, the United Kingdom, and the United States. The following countries have since joined: Japan (1964), Finland (1969), Australia (1971), New Zealand (1973), Mexico (1994), the Czech Republic (1995), Hungary (1996), Poland (1996), the Republic of Korea (1996), and Slovakia (2000).
Contribution to Emissions and Assessment of Strategies 219 25.0 Per Capita Personal Travel Activity (thousands of km) OECD North America 20.0 OECD Pacific 15.0 OECD Europe 10.0 Eastern Europe FSU 5.0 Middle East Other Asia Latin America India China Africa 0.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 Per Capita Real Income (thousands of US$, Purchasing Power Parity Basis) FIGURE B-4 Per capita personal travel activity versus per capita real income. (Source: Data generated by IEA/SMP Spreadsheet Model.) growth, (c) the share of population residing in urban areas, and (d) the spatial organization of urban areas (also called âurban formâ). In addi- tion, a potentially salient factor is the impact of telecommuting and the Internet on travel demand. Level and Rate of Growth of Real per Capita Income Transportation activity both drives and is driven by the level and rate of growth of real per capita income. This should not be surprising. Transportation services are a major enabler of economic growth, and as people become wealthier, they find more reasons to travel. Figure B-4 shows the relationship between real gross domestic product (GDP) per capita and per capita personal travel in 2000 for the countries/regions included in the SMP report.13 In 2000, the average resident of an OECD country traveled 5.7 times as many kilo- meters per year as did the average resident of a non-OECD countryâa slightly lower ratio than that between the average real per capita incomes of the two country groupings. 13 Similar data for goods transport activity are not provided because the information on waterborne originâdestination pairs needed to assign that important transport activity to countries/regions is lacking.
220 Potential Impacts of Climate Change on U.S. Transportation 3.0% Projected Change in per Capita Personal Transport Demand China 2.5% Eastern Europe 2.0% FSU Latin America 1.5% India OECD Europe 1.0% Other Asia OECD North OECD Pacific 0.5% America Africa 0.0% Middle East -0.5% 0.0% 0.5% 1.0% 1.5% 2.0% 2.5% 3.0% 3.5% 4.0% 4.5% Projected Change in Real Gross Domestic Product per Capita (Purchasing Power Parity Basis) FIGURE B-5 Projected change in real per capita personal transport demand versus projected change in real gross domestic product per capita (purchasing power parity basis), 2000â2050. (Source: Data generated from IEA/SMP Spreadsheet Model.) Figure B-5 shows the relationship between the projected rate of change in real GDP per capita and the projected rate of change in per capita per- sonal travel over the period 2000â2050. Note the difference in the relative positions in the two exhibits of the three OECD regions and those non- OECD countries/regions in which economic growth is projected to grow the most rapidly. Given the relationship between real per capita income growth and per capita personal (and probably also goods) transport activity, it is obvious that if the former were slower, so would be the latter. However, most peo- ple (especially those living in countries where real per capita GDP is relatively low today but is projected to grow rapidly) would find highly unpalatable a strategy of deliberately slowing growth in real GDP per capita in order to slow the growth of travel activity. This does not mean that the link between real per capita income growth and per capita travel activity is immutable. But it does mean that if this link is to be weakened,
Contribution to Emissions and Assessment of Strategies 221 the measures employed will have to be less draconian than limiting eco- nomic growth. This issue is revisited below. Rate of Population Growth The projected trends in real per capita income growth, if realized, will be a powerful force serving to increase transport activity. However, another factor that has exerted a powerful influence in increasing transport activity in the recent pastâpopulation growthâwill be waning in importance in the future. Population growth rates are falling everywhere. This should not be sur- prising. Figure B-6 illustrates how extraordinary population growth during the last half of the 20th century truly was. This explosion of world popula- tion was caused by reduced mortality in the less developed regionsâa reduction that was not accompanied (at least initially) by declining fertil- ity rates in these regions. One major factor enabling this rapid growth in population was the increased ability to transport food, especially by ship and rail. The growth in population, together with improved transport, stimulated the demand for finished goods. During the last quarter of the Millions Billions 100 10 Population increment 80 8 Population size 60 6 Annual increments 40 4 20 Population size 2 0 0 1750 1800 1850 1900 1950 2000 2050 FIGURE B-6 Long-term world population growth, 1750â2050 (projected). (Source: UN 1999, Figure 1, p. 7. Reprinted with permission of the United Nations Population Division.)
222 Potential Impacts of Climate Change on U.S. Transportation 20th century, the rate of population growth began to fall, largely because of declines in fertility. As with real per capita income growth, the pattern of population growth over the period 2000â2050 will differ by region. As a result of sharp fertility declines, the average age of the population in many countries will be rising. In 1950, the median age of the worldâs population was 23.5 years. It is estimated to have increased to 28.1 years by 2005, and by 2050 is projected to reach 37.8 years.14 Share of Population Residing in Urban Areas Most personal travel is within or between urban areas. Urban areas depend on transport systems to supply them with food, energy, raw materials, and finished goods. It is not surprising, therefore, to find a link between urbanization and trans- port activity. In 1950, about half of the people in the more developed regions lived in cities15 (see Table B-3). By 2000 this figure had increased to nearly 75 percent.16 However, the growth in urbanization is tapering off in the coun- tries of the more developed regions. In the next 30 years, the percentage of people living in urban areas in these countries will increase only to 82 per- cent. The total urban population in these regions will experience a relatively small increase, with most of that increase occurring in North America (the United States and Canada).17 In contrast, urban populations in countries located in the less developed regions will grow rapidly. Spatial Organization of Urban Areas (Urban Form) The spatial organi- zation of urban areas exerts an independent influence on the total volume of personal and goods transport activity, as well as on modal choice. When 14 The median age of the U.S. population, which was 30.0 years in 1950, is estimated to have reached 36.1 years by 2005 and is projected to grow to 41.1 years by 2050 (UN 2005, Table VIII-12). 15 The categorization of countries into those located in âmore developed regionsâ and âless developed regionsâ is used by the United Nations âfor statistical convenience and [does] not necessarily express a judgment about the stage reached by a particular country or area in the developing processâ (UN 2005, ii). Generally speaking, countries in the more developed regions are members of the OECD, while those in the less developed regions are not. But there are some important exceptions. Russia and the European portions of the former Soviet Union are included in the more developed regions but are not OECD members. Mexico, South Korea, and Turkey are OECD members but are designated by the United Nations as being in the less developed regions. 16 In 2000, 80 percent of the U.S. population lived in metropolitan areas as defined by the U.S. Census. 17 The figure for the United States is projected to increase to 87 percent by 2030 (UN 2003, Tables A-3 and A-5).
Contribution to Emissions and Assessment of Strategies 223 TABLE B-3 World Urbanization Trends, 1950â2030 (Projected) More Developed Regions Less Developed Regions Northern Regional Regional Europe Americaa Japan Total China India Total Urban Population (billions) 1950 0.28 0.11 0.03 0.43 0.07 0.06 0.31 1975 0.45 0.18 0.06 0.70 0.16 0.13 0.81 2000 0.53 0.25 0.08 0.88 0.60 0.28 1.97 2030 (projected) 0.55 0.35 0.09 1.01 0.88 0.59 3.93 Change 2000â2030 0.02 0.10 0.01 0.13 0.28 0.31 1.96 Average Annual Rate of Change (%) 1950â1975 1.9 2.0 3.2 2.0 3.4 3.1 3.9 1975â2000 0.7 1.3 1.1 0.9 5.4 3.1 3.6 2000â2030 0.1 1.2 0.2 0.5 1.3 2.5 2.3 (projected) Percentage Urban 1950 51 64 35 53 13 17 18 1975 66 74 57 67 17 21 27 2000 73 79 65 74 36 28 41 2030 (projected) 80 87 73 82 61 41 57 Change 2000â2030 7 8 8 8 25 13 16 Northern America = Bermuda, Canada, Greenland, Saint-Pierre-et-Miquelon, and United States. a Source: UN 2003, Tables A.2, A.3, and A.6. (Reprinted with permission of the United Nations Population Division.) people talk about eliminating âunnecessaryâ travel or âunlinkingâ eco- nomic growth from transport activity, they generally are referring to the deliberate alteration of urban areasâ spatial organization to influence the total volume of personal and goods transport activity, the choice of modes by which the demand for that activity is fulfilled, and the capacity utiliza- tion rates (i.e., load factors) of the vehicles being used. How the Spatial Organization of Urban Areas Affects and Is Affected by Transport Activity and Modal Availability Throughout history, the size and shape of cities have been constrained by the ability of their transport systems to supply them with food and raw materials, to enable their res- idents to congregate in numbers sufficient to convert these raw materials
224 Potential Impacts of Climate Change on U.S. Transportation into finished goods efficiently and to conduct other business requiring face-to-face interaction, and to transport their finished goods to distant markets. The development of inexpensive waterborne transportation eased the first and third of these constraints. But cities were still severely limited in size by their ability to move people from their homes to work and back on a daily basis. Until roughly the middle of the 19th century, the area of a city like London was constrained by the distance people could walk from home to work.18 The development of railways linking the suburbs and the central business district (CBD), together with the development of means of mov- ing masses of people underground within the CBD, enabled Londoners and residents of other major cities to live much greater distances from their work. However, the availability of these modes of high-speed pub- lic transport did not necessarily change the need for peopleâs activities other than commuting to be located within a relatively short distance of where they lived. Only when automobile availability became widespread did the location constraints on these other activities ease. Indeed, noncommute activities now account for the majority of personal trips and miles traveled in most high-income countries. Table B-4 illustrates this point for the United States with data from the 2001 National Personal Transportation Survey. The spatial organization of urban activity also affects and is affected by goods transport. Before the advent of the automobile and the truck, nearly all large cities had a single, relatively compact CBD where a large share of the cityâs employment was concentrated. (In the terminology used by urban planners, these cities were âmonocentric.â) The location of the CBD was usually determined by its proximity to waterborne (and later to rail) transportation. The development of the motorized truck freed CBDs from these constraints. Thus, the widespread ownership of motorized road vehicles allowed workers to both live and work almost anywhere they wished within a metropolitan region. The resulting decline in average residential and employment densities has undermined the viability of public transport, 18Indeed, the first traffic count of people entering the 1 square mile City of London between 8 a.m. and 8 p.m. (1854) found that horse-drawn omnibuses were the means by which the largest number of people (44,000) were transported into town; 31,000 arrived by train; and 26,000 entered by using private carriages or hackney cabs. But all these modes of transport were dwarfed by the 200,000 who walked.
Contribution to Emissions and Assessment of Strategies 225 TABLE B-4 Characteristics of U.S. âDailyâ or âShort Distanceâ Personal Travel, 2001 Trips Kilometers Annual travel (per capita) 1,481 24,459 Purpose of travel Commuting/business 18% 26% School, church 10% 6% Shopping 19% 13% Family, personal business, escort 25% 20% Social/recreational, vacation, visiting friends, and other 28% 35% Source: CRA International compilation from the 2001 National Household Travel Survey (BTS 2001). especially rail-based public transport. Only cities that have managed to maintain strong CBDs and that developed high-speed public transport systems before the automobile came to dominate personal travel have managed to keep the share of commuting travel by private car relatively low.19 And this is only true for workers traveling to work in the CBD; those with jobs outside the CBD generally commute to work by car. Magnitude of the Impact of the Spatial Organization of Urban Areas on Transport Activity and Modal Choice Studies have demonstrated a statis- tically significant relationship between the spatial organization of urban areas and the volume of personal travel activity. But just how quantitatively significant is this relationship? Two recent U.S. studies provide information on this question. A Transportation Research Board policy study is also under way that is examining the relationships among development pat- terns, vehicle miles traveled (VMT), and the energy conservation benefits of denser development patterns. Ewing et al. (2002) ranked 83 U.S. cities in terms of a âsprawl indexâ composed of four components: residential density; the neighborhood mix of homes, jobs, and services; the strength of activity centers and down- towns; and the accessibility of street networks. They compiled information on the rate of vehicle ownership, the share of commuters taking transit to work, the share of commuters walking to work, the average commute time, 19 The share of commuters working in Manhattan who drive to work is just over 10 percent. The data for commuters working in central London are similar.
226 Potential Impacts of Climate Change on U.S. Transportation TABLE B-5 How Personal Transportation Demand Is Influenced by Urban Form Vehicle Miles Sprawl Vehicles Transit to Walk to Commute Traveled per MSA/PMSA Name Index per 100 HH Work (%) Work (%) Time (min) HH (mi/day) Averageâ10 most 58.86 180 2.1 1.92 26 70.18 sprawling Averageâ10 least 132.29 162 7.0 3.56 26 54.45 sprawling (excluding outliers) Differenceâ10 most 73.43 â18 4.9 1.64 0 â15.73 sprawling and 10 least sprawling (excluding outliers) Excluding outliers Jersey City, N.J. PMSA 162.27 93 34.2 8.71 33 N/A New York, N.Y. PMSA 177.78 74 48.5 9.61 39 40.19 Note: HH = household; MSA = metropolitan statistical area; PMSA = primary metropolitan statistical area. Source: Derived from Ewing et al. 2002, Appendix 3. and the vehicle miles traveled per household per day for each of these cities. Table B-5 shows the average of these transport indicators and the average sprawl index20 for the 10 âmost sprawlingâ and the 10 âleast sprawlingâ cities. The latter category excludes two clear outliersâNew York City and Jersey City21âwhich are shown separately in Table B-5. The range in the researchersâ sprawl index between these two groups of citiesâ73.43âis almost three standard deviations. Over this range, there is an 11 percent difference in the number of cars per hundred households (180 versus 162) and a 29 percent difference in the number of VMT per household per day (70.18 versus 54.45). For both groups of cities, the share of commuters taking public transit to work averages below 10 percent, and the share of commuters walking to work averages less than 5 percent. The average daily one-way commute time in both groups of cities is identicalâ 26 minutes. 20 The sprawl index is scaled so that 25 units is equal to one standard deviation. The index ranges in value from 14.22 to 177.78, with a lower value indicating greater sprawl. 21 The New York City region (which includes Jersey City) accounts for approximately 40 percent of all U.S. public transport trips.
Contribution to Emissions and Assessment of Strategies 227 The second study, conducted by Bento et al. (2005), covered 114 U.S. urban areas. Instead of developing a sprawl index, these researchers used the actual values of different variables that they believe reflect the spatial organization of these urban areas. They used these characteristics, plus other control variables, to predict the average VMT per household, the average probability of driving to work by workers, and the average annual commute miles for the âaverage U.S. householdâ if it resided in each urban area. Table B-6 shows these predicted values for six U.S. urban areas: Atlanta, Boston, Chicago, Houston, New York, and San Diego. The difference in predicted average annual VMT per household between Atlanta and Boston (the latter being the âmost compactâ urban area other than New York City) is 25 percent (16,899 versus 12,704 VMT). The pre- dicted average probability of driving to work by workers does not fall below 70 percent for any city except New York City. The average number of commute miles driven per year ranges between 4,500 and 5,600 miles. The differences in transport activity and modal choice among U.S. urban areas reported in the above two studies are not trivial, but they need to be placed in perspective. Cities change slowly, and their changes are heavily path dependent. Transforming a city with the spatial organi- zation of Atlanta into one with the spatial organization of Boston would be a tremendous task requiring many decades, if it could be accom- plished at all. Making marginal changes over time might be practical, but even this would not be easy. Moreover, marginal changes are likely to yield only marginal results. Impact of Telecommuting and the Internet on Travel Demand Some have argued that the development of telecommuting and the Internet could reduce travel demand significantly. This does not appear to be hap- pening. Mokhtarian (2003) summarizes the evidence as follows: Overall, substitution, complementarity, modification, and neutrality within and across transportation modes are all happening simultane- ously. The net outcome of these partially counteracting effects, if current trends continue, is likely to be faster growth in telecommuni- cations than in travel, resulting in an increasing share of interactions falling to telecommunications, but with continued growth in travel in absolute terms. The empirical evidence to date is quite limited in its ability to assess the extent of true causality between telecommunica- tions and travel, and more research is needed in that area. At this point, what we can say with confidence is that the empirical evidence
TABLE B-6 How Transport Demand Is Influenced by Urban Form Characteristic Minimuma Maximuma Atlanta Boston Chicago Houston New York San Diego Lane density (area of roads per 100 square 1.6 10.6 3.9 4.3 4.7 5.2 5.3 4.2 miles of land) Land area (km2) 135 7,683 2,944 2,308 4,104 3,049 7,683 1,788 Population 158,553 16,044,012 2,157,806 2,775,370 6,792,087 2,901,851 16,044,012 2,348,417 Density (people/km2) 446 2,240 733 1,202 1,655 952 2,088 1,314 Rail transit supply (10,000 mi/km2) 0 5.7 0.7 1.8 1.9 0.0 5.7 0.2 Nonrail transit supply (10,000 mi/km2) 0.1 4.3 1.0 1.3 2.8 1.4 3.0 1.6 Jobsâhousing balance (standardized) 0.12 0.58 0.44 0.28 0.35 0.44 0.41 0.58 Population centrality (standardized) 0.11 0.22 0.11 0.17 0.15 0.13 0.20 0.20 City shape 0.04 0.99 0.26 0.82 0.48 0.80 0.73 0.36 Predicted average vehicle miles traveled 16,899 12,704 14,408 15,685 9,453 16,493 per household Predicted average probability of driving to 0.87 0.73 0.74 0.90 0.40 0.84 work by workers Predicted average commute miles driven 5,450 4,565 4,620 5,641 2,496 5,247 a Refers to sample of 114 urban areas. Source: Bento et al. 2005, p. 477. (Copyright 2005 by the President and Fellows of Harvard College and the Massachusetts Institute of Technology. Reprinted with the permission of the MIT Press Journals.)
Contribution to Emissions and Assessment of Strategies 229 for net complementarity is substantial, although not definitive, and the empirical evidence for net substitution appears to be virtually nonexistent. (p. 43) However, she issues the following caution: The caveat, âif current trends continue,â is a nontrivial one. My expectations for the future are largely predicated on the assumption that the real price of travel will continue to decline or at least remain relatively stable. Should the price of travel escalate markedly . . . the substitutability of telecommunications will obviously become more attractive. Shifts towards telecommunications substitution may also occur for reasons such as an increasing societal commitment to more environmentally benign or sustainable communications modes, but experience suggests that such impacts will be modest at best. (p. 54) Personal and Goods Transport Demand: Summary Growth in personal and goods transport demand is the most important single factor driving the increase in transport-related GHG emissions. This growth is being propelled by growth in real GDP per capita, by population growth, by urbanization, and by the spatial organization of urban areas (urban form). The latter factor is also causing shifts in modal use (public transport to LDVs for personal transport and rail to truck for goods trans- port) that likewise serve to boost transport-related GHG emissions. The correlation between economic growth and increased transport demand has proved to be very robust, but it is not immutable. Reducing it would be a slow process that would require substantial changes in almost every aspect of peopleâs lives. This does not mean that efforts to induce change should not be made; it does mean that such efforts would not likely bear immediate fruit. Reducing Vehicle Energy Consumption per Unit of Transport Activity (I ) Hundreds (if not thousands) of studies describe and analyze the poten- tial of various technologies to reduce the fuel consumption of transport vehicles. Most of these studies focus on personal vehiclesâby far the most numerous road vehicles. The majority of the studies also give the
230 Potential Impacts of Climate Change on U.S. Transportation greatest attention to improvements in power train technologies. No attempt is made here to provide an encyclopedic account of the results presented in this vast body of material. Rather, the discussion is confined to broad categories of technologies. Addressed in turn are technologies with the potential to reduce the fuel consumption of road vehicles, those with the potential to reduce the fuel consumption of nonroad vehicles, factors influencing the extent to which the potential of a technology to reduce transport-related GHG emissions is realized, and the impact of vehicle capacity utilization (load factor) on energy use. Technologies with the Potential to Reduce the Fuel Consumption of Road Vehicles22 Road vehicles (automobiles, trucks of all sizes, buses, and powered two- and three-wheelers) account for 76 percent of all transport energy use worldwide and for 82 percent of U.S. transport energy use. LDVs (cars, light trucks, and powered two- and three-wheelers) account for the lionâs share of this totalâ46 percent worldwide and 62 percent for the United States. Most of the remainderâ24 percent worldwide and 20 percent for the United Statesâis accounted for by medium and heavy freight trucks. Engine Technologies Road vehicles utilize one of two fundamental engine technologiesâspark ignition or compression ignition. Other tech- nologies, such as gas turbines, have been tried, but have proved unsatisfactory for powering road vehicles. There are, however, a very lim- ited number of vehicles powered solely by electric batteries. Spark-Ignition Internal Combustion Engines Most spark-ignition engines in use today are fueled by petroleum gasoline, but they also can run on syn- thetic gasoline derived from gas-to-liquid processes, on ethanol (or blends of gasoline and ethanol), on compressed natural gas, on liquefied petro- leum gases, or on hydrogen. In the intake system of spark-ignition engines, air is mixed with small amounts of fuel. In the past, this process was carried out in the carbure- tor, where the fuel was drawn into the airflow mechanically. To meet more stringent emissions requirements, the carburetor has been replaced in nearly all engines by port-injection systems or direct-injection sys- tems. The latter are particularly effective in reducing fuel consumption 22 The material in this section is an edited version of IEA 2006, Chapter 5.
Contribution to Emissions and Assessment of Strategies 231 and CO2 emissions; this is especially true for several combustion tech- nologies now in development, such as lean-burn technologies.23 Conventional engine architectures use valves activated mechanically by one or more camshafts to control the gas flow into the combustion chamber and the expulsion of the exhaust gases. Variable valve control is an advanced system that allows better management of valve timing and substantially reduces the need for the throttle plates in gasoline engines. Some mechanical systems for valve timing have already been introduced. Other systems, based on electromagnetic or electrohydraulic actuation technologies, are currently being developed. Variable valve control can also enable modular use of the engine, completely obviating the need for some of the cylinders when little engine power is required. This solution diminishes fuel consumption even further and has already been intro- duced in some large cars. Controlled auto ignition (CAI) is another new combustion process being actively explored to improve fuel economy and lower the exhaust emissions of spark-ignition internal combustion engines. CAI engines use a highly diluted mixture of fuel, air, and residual gases that can auto- ignite in a four-stroke engine without preheating of the intake air or an increase in the compression ratio. Technologies that reduce or eliminate pumping losses and throttled operations, combined with turbochargers and technologies that help con- tain knock, can result in significant reductions in engine size, also allowing substantial fuel economy improvements and reductions in CO2 emissions. Many in the automotive industry believe engine downsizingâincluding the use of turbochargersâcan reduce engine displacement size by up to 30 percent. Downsizing the engine also has a positive effect on the whole vehicle design, reducing vehicle inertia and therefore engine load. Almost all recent-model vehicles equipped with a spark-ignition engine are fully capable with low-level ethanol fuel blends, such as E5 or E10 (5 percent and 10 percent ethanol blends, respectively). To use blends of more than 10 percent ethanol, some engine modifications may be nec- essary because of ethanolâs low compatibility with certain materials and elastomer components. Using compatible materials would eliminate 23A lean-burn engine is designed to operate with a very high air-to-fuel ratio under light-load conditions. When little power is required, lower amounts of fuel are injected into the combustion chamber, only in the area around the spark. This reduces the need for throttling and limits NOx.
232 Potential Impacts of Climate Change on U.S. Transportation these problems, and the use of such materials is already common in some countries, such as the United States and Brazil. The cost of making vehi- cles fully compatible with E10 is negligible, and the cost remains very low for full compatibility with E85 (an 85 percent ethanol fuel blend.) If engines were designed exclusively for pure ethanol or ethanol-rich blends, their costs would be roughly the same as today, but their fuel economy (expressed in liters of gasoline equivalent per 100 km) would be better than that of engines designed for conventional gasoline with the same performance.24 Similar effects would be seen in CO2 tailpipe emis- sions. These improvements are possible because the high octane number of ethanol-rich blends, along with the cooling effect from ethanolâs high latent heat of vaporization, would allow higher compression ratios in engines designed for ethanol-rich blends, especially those using the most advanced injection systems available, such as direct-injection systems. Technologies such as direct injection and turbochargers that could lead to downsizing of spark-ignition engines also favor the introduction of ethanol as a transportation fuel. Compression-Ignition Engines Compression-ignition engines (commonly known as diesel engines) are similar to four-stroke spark-ignition engines, with a few essential differences. One difference is that they do not need to be controlled by a throttle. Instead, the power output is controlled by the amount of fuel injected into the cylinder, without airflow limitation. This characteristic reduces the pumping losses that occur in the aspiration phase in spark-ignition engines. Diesel engines do not need spark plugs. The air- fuel mixture used in these engines self-ignites when the fuel is injected into the combustion chamber. As a result, diesel engines can run lean and reach much higher compression ratios than conventional spark-ignition engines. Diesel indirect-injection engines (the conventional injection technol- ogy used in compression-ignition engines until a few years ago) were characterized by fuel delivery in a prechamber designed to ensure proper mixing of the atomized fuel with the compression-heated air. Precise control of fuel delivery was not easy to achieve in these systems. In recent years, indirect-injection systems have been replaced by common-rail sys- 24According to the Transportation Energy Data Book, 25th edition (Davis and Diegel 2006, B-3), the combustion of a gallon of ethanol produces only about two-thirds the heat of a gallon of gasoline (75,670 Btu versus 115,600 Btu). This means a gallon of âgasoline equivalentâ ethanol consists of about 1.5 gallons of actual ethanol.
Contribution to Emissions and Assessment of Strategies 233 tems. These systems still use a pump to store fuel at very high pressure in a reservoir (the common rail), which is connected to the combustion chamber by fuel injectors. Rail systems permit the activation of the injec- tors rather than the pump, eliminating the buildup of pressure before each individual injection. This makes it possible to control very precisely the amount of fuel injected and the timing of each injection, thereby maximizing performance and optimizing fuel use. The fact that diesel engines can work with higher compression ratios than gasoline engines and without a throttle favors the use of intake-air compressors, usually in the form of turbochargers. Such compressors are generally coupled with intercoolers and aftercoolers to increase the den- sity of the air entering the combustion chamber. Turbocharged diesel engines, working with common rail and direct injection, are now an estab- lished technology. They equip most of the light-duty diesel vehicles sold in Europe and virtually all new heavy-duty trucks sold around the world.25 Two important barriers to the increased use of diesel engines have been their relatively high emissions of particulate matter and NOx. A modern exhaust system for diesel engines includes a two-way oxidation catalyst and, in the most recent versions, a particulate filter.26 The two-way oxidation catalyst is similar to the catalytic converter used in gasoline- fueled cars. It converts unburned hydrocarbons and CO into CO2 and water. These converters are not as effective as those used in gasoline-fueled vehicles. On the other hand, CO and hydrocarbon emissions from com- pression-ignition engines are inherently low because of the leaner fuel mixture. Oxidation catalysts reduce particulate mass by as much as 50 per- cent. The problem of ultrafine particulates, one of the most dangerous emissions in terms of health effects, remains unresolved at this point. The aftertreatment of NOx emissions in diesel engines presents a diffi- cult technical challenge because of the oxygen-rich state of the exhaust under lean conditions. The formation of NOx can be reduced by using cooled intake-air compression (whereby an intercooler and aftercooler lower the temperature of the air-to-fuel mix in the cylinder) and by exhaust 25 Variable valve control, already described for gasoline engines, also offers improvements in diesel engines, although its ability to reduce fuel consumption is lower for compression-ignition engines than for spark-ignition engines because the latter suffer from higher pumping losses. 26 A diesel particulate filter may take the form of a ceramic honeycomb monolith. It may also consist of sintered metal, foamed metal structures, fiber mats, or other materials. It removes particulate from the diesel exhaust by physical filtration, capturing the particulate matter on its walls.
234 Potential Impacts of Climate Change on U.S. Transportation gas recirculation. Research in the field of aftertreatment systems continues, including efforts to integrate NOx reduction with particulate filters. Hybrid Vehicles The term âhybridâ refers to any vehicle that can use dif- ferent energy sources in combination. Currently, the term usually refers to hybrid-electric vehicles,27 which are powered by a drivetrain that com- bines a conventional internal combustion engine (powered by gasoline, diesel, or an alternative fuel) and an electric motor. Hybrid-electric vehi- cles can be built in a range of engine architectures with varying sizes for the combustion engine and electric motor, each of which involves differ- ent trade-offs in terms of cost, efficiency, and performance. In series hybrids, an electric motor drives the wheels and derives its energy from a battery or an engine, generally an internal-combustion engine, used as a power generator.28 The power generator supplies the aver- age power required to operate the vehicle and accessories, while a battery stores the excess energy and provides it when needed. Like electric vehicles, series hybrids may use regenerative braking to recharge the battery. A fur- ther efficiency gain is achieved by the fact that the engine is largely uncoupled from the load because of road conditions and can be kept work- ing at a range of operating points where its efficiency is high. This type of engine use offers advantages for the aftertreatment of exhaust gases. In parallel hybrids, motion is delivered to the wheels by both an internal combustion engine and an electric motor. The internal combustion engine is no longer used exclusively as a power plant, but works jointly with the electric motor to deliver movement to the vehicle. âMildâ parallel hybrids have an electric motor that acts as a starter and can serve as an alternator during braking (regenerative braking), while an internal combustion engine powers the drivetrain. In mild hybrid configurations, the electric motor may also provide extra torque and extra power when needed. The electric motor used in mild hybrids is usually located between the engine and the trans- mission or, in âlightâ designs, in the same position as a standard alternator. Full hybrids can operate in internal-combustion mode, in hybrid mode, or even in all-electric mode, the latter being used mainly for cold starts and for urban driving at ranges below 50 km. The electric energy is stored in large batteries during the periods of internal combustion engine 27There are hybrid vehicles that use hydraulic fluid rather than batteries as a power âaccumulator.â 28Diesel-electric railroad locomotives are series hybrids. Their diesel engine drives a generator that provides power to electric traction motors.
Contribution to Emissions and Assessment of Strategies 235 driving and regenerative braking. In some cases, electricity is stored by charging the battery from the grid (plug-in hybrids). Hybrid drivetrains are a promising technology not only for LDVs but also for heavy- and medium-duty vehicles that operate locally and for urban buses. Hybrid solutions are not particularly suitable for heavy- duty trucks and intercity buses because the driving cycle of those vehicles is characterized by long driving periods at steady speeds. The main barrier to greater market penetration of hybrid vehicles is their cost, which is still higher than that of competing vehicles, notably diesel. In their most advanced configurations, diesel vehicles offer fuel economies not far behind those of hybrids. Reducing the cost, weight, and size of batteries is the greatest technology challenge facing hybrid development. Diesel hybrids achieve smaller reductions in fuel consumption rela- tive to hybrids that incorporate gasoline engines; nevertheless, full diesel hybrids may be the most efficient vehicles in the long run. Diesel hybrid engines will be best suited to urban buses and medium freight trucks, although further improvements in battery technology are needed for this type of application. Table B-7 shows estimates of CO2 emissions from new midsized U.S. passenger cars equipped with the above engine technologies identified during the three time intervals 2003â2015, 2015â2030, and 2030â2050. On the basis of a 2003â2015 gasoline internal combustion engine vehicle equal to 100, Table B-8 shows an index of the emissions from each engine type during each of the three periods. Fuel Cell Vehicles A fuel cell is an electrochemical device that converts hydrogen and oxygen into water and produces electricity in the process. Fuel cell vehicles are propelled by electric motors, with electricity pro- duced within the vehicle. Proton-exchange-membrane (PEM) fuel cells are particularly suited to powering passenger cars and buses because of their fast start-up time, favorable power density, and high power-to-weight ratio. Fueled with pure hydrogen from storage tanks or onboard reformers, PEM fuel cells use a solid polymer as an electrolyte and porous carbon electrodes with a platinum catalyst. They operate at relatively low temperatures of around 80Â°C. This has the advantage of allowing the fuel cell to start quickly, but cooling of the cell is required to prevent overheating. The platinum cata- lyst is costly and extremely sensitive to CO poisoning. Research efforts are
TABLE B-7 Tailpipe CO2 Emissions for Midsized U.S. Passenger Cars Using Different Engine Technologies (g/km) Time Period Engine Technology 2003â2015 2015â2030 2030â2050 Spark ignition Gasoline ICEa 130â234 122â219 114â204 Dedicated ethanol ICEb 120â215 112â200 103â185 Flexible fuel vehicle ICEc 133â239 125â224 116â209 Light hybrid, gasoline ICEd 119â214 111â199 103â185 Mild hybrid, gasoline ICEd 108â194 99â178 94â168 Full hybrid, gasoline ICEd 99â178 94â169 89â160 Compression ignition Diesel ICEe 108â193 105â188 102â183 Light hybrid, diesel ICEf 96â173 94â168 91â163 Mild hybrid, diesel ICEf 87â155 83â149 80â143 Full hybrid, diesel ICEf 83â148 80â143 77â138 Note: These estimates refer to a midsized vehicle and assume that roughly half of the potential improvements due to advanced vehicle technologies will improve vehicle fuel economy. (In the case of hybrids, this share rises to 100 percent, but hybrid power trains could also be used to increase performance rather than fuel economy.) The estimates also assume that considerable learning and optimization will occur between 2010 and 2050, in addition to large-scale production of the vehicles. Footnotes below indicate assumptions concerning what technologies are introduced and when. ICE = internal combustion engine. a2003â2015: Conventional engine, stoichiometric combustion, increased use of variable valve control. 2015â2030: Turbocharged engine with direct injection and variable valve control, engine downsizing. Progressive introduction of advanced combustion technologies with NOx traps. 2030â2050: Downsized turbocharged engine with direct injection and variable valve control using advanced combustion technologies (CAI) with NOx traps. b2003â2015: Conventional engine, stoichiometric combustion. 2015â2030: Turbocharged engine combustion with direct injection and variable valve control. Progressive introduction of advanced combustion technologies needing NOx traps, pro- gressive downsizing. 2030â2050: Turbocharged downsized engine with direct injection and variable valve control, using advanced combustion technologies with NOx traps. c2003â2015: Conventional engine, stoichiometric combustion. 2015â2030: Ethanol-hybrid turbocharged engine with direct injection and variable valve control, downsizing. Progressive introduction of advanced combustion technologies needing NOx traps. 2030â2050: Downsized ethanol-hybrid turbocharged engine with direct injection and variable valve control using advanced combustion technologies with NOx traps. d2003â2015: Wide introduction of starter-alternator systems, mild hybrid engines on some models, full hybrids mainly on large LDVs. 2015â2030: Higher penetration of mild hybrids, even on small vehicles, wide diffusion of full hybrids on large LDVs. Large hybrid shares for minibuses and medium freight trucks. ICE improved in light hybrids, slightly less for mild and full hybrids. 2030â2050: A large share of ICE vehicles sold on the market equipped with hybrid systems. ICE improved in light hybrids, slightly less so for mild and full hybrids. e2003â2015: Second-generation common rail, progressive downsizing. 2015â2030: Turbocharged downsized engine, vari- able valve control, possibly heat recovery, particulate filter and NOx trap (or selective catalytic reduction systems, especially on large engines). 2030â2050: Turbocharged downsized engine, variable valve control, and possibly heat recovery. Particulate filter and NOx trap (or selective catalytic reduction systems, especially on large engines). f2003â2015: Introduction of starter-alternator systems; hybrid motorizations on large LDVs; initial diffusion of hybrids in buses, minibuses, and freight trucks. 2015â2030: Penetration of mild hybrids on small vehicles, wider diffusion of full hybrids on large vehicles. Larger shares of hybrid buses, minibuses, freight trucks. ICE improved as for diesel engines in light hybrids, slightly less so for mild and full hybrids. 2030â2050: Further cost reductions leading to large shares of new vehicles equipped with hybrid systems. ICE improved as for diesel engines in light hybrids, slightly less so for mild and full hybrids. Source: IEA 2006, Chapter 5, material taken from Table 5.2, p. 297; Tables 5.3â5.4, pp. 300â301; Table 5.5, p. 309; and Tables 5.6â5-7, pp. 318â319. Reprinted with permission of OECD/IEA.
Contribution to Emissions and Assessment of Strategies 237 TABLE B-8 Index of CO2 Emissions from a Midsized New U.S. Passenger Car Powered by Engine Type Shown During Period Indicated (g/km; 2003â2015 Gasoline-Powered ICE = 100) Time Period Engine Technology 2003â2015 2015â2030 2030â2050 Spark ignition Gasoline ICE 100 94 88 Dedicated ethanol ICE 92 86 79 Flexible fuel vehicle ICE 102 96 89 Light hybrid, gasoline ICE 92 85 79 Mild hybrid, gasoline ICE 83 76 72 Full hybrid, gasoline ICE 76 72 68 Compression ignition Diesel ICE 83 81 78 Light hybrid, diesel ICE 74 72 70 Mild hybrid, diesel ICE 67 64 62 Full hybrid, diesel ICE 64 62 59 Note: ICE = internal combustion engine. Source: Derived from Table B-7. focusing on high-temperature membranes that would allow the use of lower-cost and more robust catalyst systems. The current cost of PEM fuel cells exceeds $2,000 per kilowatt (kW), but costs could be cut to as low as $100 per kW through mass production and experience with the technology. This reduction in cost might not be sufficient, however. It is believed that the cost of fuel cells must fall to below $50 per kW to make them competitive. Achieving this cost reduc- tion would require fundamental advances in materials technology and the achievement of higher power densities for fuel cells. Onboard fuel storage is a major challenge. Existing onboard storage options do not yet meet the technical and economic requirements for making them competitive. Safety is also believed to be an issue. Gaseous storage at 350 to 700 bar29 and liquid storage at â253Â°C are commercially 29 A bar is a unit of pressure. It is equivalent to 14.5 pounds per square inch (psi). Atmospheric pressure is 14.7 psi. So 350 to 700 bar would be approximately equal to 350 to 700 times atmospheric pressure, or 5,000 to 10,000 psi.
238 Potential Impacts of Climate Change on U.S. Transportation available but are very costly. Solid storage (for example, using hydrides) offers potentially decisive advantages but is still under development, with a number of materials being investigated. At present, in the absence of further breakthroughs, gaseous storage at 700 bar appears to be the technology of choice for passenger cars. However, the cost of the tank is $600 to $800 per kilogram (kg) of hydro- gen (H2), and 5 kg of storage capacity is needed to provide adequate range for the vehicle. Depending on the pace of technological development, the stack cost of a PEM fuel cell could decline to $35 to 70 per kW by 2030. If this were to happen, the cost of a fuel cell vehicle at that time would exceed that of a conventional internal combustion engine vehicle by $2,200 to $7,600. Determining CO2 emissions from hydrogen-powered fuel cell vehicles is not straightforward. The fuel cells themselves do not produce any CO2 emissions. However, as discussed below, the processes used to produce the hydrogen fuel can emit large quantities of CO2, making the âwell-to- wheelsâ emissions from such vehicles comparable with (and sometimes even higher than) those from conventional gasoline or diesel internal combustion engines. Nonengine Technologies While reductions in engine energy use attract by far the most attention, other important technologies have the poten- tial to reduce the amount of energy required to propel a vehicle. Transmission Technologies Engines have ideal ranges of speed at which they can operate. Operating outside these ranges increases fuel con- sumption and engine wear. A vehicleâs transmission allows its engine to operate more closely to its ideal speed while permitting its driving wheels to operate at the speed the driver chooses. Transmissions use gears to reduce the speed of the engine to the speed of the wheels. The larger the number of gears, the more likely it is that the engine will be able to operate at its ideal speed across a wide range of vehicle speeds and power requirements. But more gears mean more com- plexity and more internal friction. Until the 1970s, most LDVs in the United States had three forward gearsâlow, medium, and high. When fuel prices increased substantially in the late 1970s and early 1980s, vehicles generally added a fourth gear, sometimes called âoverdrive.â More recently, transmissions have added electronic controls and additional speeds. Today, transmissions with six
Contribution to Emissions and Assessment of Strategies 239 forward speeds are being introduced, and seven-speed transmissions are not unheard of. Some vehicles are now equipped with a continuously variable trans- mission (CVT). CVTs use a system of belts and adjustable-diameter pulleys to permit an infinite number of forward gear ratios. The engine operates at a near-constant speed while the transmission adjusts contin- ually to produce the speed required for the wheels. CVTs have not yet achieved the power-handling capability to be used on the full range of LDV sizes and weights. Also, they have suffered from reliability problems. However, their use has been forecast to increase in the years ahead. Technologies to Reduce Vehicle Weight The lighter a vehicle, the less fuel it consumes. Vehicle mass can be reduced either by decreasing the size of the vehicle or by changing the materials from which it is made. Lighter cars can be propelled by lighter engines. A light power train, in turn, requires less structural support and allows further reductions in the weight of the vehicle frame, suspension, and brakes. Steel is currently the main automotive material. Over the past decade, steel made up an average of 55 percent of the weight of a fully fueled car without cargo or passengers. Most of the remaining weight is accounted for by iron (10 percent), aluminum (6 to 10 percent), and plastics. Cost and the eventual need for large investments to modify the vehicle pro- duction process are the main barriers to the increased use of lightweight materials. High-strength steel can cost as much as 50 percent more than traditional steels, but less of the material is needed to achieve the same performance.30 Lighter materials such as aluminum and magnesium cost more than conventional mild steel, but their greater use could lead to improved manufacturing processes, thereby reducing manufacturing costs. Composite materials have extremely attractive properties but cost a great deal more than metals. Another source of weight reduction is the replacement of mechanical or hydraulic systems by electrical or electronic systems. Steering can be accomplished by electric motors actuated by joysticks rather than by mechanical linkages between the steering wheel and the wheels of the car. 30 The type of steel used in vehicles has been changing. In 1977, 60 percent of the weight of the average U.S. domestic car consisted of steel, 90 percent of which was conventional steelâincluding cold-rolled and precoated steel. By 2003, the total steel share had fallen to 54 percent, with only 75 percent of that total consisting of conventional steel. During both years, the remainder is accounted for by high-strength steel, stainless steel, and other steels.
240 Potential Impacts of Climate Change on U.S. Transportation This is known as âsteering by wire.ââBraking by wireâ also has been devel- oped, as has âshifting by wireâ and âthrottle by wire.â Lightweight technologies are being introduced progressively and will continue to be developed. Tire Technologies The energy requirements of the power train can be reduced through the use of energy-efficient tires. For an LDV, fuel con- sumption can be reduced by 3 to 4 percent through the use of currently available low-rolling-resistance tires. An additional reduction of 1 to 2 per- cent in fuel consumption can be achieved by accurately monitoring tire pressure. Currently available technologies can automatically sense low pressure and inform the driver. Technologies to Improve Vehicle Aerodynamics Aerodynamic drag, which is proportional to the square of a vehicleâs speed, is the main factor deter- mining a vehicleâs need for power at high speeds. At this time, aerodynamic issues affect most seriously long-haulage heavy-duty trucks and intercity buses, and significant improvements are possible for such vehicles. At high- way speeds, aerodynamic losses are estimated to account for 21 percent of the energy use of a heavy-duty truckâtrailer combination unit. Technologies to Reduce the Energy Requirements of Onboard Equipment The energy consumption of air conditioners and other onboard appliances can account for up to half of a vehicleâs fuel consumption under certain con- ditions. A number of efforts are under way to reduce the energy used by these devices. A particular focus has been on reducing the energy required to operate a vehicleâs air conditioning system. Installation rates for air con- ditioners, already approaching 100 percent in both North America and the OECD Pacific region, are growing rapidly in Europe. Fewer than 15 per- cent of LDVs sold in France in 1995 were equipped with air conditioning. By 2000, that rate had risen to 60 percent, and it is expected to reach nearly 100 percent by 2010. A significant barrier to greater market penetration of energy-efficient onboard components is the fact that the energy consumed by these appli- ances is not always captured in current vehicle tests.31 This reduces the incentive for manufacturers to use such devices. The public is largely unaware of the fuel use of onboard appliances and the cost entailed: 1 kW- 31 We understand that this is not the case for the United States.
Contribution to Emissions and Assessment of Strategies 241 hour (kWh) of electricity generated on board costs just slightly less than 1 liter of gasoline, exceeding by far the cost of electricity generated in cen- tral power plants. Technologies with the Potential to Reduce Fuel Consumption by Nonroad Vehicles While LDVs are, in the aggregate, the largest consumers of trans- port fuel and emitters of GHGs, vehicles such as medium and heavy trucks, commercial aircraft, locomotives, and large waterborne vessels actually use much more energy per vehicle each year. In 2003, the average U.S. passenger car traveled about 12,000 miles and used about 550 gallons of fuel. The average combination truck (i.e., a tractor unit with one or more trailers) traveled about 62,000 miles and used about 12,000 gallons of fuel. The average commercial aircraft traveled about 900,000 miles and used about 2.3 million gallons of fuel.32 This appendix has already described technologies applicable to medium and heavy freight trucks and to buses. The discussion now turns to vehicles that do not travel on roadsâaircraft, waterborne vessels, and railroad locomotives. Aircraft Commercial aircraft account for 12 percent of transport energy use worldwide and 8 percent of that in the United States. Since the 1960s, turbine engines fueled by a light petroleum product known as jet fuel have powered virtually all new commercial aircraft. While the combustion process of these turbine engines is quite efficient, the energy required to lift an aircraft and its payload off the ground and propel it long distances at high speeds is formidable. In fact, a large share of the payload trans- ported by any aircraft is its own fuel. Not surprisingly, fuel usage and fuel costs are therefore an extremely important component of the total oper- ating cost of an air transport system, comparable in magnitude with crew costs and ownership and investment costs. In a review of historical and projected future trends in aircraft energy use, Lee et al. (2001) analyze the relative contribution of different tech- nological improvements and operational factors to reducing the energy intensity of commercial aircraft during the period 1971â1998. As mea- sured by megajoules per revenue passenger kilometer, this energy intensity has declined by more than 60 percentâan average decline of about 3.3 per- cent a year. 32 These data were calculated from data provided in the Transportation Energy Data Book (Davis and Diegel 2006) and National Transportation Statistics 2004 (BTS 2005).
242 Potential Impacts of Climate Change on U.S. Transportation Three technological factorsâreduced specific fuel consumption, an increase in aerodynamic efficiency, and improved structural efficiencyâ have been responsible for much of this decline. Engine efficiency improved by about 40 percent between 1959 and 1995, with most of the improve- ment being achieved before 1970 with the introduction of high-bypass engines. Other factors include higher peak temperatures within the engine, increased pressure ratios, and improved engine component efficiencies. Aerodynamic efficiency has increased by approximately 15 percent histor- ically, driven by better wing design and improved propulsionâairframe integration. Improvements in structural efficiency have contributed less, despite some improvements in the materials used to construct aircraft. As has also been true for motor vehicles, reductions in aircraft weight pro- duced by these improved materials have largely been traded off for other technological improvements and passenger comfort. Lee et al. (2001) project that over the next several decades, the energy intensity of commercial aircraft will continue to decline, but at a slower rateâ1.2 to 2.2 percent per year, compared with the 3.3 percent average annual decline experienced over the past several decades. Waterborne Vessels Waterborne transport, including ocean shipping, coastal shipping, and inland waterway transport, accounts for 10 percent of transport energy use worldwide and for 4 percent of U.S. transport energy use. (The U.S. figure includes recreational uses; the world figure does not.) Almost all commercial vessels are powered by diesel engines. The engines used in large oceangoing ships are the largest ever built. These giant diesels can have up to 14 cylinders, each with a bore of 980 mm and a stroke of 2,660 mm, giving the engine a displacement of nearly 1,000 liters. Most of these very large engines are classified as âslow speed.â That is, they operate at about 100 revolutions per minute and are coupled directly to the shipâs propeller, eliminating the need for reduction gears. The diesel engines powering towboats or self-propelled barges on inland waterways are much smallerâabout the size of a large diesel- electric locomotive, though there may be more than one such engine. Large towboats on U.S. inland waterways are rated at over 10,500 horse- power. Fuels used by waterborne transport vehicles are âheavyâ grades of diesel fuel and an even âheavierâ petroleum product known as âresidual fuel oil.â Typically, these fuels are higher (often much higher) in sulfur relative to other transport fuels.
Contribution to Emissions and Assessment of Strategies 243 TABLE B-9 Marine Emissions, 1996 Gas Component Range of Estimated Emissions (Mt) Carbon monoxide 0.7â1.1 Nonmethane volatile organic compounds â Methane â Nitrous oxide â Carbon dioxide 436â438 Sulfur dioxide, total 5.2â7.8 Generated by combustion of residual fuel oil 5.0â7.0 Generated by combustion of distillate 0.2â0.8 Nitrogen oxides 10.1â11.4 Source: IMO 2000, p. 11. (Reprinted with permission of IMO.) A report to the International Maritime Organization published in March 2000 details the energy use and emissions characteristics of ocean- going vessels as of 1996 (IMO 2000). Table B-9 shows the emissions estimated to result from the 138 million tonnes of distillate and residual fuel consumed during that year by these ships. The same report identifies and evaluates the impact of a range of technical and operational mea- sures that could be applied to new and existing ships to reduce energy use and CO2 emissions. Table B-10 summarizes the reportâs findings con- cerning technical measures that might be applied. Railroad Engines33 Railroad engines account for 3 percent of transport energy use worldwide and for 2 percent of transport energy use in the United States. Most railroad engines use electricity generated externally or diesel fuel carried on board as their primary energy source. For the world as a whole, 27 percent of energy used by railroads is externally gen- erated electricity, 59 percent is diesel, and 12 percent is coal (virtually all in China). Countries vary widely in the extent to which their railroads rely on electric power. Railroads in Canada and the United States are almost totally diesel powered. In Japan, 78 percent of the rail energy used is electrical, and in Europe, 61 percent.34 33 The International Union of Railways conducted a project, Energy Efficiency Technology for Railways, in which a range of technologies relating to railway energy efficiency including, but not limited to, engine technologies were evaluated. The project can be accessed at www.railway-energy. org/tfee/index.php. 34 The statistics in this paragraph were calculated from 2003 data provided by IEA.
244 Potential Impacts of Climate Change on U.S. Transportation TABLE B-10 Marine CO2 Reductions by Technical Measure Fuel/CO2 Savings Measure Potential (%) Subtotala (%) Totala (%) New ships 5â30 Optimized hull shape 5â20 5â30 Choice of propeller 5â10 Efficiency optimized 10â12b 14â17b 2â5c 6â10c Fuel (HFO to MDO) 4â5 Plant concepts 4â6 8â11 Fuel (HFO to MDO) 4â5 Machinery monitoring 0.5â1 0.5â1 Existing ships 4â20 Optimal hull maintenance 3â5 4â8 Propeller maintenance 1â3 Fuel injection 1â2 5â7 Fuel (HFO to MDO) 4â5 Efficiency rating 3â5 7â10 Fuel (HFO to MDO) 4â5 Efficiency rating + TC upgrade 5â7 9â12 Fuel (HFO to MDO) 4â5 Note: HFO = heavy fuel oil; MDO = marine diesel oil; TC = turbocharging. aPotential for reduction from individual measures is documented by different sources; potential for combinations of measures is based on estimates only. bState-of-the-art technique in new medium-speed engines running on HFO. cSlow-speed engines when trade-off with NO is acceptable. x Source: IMO 2000, p. 14. (Reprinted with permission of IMO.) Recent years have seen major improvements in the efficiency of elec- tric locomotives, brought about by the use of AC power. In the case of diesel-powered locomotives, propulsion system developments have focused primarily on improving the power, reliability, and efficiency of the diesel engines used to generate onboard electric energy, as well as the efficiency of the electric traction engines that deliver this energy to the driving wheels. In addition, diesel locomotives have become subject to emissions standards and, in some places, to noise standards. Interest is growing in the use of fuel cells to provide auxiliary power for diesel locomotives. This would permit the main diesel engine to be shut
Contribution to Emissions and Assessment of Strategies 245 down when the locomotive is not in use but still has power needs. Idle time constitutes a surprisingly large share of the total time a diesel engine is in operation. A recent study of locomotive duty cycles on Canadian railroads found that engines were idling between 54 and 83 percent of the time. Using either fuel cells as auxiliary power units or the âhybridâ approach described above would permit engines to reduce the amount of idle time substantially. Although fuel use and emissions are much greater when a locomotive is operating at full power than when it is idling, the potential improvements in both are nontrivial. Factors Influencing the Extent to Which the Potential of a Technology to Reduce Transport-Related Greenhouse Gas Emissions Is Realized One of the most controversial issues in the debate over the use of new technologies to reduce GHG emissions is how effective these technologies will be when incorporated into actual transport vehicles in normal serv- ice. Invariably, ex post analyses of actual emissions reductions fall short (sometimes considerably short) of their original claimed potential. The discussion below reviews some of the more important factors that tend to create this result. Extent to Which a Technologyâs Potential to Reduce Energy Consumption Is Incorporated into the Vehicles in Which It Is Employed Most vehicle technologies with the potential to reduce fuel consumption offer vehicle designers a range of possibilities for how they may be used. Depending on the decisions made by the designer, the share of this potential that is actu- ally used to reduce fuel consumption can vary from zero to 100 percent. The history of LDV fuel economy in the United States since the mid- 1980s provides a textbook example. The bottom line in Figure B-7 shows the corporate average fuel economy (CAFE) of the new light vehicle fleet as tested by EPA. New LDV CAFE rose sharply between 1979 and 1982, increased slowly from 1983 through 1987, declined slowly from 1987 through 1994, and has remained nearly constant since. This does not mean that vehicle technologies related to fuel consump- tion have failed to improve since the mid-1980s. EPA uses a measure known as ton-miles per gallon as an (imperfect) reflection of changes in the energy efficiency potential of the technologies actually incorporated into vehicles. The top line of Figure B-7 tracks this indicator. It has grown relatively steadily at a rate of about 1 to 2 percent per year throughout the
246 Potential Impacts of Climate Change on U.S. Transportation 50 45 40 35 MPG or Ton-MPG 30 25 20 15 10 5 0 1975 1978 1981 1984 1987 1990 1993 1996 1999 2002 2005 Model Year Combined Fleet LDV CAFE (as tested) Combined Fleet Ton-MPG FIGURE B-7 U.S. light-duty vehicle corporate average fuel economy and technol- ogy capability. (Note: MPG = miles per gallon.) (Source: Heavenrich 2005, Table 1, p. 10.) period, reflecting the new technologies that have been introduced and dis- seminated throughout the new vehicle fleet. What explains the sharp contrast between the two lines in Figure B-7? The brief answer is that for much of the period, most of the fuel economy improvement potential has been used by vehicle designers to improve vehicle performance, not fuel economy. Figures B-8a and B-8b show, respectively, the evolution of passenger car and light truck acceleration performance (measured as 0â60 mph time) between 1975 and 2006. Figures B-9a and B-9b show similar data for the evolution of vehicle iner- tia weight.35 The sharp decline in inertia weight, rather than any radical change in vehicle technology, largely explains the dramatic improvement in new vehicle fleet fuel economy that occurred between the late 1970s and the early 1980s. By the mid-1980s, as new energy-saving technologies began to be introduced in a major way into LDVs, average vehicle weight 35 Inertia weight is defined as the curb weight of the vehicle (including fuel) plus 300 pounds.
Contribution to Emissions and Assessment of Strategies 247 30 Combined City/Highway MPG (as tested) 2005 1995 1990 2000 1985 25 1980 20 1975 15 9 10 11 12 13 14 15 0â60 Time (sec) (a) 25 Combined City/Highway MPG (as tested) 2005 20 2000 1990 1985 1995 1980 15 1975 10 9 10 11 12 13 14 15 0â60 Time (sec) (b) FIGURE B-8 (a) Car and (b) truck 55/45 laboratory MPG versus 0â60 time by model year. (Source: Heavenrich 2006, Figures 6 and 7, p. 16.)
248 Potential Impacts of Climate Change on U.S. Transportation Combined City/Highway MPG (as tested) 29 1995 2005 2000 1990 27 1985 25 1980 23 21 19 17 1975 15 3000 3200 3400 3600 3800 4000 4200 Inertia Weight (pounds) (a) Combined City/Highway MPG (as tested) 22 2005 2000 21 20 1985 1990 1995 19 18 1980 17 16 15 14 13 1975 12 3500 3700 3900 4100 4300 4500 4700 4900 Inertia Weight (pounds) (b) FIGURE B-9 (a) Car and (b) truck 55/45 laboratory MPG versus inertia weight by model year. (Source: Heavenrich 2006, Figures 8 and 9, p. 17.) began to increase, and acceleration performance, which had remained rel- atively constant (or even deteriorated somewhat) between 1975 and 1980, began to improve. The increase in average weight resulted from two interrelated factors. First, the average inertia weight within each vehicle size class grew. Second, larger size classes made up a greater share of the total vehicle market. In
Contribution to Emissions and Assessment of Strategies 249 particular, a greater share of the new LDV fleet began to be accounted for by light trucks, especially vans and SUVs (see Figure B-10). By 2006, the weight of the average LDV had exceeded its 1975 level. The acceleration performance of both fleets of vehicles had improved dramatically. The 2005 edition of the EPA report just referenced (Heavenrich 2005) estimates the impact of vehicle weight and vehicle performance on the âlaboratoryâ (or âas testedâ) fuel consumption of new U.S. passenger cars, light trucks, and all LDVs. Table B-11, adapted from this report, shows these results. The top line of the table shows the actual âlabora- toryâ (or âas testedâ) âcombinedâ fuel economy (mpg) for new model year 2005 cars, trucks, and the total LDV fleet. The next two rows show estimates of what the 2004 fuel economy would have been had the iner- tia weight and 0â60 acceleration time been what they were in 1981 and 1987. The final two rows show similar results for size (interior volume) and 0â60 acceleration time. In all cases, the model year 2004 fleet would have exhibited improved fuel economy performance, with the increase in some cases being as high as 30 percent. Automobile manufacturers assert that these developments merely reflected changes in consumer tastes as fuel prices fell sharply in the mid- 1980s and remained low in inflation-adjusted terms thereafter (until recently). Environmentalists assert that the path of fuel economy improve- 100% 90% 80% SUV Sales Fraction (%) 70% Car 60% Wagon 50% Van 40% Pickup 30% 20% 10% 0% 1975 1980 1985 1990 1995 2000 2005 Model Year FIGURE B-10 Sales fraction by vehicle type (3-year moving average). (Source: Heavenrich 2006, Figure 10, p. 18.)
250 Potential Impacts of Climate Change on U.S. Transportation TABLE B-11 Effect of Performance, Size, and Weight Distribution on Laboratory 55/45 Fuel Economy Laboratory 55/45 Fuel Percent Change from Economy (MPG) 2005 Actual Averages Scenario Cars Trucks Both Cars Trucks Both 2005 actual average 28.8 21.3 24.5 Model year 2005 averages recalculated using 1981: Weight distribution 31.9 30.2 31.0 10.8 41.8 26.6 Size distribution 28.4 21.2 24.3 â1.4 â0.5 â0.9 0â60 distribution 29.8 20.9 24.6 3.5 â1.9 0.3 Weight and 0â60 36.4 28.5 32.0 26.4 33.8 30.5 Size and 0â60 37.1 25.0 29.9 28.8 17.4 22.0 Ref. 1981 actual average 25.1 20.1 24.6 â12.8 â5.6 0.4 Source: Adapted from Heavenrich 2005, Table 24, p. 72. ments over time reflects the failure of the U.S. government to increase its vehicle energy efficiency standards once they reached their peak in the mid-1980s. Whatever the reason, technological improvements have not automatically translated into improved fuel economy over much of the period shown. Length of Time Required Between the First Commercial Use of a Technology and When Its Impact on Fuel Consumption Is Felt Throughout the Entire Vehicle Fleet New technologies are not intro- duced across a manufacturerâs entire fleet at one time. EPA has collected data showing the length of time required to achieve various rates of fleet penetration for successful new LDV technologies in the United States (Heavenrich 2006, 62). Fifty percent penetration rates of 10 years are not unusual, and 75 percent penetration rates can easily take 20 years to achieve. Transport vehicles tend to last a long time. Half the cars built during the 1990 model year were still on the road when the 2007 model year vehi- cles were first introduced. Heavy trucks last even longer. On the basis of âminimal preliminary data,â the expected median lifetime for a model year 1990 heavy truck is 29 years (Davis and Diegel 2006, Tables 3-8 and 3-10).
Contribution to Emissions and Assessment of Strategies 251 Commercial aircraft are also very long-lived. DC-3 aircraft built 50 years or more ago are still in commercial service in parts of the world. Boeing esti- mates that 8,800 of the worldâs fleet of 17,000 commercial aircraft that were operating in 2005 will still be operating in 2025 (Boeing Company 2006, 6). Table B-12 shows estimates made by MITâs Laboratory for Energy and the Environment as to how long it might take for various new LDV tech- nologies to have a significant impact on energy use and GHG emissions. How Motorists Actually Operate Their Vehicles The way vehicles are operated has a significant influence on fuel consumption. Governments test the fuel consumption performance of LDVs on dynamometers that are programmed to follow a set sequence of actions (accelerating, stop- ping, operating at high speed, operating at low speed, etc.) for specific intervals of time. Their ratings of vehiclesâ fuel consumption are based on these tests. However, vehicle operators typically do not operate their vehi- cles as implied by these test procedures. They accelerate more rapidly, drive faster, and so on. This produces a significant gap between âas testedâ and âin-useâ fuel consumption. For reporting purposes (but not for regulatory compliance purposes), EPA currently adjusts âas testedâ mpg downward by 15 percent to make it more comparable with the fuel economy vehicle users are likely to experience in practice. However, the agency believes that this adjustment factor, which is about two decades old, is outdated and proposes increasing it to approximately 22 percent. According to EPA, adoption of this new adjustment factor would result in the 2006 TABLE B-12 Timescales for New Light-Duty Vehicle Power Train Technologies Implementation Phase (years) Penetration Across Market New Vehicle Major Fleet Total Time Vehicle Technology Competitive Productiona Penetrationb for Impact Turbocharged gasoline engine 5 10 10 20 Low-emissions diesel 5 15 10â15 30 Gasoline hybrid 5 20 10â15 35 Hydrogen fuel cell hybrid 15 25 20 55 a Accounts for more than one-third of new vehicle production. b Accounts for more than one-third of all mileage driven. Source: Heywood 2006, p. 62. Copyright 2006 by Scientific American, Inc. Reprinted with permission. All rights reserved.
252 Potential Impacts of Climate Change on U.S. Transportation U.S. new vehicle fleetâs adjusted fuel economy being reduced from its âas testedâ level of 24.6 mpg (9.6 L/100 km) to 19.1 mpg (12.3 L/100 km). Using the current 15 percent adjustment factor, the adjusted fuel econ- omy for the 2006 U.S. new vehicle fleet is 21.0 mpg (11.2 L/100 km) (Heavenrich 2006, A.10âA.14). Impact of Vehicle Capacity Utilization (i.e., Load Factor) on Energy Use In the analysis thus far, reductions in energy use per vehicle kilometer have been treated as producing corresponding reductions in energy use per passenger kilometer or tonne-kilometer. The latter two measures, not the former, represent the fulfillment of transport demand. Increasing a vehicleâs average load of passengers or freight, while increasing energy use somewhat, normally leads to a reduction in the energy required to pro- duce a given volume of transport services. Fitting vehicle size to demand is an important consideration in minimizing transport energy use. Different transport modes have experienced varying degrees of success in improving the capacity utilization of their vehicles. In the case of U.S. commercial aviation, the increase in average load factor from about 55 per- cent in 1975 to the 80+ percent levels being experienced today is responsible for a major share of the industryâs energy efficiency improve- ment per passenger kilometer. The average load factor of freight trucks has also increased. However, U.S. LDV load factors have shown the opposite trend. Between 1977 and 2001, the average number of occupants per vehi- cle declined from 1.9 to 1.6 passengers, or by 14 percent (Hu and Reuscher 2004, Table 16, p. 31). This helps explain why the number of Btuâs required to propel the average U.S. passenger car 1 mile fell from 9,250 in 1970 to 5,572 in 2003 (i.e., by 40 percent), while the number of Btuâs required to move one passenger 1 mile fell only from 4,868 to 3,549 (i.e., by 27 percent) over the same period (Davis and Diegel 2006, Table 2-11, p. 2â13). The percentage of a vehicleâs available capacity that can be used is the result of a complex trade-off between cost and convenience. No form of commercial transport can operate at 100 percent of capacity all the time. But as operating costs increase, people are willing to sacrifice convenience to reduce cost, and load factors rise. Public transport systems are espe- cially sensitive to this trade-off. As noted above, low residential densities and the decline of CBDs as the location of most jobs have reduced the number of people wishing to travel from one given point to another, especially during peak hours. Maintaining a level of service frequency and service coverage necessary to make public transport services attrac-
Contribution to Emissions and Assessment of Strategies 253 tive has collided with the need to use larger vehicles to reduce per seat labor, energy, and capital costs. Vehicle Fuel Consumption: Summary New technologies have the potential to reduce substantially the energy used by transport vehicles. The time required to develop, commercialize, and disseminate new vehicle technologies probably is shorter than the time required to alter the fundamental drivers of personal and goods transport demand, but it is still measured in decades. In addition, there is the problem of ensuring that the potential of new technologies to reduce energy consumption in transport is actually realized. As the example of U.S. LDVs after the mid-1980s shows, there is no guarantee that this will occur. Fuel consumption is but one of many attributes of vehicle per- formance. Unless conditions are right (or can be made right), it is possible that some (or even all) of this potential will end up improving these other performance attributes rather than reducing vehicle fuel consumption. Altering the Greenhouse Gas Emissions Characteristics of Transport Fuel (F ) Gaseous and liquid transport fuels can be produced from a wide range of primary energy sources (see Figure B-11). Depending on the feedstock used and the production method employed, CO2 emissions (sometimes referred to as âwell-to-tankâ or WTT emissions) can vary widely, some- times even being negative. As noted above, changes in the GHG emissions characteristics of transport fuels have not contributed much one way or the other to changes in transport-related GHG emissions over the past several decades. This is due to the present overwhelming dominance of petroleum-based fuels in transport and to the fact that all petroleum- based transport fuels emit approximately the same amount of CO2 per unit of energy they provide (see Table B-13). In the future, however, WTT emissions are likely to have much greater significance in determining total transport-related GHG emissions. Figure B-12, from the SMPâs final report, shows estimates of the âwell-to-tank,â âtank-to-wheelsâ (TTW, sometimes also called âtailpipe emissionsâ), and âwell-to-wheelsâ (WTW) emissions (the sum of WTT and TTW emissions) generated by a wide range of vehicleâfuel combina- tions. The figure illustrates that for transport fuels such as hydrogen and for power train technologies such as fuel cells, the WTT portion totally dominates total transport-related CO2 emissions.
254 Potential Impacts of Climate Change on U.S. Transportation FIGURE B-11 Possible transport fuel pathways (CNG = compressed natural gas; DME = dimethyl ether; FC = fuel cell; FT = FischerâTropsch; ICE = internal combustion engine; LPG = liquefied petroleum gas). (Source: World Business Council for Sustainable Development 2004, Figure 3.1, p. 67.) TABLE B-13 CO2 Emissions per Liter (Gasoline Equivalent) CO2 Emissions Index Fuel (kg/liter) (Gasoline = 100) Gasoline 2.416 100 Diesel (distillate) 2.582 107 Jet fuel 2.491 103 Ethanol 2.484 103 Biodiesel 2.672 111 Residual fuel (bunker fuel) 2.697 112 Source: Data generated from IEA/SMP Spreadsheet Model.
Contribution to Emissions and Assessment of Strategies 255 Gasoline 2010 ICE Gasoline DI ICE Advanced Gasoline ICEa Ethanol Sugar Beet ICEb Ethanol Poplar Plantation ICEb Diesel DI ICE Advanced DI Diesel ICEa RME Biodiesel DI ICEb FT-Diesel remote-NG DI ICE FT-Diesel Residual Wood DI ICE CNG EU-NG-Mix ICE LH2 EU-NG-Mix ICE Gasoline DI HEV Diesel DI HEV Gasoline FC Methanol remote-NG FC CGH2 Residual Wood FCc CGH2 EU-NG-Mix onsite FC CGH2 EU-EI-Mix onsite FC LH2 EU-NG-Mix FC -150.0 -100.0 -50.0 0.0 50.0 100.0 150.0 200.0 250.0 300.0 Greenhouse Gas Emissions (g/km) Well-to-Tank Tank-to-Wheels Well-to-Wheels a Estimated by the Institute for Internal Combustion Engines. b Estimated by British Petroleum, from General Motors data. c Net output from energy use in conversion process. FIGURE B-12 Well-to-wheels (well-to-tank + tank-to-wheels) GHG emissions for various fuel and propulsion system combinations (CGH2 = gaseous hydrogen; CNG = compressed natural gas; CO2 = carbon dioxide; DI = direct injection; EU = European Union; FC = fuel cell; FT = FischerâTropsch; HEV = hybrid electric vehicle; ICE = internal combustion engine; LH2 = liquid hydrogen; NG = natural gas; RME = rapeseed methyl ester). (Source: World Business Council for Sustainable Development 2004, adapted from Figure 3.3, p. 77.) The wide range in WTT emissions for the various fuels illustrated in Figure B-12 results largely from three factors: â¢ The growing of biomass used to manufacture biofuels (and possi- bly hydrogen) removes CO2 from the atmosphere. Gathering and processing the biomass into fuel takes energy and results in the emission of CO2. This offsets some of the CO2 removed from the
256 Potential Impacts of Climate Change on U.S. Transportation atmosphere by the growing of the biomass. But under plausible assumptions, net WTT CO2 emissions for biomass-derived fuels can still be negative. â¢ The production process for some biofuels (e.g., ethanol from corn) generates coproducts that can displace other products that require energy to produce and whose production emits CO2. How these âcoproduct creditsâ are allocated has a major impact on a biofuelâs costs, the energy required to produce it, and its WTT CO2 emissions (Farrell et al. 2006). â¢ The production of transport fuels from nonpetroleum fossil car- bon sources (e.g., coal or natural gas) generates substantial CO2 emissions. However, if these emissions can be sequestered, the WTT emissions from the production of these fuels can be reduced to nearly zero. Analysts differ on how each these factors should be treated in âscoringâ the WTT emissions characteristics of different transport fuels produced by different processes from different primary energy sources. Therefore, anyone reviewing the literature on this topic can expect to encounter a range of estimates. The important thing now universally acknowledged is that WTT emissions must be incorporated into any estimates of future transport-related CO2 emissions. Fueling Infrastructure A vast supply infrastructure has developed to deliver petroleum-based transport fuels to the vehicles that utilize them. As noted earlier, motor vehicles can use some alternative fuel blends (e.g., E5 and E10) without major modifications either to their engines or to their fuel systems. The same is true of the current fuel supply infrastructure. Todayâs petroleum product pipelines routinely carry gasoline, diesel fuel, jet fuel, and propane. They also can carry âmildâ blends of gasoline and biofuels (such as E5 and E10). But they cannot carry blends consisting of a majority of biofuels (such as E85) or 100 percent ethanol. The only gaseous transport fuel carried by pipeline is natural gas. Other gaseous transport fuels (in particular, hydrogen) would require dedicated pipelines. Another very important part of the transport fuel infrastructure is the fueling stations that actually deliver fuel to vehicles. Most of these are
Contribution to Emissions and Assessment of Strategies 257 not directly connected to a pipeline. Instead, they are supplied by tank trucks that haul fuel from a distributing point (that is connected to a pipeline) to individual fueling stations. One of the most formidable challenges facing any new transport fuel would be the establishment of an infrastructure capable of distributing it widely. The enormous fixed costs involved in establishing such an infrastructure mean it would not be established without assurance that the demand for the products it would transport would be forthcoming. Yet the vehicles that would be the source of this demand would not be built and purchased without assurance that fuel to power them would be available. Efforts are being made in some states to establish âhydrogen high- ways.â These are routes along which enough hydrogen refueling stations have been established to permit drivers of hydrogen-fueled vehicles to travel on them. These stations are supplied by tanker trucks. While this could help build initial demand for hydrogen as a transport fuel, it is not a long-term solution to the fuel infrastructure problem. HOW MUCH AND OVER WHAT TIME PERIOD MIGHT TRANSPORT-RELATED GHG EMISSIONS BE REDUCED? This appendix has described a wide range of technological and non- technological means of reducing transport-related GHG emissions. In this final section, the committee attempts to indicate how much transport-related GHG emissions might be reduced given the trends thus far described. As stated at the outset, the fundamental challenge is to reduce the emissions produced per unit of transportation services provided more rapidly than the demand for transportation services grows. While it may be possible to reduce the rate of transportation demand growth some- what without harming economic growth unacceptably, the committee is aware of no forecast that projects that transportation demand will fail to grow relatively rapidly in the decades ahead, especially in many of the worldâs less developed countries. The bulk of the responsibility for reduc- ing emissions will therefore fall on improved vehicle technologies and low-carbon or carbon-free fuels. There is considerable uncertainty about what it might cost to com- mercialize and widely disseminate many of the more advanced vehicle
258 Potential Impacts of Climate Change on U.S. Transportation technology and fuel solutions. Given what is known about projected demand growth, however, it is possible to simulate what might be feasi- ble trajectories of advances in vehicle technology and fuel substitution. To obtain a better sense of the potential impact of various technolo- gies and fuels in reducing transport-related GHG emissions, the SMP conducted a number of simulations using its spreadsheet model. The benchmark was the SMP reference case projection showing total trans- port-related CO2 emissions doubling between 2000 and 2050, with most of the growth in emissions occurring in the countries of the developing world. While other analyses have examined this issue for individual developed countries or regions, to the committeeâs knowledge, the SMP was the first to examine it for the world as a whole. In these simulations, the focus was on total road transport. The exer- cise did not examine the technical or economic feasibility of any of the actions being simulated. It was intended merely to help the SMP under- stand the impact on GHG emissions from road vehicles if the actions described were taken. This enabled the SMP to compare its results with those of other studies that likewise did not consider technical or economic feasibility in deriving their results. Single-Technology Simulation The SMP began by examining the impact of single technologies on CO2 emissions from road transport worldwide. Figure B-13 shows results for five such technologiesâdieselization, hybridization, fuel cells, âcarbon- neutralâ hydrogen, and biofuels. It was assumed that each power train technology would achieve as close to 100 percent global sales penetration as possible given the characteristics of the technology and that each fuel would become as close to 100 percent of the global road transport fuel pool as its characteristics would permit. The SMP emphasized that these single-technology examples were purely hypothetical. It is highly unlikely in practice that any single tech- nology would achieve 100 percent penetration. Also, the examples cannot be added together. Differences in the timing of the implementation of these technologies and fuels in the developed and developing worlds were largely ignored. For both diesels and advanced hybrids, it was assumed that 100 per- cent sales penetration would be reached by 2030 and that these technologies
Contribution to Emissions and Assessment of Strategies 259 12 Gigatonnes CO2-Equivalent GHGs 10 Reference Case 8 Diesels Hybrids 6 Fuel Cells (hydrogen from natural gas) 4 Fuel Cells (zero-carbon hydrogen) Advanced Biofuels (all road 2 vehicles) 0 2000 2010 2020 2030 2040 2050 FIGURE B-13 Hypothetical potential of individual technologies to lower road transport well-to-wheels GHG emissions relative to the SMP reference case. (Source: World Business Council for Sustainable Development 2004, Figure 4.7, p. 113.) would be used in LDVs and medium-duty trucks.36 In the case of fuel cell vehicles, it was assumed that 100 percent sales penetration would be reached by 2050.37 It was also assumed that the hydrogen used in these vehicles would be produced by reforming natural gas and that carbon sequestration would not be involved. The estimate of the impact of car- bon-neutral hydrogen was generated by changing the WTT emissions characteristics of the hydrogen used in the fuel cell case just described. To focus on the impact of biofuels, it was assumed that these fuels would be used in a world road vehicle fleet similar in energy use characteristics to the SMP reference fleet. Diesel internal combustion engine technology (using conventional diesel fuel) was assumed to have an 18 percent fuel consumption benefit compared with the prevailing gasoline internal 36 A very high proportion of heavy trucks and buses are already diesel powered. The SMP assumed that hybrid technology would not see significant use in heavy-duty over-the-road trucks and buses because of their operating characteristics. Public transport buses are already being viewed as prime candidates for hybridization. These were not included in the SMPâs calculation, but their omission makes relatively little difference in the results. 37 The SMP made the same assumptions concerning the types of vehicles to which fuel cells might be applied as it did for hybrids.
260 Potential Impacts of Climate Change on U.S. Transportation combustion engine technology during the entire period. The fuel con- sumption benefit relative to gasoline internal combustion engine technology was assumed to be 36 percent for diesel hybrids, 30 percent for gasoline hybrids, and 45 percent for fuel cell vehicles. From this single-technology assessment, it is evident that even if implemented worldwide, diesels and hybrid internal combustion engines fueled with conventional gasoline and diesel fuel or fuel cells fueled with natural gasâderived hydrogen could no more than slow the growth in road transport CO2 emissions during the period 2000â2050. Only the use of carbon-neutral hydrogen in fuel cells and advanced biofuels in inter- nal combustion engineâpowered vehicles could largely or totally offset the increase in CO2 emissions produced by the growth in road travel dur- ing the period 2000â2050. This does not mean that vehicle energy use characteristics are irrele- vant. They might not have a major impact on the trajectory of road vehicle GHG emissions over the very long term, but they would have a major impact on the amount of low-carbon or carbon-neutral fuel that would have to be produced to power the worldâs road vehicle fleet. This means they could have a very important impact on the cost of signifi- cantly reducing GHG emissions from road vehicles.38 On the basis of these results, the SMP concluded that it is only through a combination of fuel and power train solutions that significant CO2 reduction can be attained. No single-technology pathway merits selection as the sole long-run solution. Combined-Technology Simulation Since the substantial reduction of CO2 emissions from road vehicles is likely to depend on the widespread adoption of several advanced vehicle and fuel technologies, as well as other factors, the SMP decided to exam- ine the combined impact of several actions, including the following: â¢ Fuels that are carbon neutral (defined by the SMP as ones that reduce WTW CO2 emissions by at least 80 percent); â¢ Power trains that are highly energy efficient; 38 The fuel economy benefit relative to gasoline internal combustion engine technology was assumed to be 36 percent for diesel hybrids, 30 percent for gasoline hybrids, and 45 percent for fuel cell vehicles.
Contribution to Emissions and Assessment of Strategies 261 â¢ A change in the historical mix-shifting trend to larger vehicle cat- egories; and â¢ Improved traffic flow and other changes in transport activity resulting from better integration of transport systems, enabled, at least in part, by information technology. The SMP set an illustrative target of reducing annual worldwide CO2 emis- sions from road transport by half by 2050. This is equivalent to a decline in yearly CO2 emissions of about 5 gigatonnes from levels that the SMP refer- ence case projects would otherwise be reached and, by coincidence, returns annual road vehicle CO2 emissions in 2050 to about their current levels. For illustrative purposes, the CO2 reduction target was divided into six increments. The timing and size of each increment are not fixed and ultimately would be decided on the basis of sustainability and investment choices at the national, regional, and global levels. The purpose of the analysis was to illustrate what might be achieved if ambitious changes were made beyond those in the SMP reference case, with no judgment as to cost or the probability of each step being taken. Increment 1. Dieselization: It was assumed that dieselization of LDVs and medium-duty trucks would rise to around 45 percent globally by 2030 (that is, to about current European levels). Diesel engines were assumed to consume about 18 percent less fuel (and emit 18 percent less CO2) than current gasoline internal combustion engines. Increment 2. Hybridization: It was assumed that the hybridization (gasoline and diesel) of LDVs and medium-duty trucks would increase to half of all internal combustion engine vehicles sold by 2030. Gasoline hybrids were assumed to consume an average of 30 percent less fuel than current gasoline internal combustion engines, and diesel hybrids were assumed to consume an average of 24 percent less fuel than current diesels.39 Increment 3. Conventional and advanced biofuels: It was assumed that the quantity of biofuels in the total worldwide gasoline and diesel pool would rise steadily, reaching one-third by 2050. Conventional biofuels (those yielding a 20 percent CO2 unit efficiency benefit) were capped at 39 It is generally acknowledged that, because of the dieselâs initial superior energy efficiency, any additional benefit from hybridizing a diesel is likely to be smaller than that from hybridizing a gasoline engine.
262 Potential Impacts of Climate Change on U.S. Transportation 5 percent of the total pool. The balance was assumed to be advanced bio- fuels (those yielding at least an 80 percent CO2 unit efficiency benefit).40 Increment 4. Fuel cells using hydrogen derived from fossil fuels (no carbon sequestration): It was assumed that mass market sales of LDVs and medium-duty trucks would start in 2020 and rise to half of all vehi- cle sales by 2050. It also was assumed that fuel cellâequipped vehicles consume an average of 45 percent less energy than current gasoline inter- nal combustion engines. Increment 5. Carbon-neutral hydrogen used in fuel cells: It was assumed that hydrogen sourcing for fuel cells would switch to centralized production of carbon-neutral hydrogen over the period 2030â2050 once hydrogen LDV fleets had reached significant penetration at the country level. By 2050, 80 percent of hydrogen would be produced by carbon- neutral processes. The first five increments reflect the inherent properties of different vehicle technologies and fuels. Actual reductions in CO2 emissions will be determined not only by these properties but also by the mix of vehicles purchased by consumers and businesses and by how these vehicles are used on a daily basis. To reflect these two factors, two more increments were included. Increment 6. Additional improvement in fleet-level vehicle energy efficiency: The SMP reference case projects an average improvement in the energy efficiency of the on-road LDV fleet of about 0.4 percent per year, with new vehicle sales showing an average 0.5 percent per year improve- ment in fuel economy. The improvement potential embodied in actual vehicles is around 1.0 percent per year, but about half of this potential improvement is offset because of vehicle purchasersâ preferences for larger and heavy vehicles. In developing this increment, the SMP assumed that preferences relating to the mix of vehicles chosen by purchasers and the performance of these vehicles would change somewhat, leading to an additional 10 percent average annual in-use improvement relative to the reference case (i.e., average annual fleet-level improvement would rise from about 0.4 percent to about 0.6 percent). 40 This implies that these advanced biofuels are either gasoline from lignocellulosic sugar fermentation or diesel from biomass gasification/FischerâTropsch synthesis.
Contribution to Emissions and Assessment of Strategies 263 Increment 7. A 10 percent reduction in emissions due to better traf- fic flow and other efficiencies in road vehicle use: It was assumed that the gap between on-road energy-use performance and the technological improvements embodied in vehicles would narrow. How might this hap- pen? For one thing, there are a number of opportunities relating to the increased use of information technology in transport systems that might enable the better management of travel demand. Improved routing infor- mation might permit trips to be shortened, while improved information about road conditions might reduce the amount of time motorists spend in their vehicles while idling in traffic. For another thing, more accurate and current information about when public transport vehicles will arrive and how long they will take to get to their destinations might encourage additional use of public transport. Individually, none of these improve- ments would be major, and almost certainly there would be offsets. But in combination, the SMP assumes that such factors could produce an additional 10 percent reduction in road vehicle CO2 emissions. Figure B-14 shows the results of the SMP combined-technologies analysis just described. It confirms the impression conveyed by the three single-technology analyses discussed above that the widespread adoption of a combination of vehicle and fuel technologies (plus other factors) would be required to return 2050 CO2 emissions from road vehicles to their 2000 level. SUMMARY Any global warming that will be experienced during the next several decades will largely be the result of GHG emissions that have already occurred. As the main body of this report points out, regardless of what else it might do, Americaâs transport sector will have to adjust to the con- sequences of this warming. But the transport sector in general, and Americaâs transport sector in particular, is a significant source of GHG emissions. If future warming is to be limited, GHG concentrations in the atmosphere must be stabilized. This will require reducing GHG emis- sions not merely to below what they might otherwise be if present trends were to continue but to well below current levels. The transport sector will have to contribute to this reduction. This appendix has identified several approaches by which transport- related GHG emissions might be reduced. A common characteristic of
264 Potential Impacts of Climate Change on U.S. Transportation 12 Gigatonnes CO2-Equivalent GHGs 10 8 6 4 2 0 2000 2010 2020 2030 2040 2050 Diesels (LDVs) Hybrids (LDVs and MDTs) Biofuels (80% low GHG sources by 2050) Fuel Cells (fossil hydrogen) Fuel Cells (80% low-GHG hydrogen by 2050) Mix Shifting Yielding 10% Vehicle Efficiency Improvement 10% Vehicle Travel Reduction (all road vehicles) Remaining GHGs FIGURE B-14 Combined-technology case. (Source: Adapted from World Business Council for Sustainable Development 2004, Figure 4.11, p. 117.) these approaches is that they take considerable time to be fully effective. This means that if transport-related GHG emissions are to be reduced to below their current levels by 2050, steps must be taken now to begin to implement certain of these approaches. REFERENCES Abbreviations BTS Bureau of Transportation Statistics IEA International Energy Agency IMO International Maritime Organization IPCC Intergovernmental Panel on Climate Change UN United Nations
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