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This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions, American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected]. ©Sigma Xi, The Scientific Research Society and other rightsholders The Other Climate Threat: Transportation

A global travel surge is inevitable, but runaway growth of mobility-related CO2 emissions is not

By Andreas Schäfer, Henry D. Jacoby, John B. Heywood and Ian A. Waitz

fter the Wright brothers invented portation already accounts for about 23 regardless of income or geography, Athe first powered airplane and percent of energy-related carbon diox- people on average spend a roughly Henry Ford manufactured the first af- ide emissions, estimated at 31.5 billion constant amount of time on travel. At fordable car, transportation innovation metric tonnes worldwide in 2008. Pas- low income levels, for instance, resi- motored forward. Steady improve- senger travel accounts for 60 percent of dents in African villages spend slightly ments in technology and reductions in transportation’s share of emissions. With more than one hour per day traveling, cost enabled unprecedented gains in coming global economic development, mainly to obtain water and firewood, mobility. That, in turn, assisted extraor- the relative importance of transportation and to commute to and from agricul- dinary global economic growth. A cen- CO2 emissions will only increase. Imag- tural work. Much wealthier residents tury later, however, one early transpor- ine the additional impact of hundreds of in automobile-dependent societies tation practice persists. Oil-based fuels millions of new commuters in China and such as Japan, Western Europe and the still power the world’s transport fleets. India driving to work, and the residents United States—where half of all trips These fuels made sense for many of today’s industrialized world increas- are leisure-oriented—spend a similar decades for many reasons. Oil prod- ingly relying on air travel. Then envision amount of time en route. ucts, liquid at common temperatures the rest of the world trying to catch up. A second characteristic is the con- and pressures, are easily handled and To prevent unrestrained growth in nection between income and mobility: stored. They possess the highest energy these emissions, transportation trends As per capita gross domestic product content per volume of all practical fuels worldwide must be better understood. (GDP) grows, so does per capita pas- and per weight of all liquid fuels, im- For that reason, we’ve assembled a senger kilometers traveled (PKT). This posing the smallest penalties for energy unique data set describing the traffic pattern holds regardless of culture, po- storage—a quality especially valuable volume of world regions since 1950 for litical system or stage of economic de- for aircraft. And for many years, pe- each major transport mode. With this velopment. Neither economic ups and troleum has been affordable and abun- database, we studied the fundamental downs nor changes in fuel price have dant. Some analysts argue it is likely to relationships that transformed the global fundamentally changed this relation- remain so for a long time to come. transportation system in the past. Char- ship over long periods. To travel more Unfortunately, when burned, gaso- acterizing such forces and constraints within a stable time budget, people line, diesel and jet fuel emit carbon di- has helped us understand the likely fu- shift to faster modes of transport, to oxide, the most abundant human-made ture course of transportation. motor scooters and public transport in greenhouse gas. Thus, oil-based fuels We have also analyzed the ways poorer places or to airplanes and high- are a prime player in climate change. Be- that industry and consumer choices speed trains in richer ones. cause of the enormous scale of our glob- may influence greenhouse gas emis- Indeed, there appear to be three al transport systems, motorized trans- sions. Such insights give clues to which distinct phases of motorized mobility vehicle designs, aircraft technologies development. Below an annual mobil- Andreas Schäfer is director of the Martin Center for and alternative fuels can most effec- ity level of 1,000 kilometers per person Architectural and Urban Studies at the University tively reduce those emissions. All this (fewer than 3 kilometers per day), low- of Cambridge and a research affiliate at the Massa is needed to understand which public speed public transportation accounts for chusetts Institute of Technology. Henry D. Jacoby policy strategies should be pursued to nearly the entire traffic volume. These is codirector of the MIT Joint Program on the Sci- achieve needed change. Because many modes include urban public transport ence and Policy of Global Change and professor of greener technologies and fuels are more systems, commuter railways, intercity management in the Sloan School of Management at expensive than what’s prevalent today, trains and intercity buses. At the next MIT. John B. Heywood is Sun Jae Professor in the reducing transportation emissions is stage of development—a yearly travel Department of Mechanical Engineering and direc- not a problem the marketplace can be demand of between 1,000 and 10,000 Images AFP/Getty Dovic/Corbis, Muriel Leibowitz, Bruce Images, Getty via Universal El McConnell/Alamy, Andrew left: upper from clockwise tor of the Sloan Automotive Laboratory at MIT. Ian expected to solve alone. kilometers per capita (about 3 to 30 ki- A. Waitz is the Jerome C. Hunsaker Professor and department head of MIT’s Department of Aeronau- lometers per day), the use of low-speed tics and Astronautics. Address for Schäfer: Depart- Our Predictable Travel Habits public transport modes declines to be- ment of Architecture, University of Cambridge, Central to our projections of future tween 10 and 30 percent of total PKT 1-5 Scroope Terrace, Cambridge, CB2 1PX, U.K. travel demand are two characteristics and the use of light-duty vehicles rises. Internet: [email protected] of aggregate human behavior. First, These vehicles consist primarily of auto-

© 2009 Sigma Xi, The Scientific Research Society. Reproduction 476 American Scientist, Volume 97 with permission only. Contact [email protected]. Figure 1. People around the world use whatever type of transportation they can afford to get where they want to go. As they grow richer, they pay more to travel farther faster. At bottom left, Cambodians on motorbikes, some doubled up on the same vehicle, sit stuck in a traffic jam in Phnom Penh. Above, vans, buses and pedestrians crowd a main thoroughfare in Khartoum, Sudan. To the right, traffic is even more intense in the southern Mexican city of Naucalpan. Below, near-gridlock strikes Hartsfield-Jackson Atlanta International Airport in the United States during a weekday evening. Below that, the train at left makes the trip between Alsace and Paris in two hours and 20 minutes. France inaugu- rated the new high-speed train line in 2007.

© 2009 Sigma Xi, The Scientific Research Society. Reproduction www.americanscientist.org 2009 November–December 477 with permission only. Contact [email protected]. 5BO[BOJBWJMMBHFT 4PVUI,PSFB +BQBO trialized world is projected to double (IBOBWJMMBHFT (FSNBOZ 'SBODF or almost triple and retain the highest 1BMFTUJOF 4JOHBQPSF 1BSJT per-person mobility levels. Within that 3PNBOJB 4QBJO 4XJU[FSMBOE sector, North America would remain 8BSTBX 1BSJT (SFBU#SJUBJO 4ÈP1BVMP 5PLZP /PSXBZ the region with the highest per-person 4PVUI"GSJDB 'JOMBOE 6OJUFE4UBUFT mobility levels, rising from 27,400 kilometers per year in 2005 to 39,000 to 48,000 kilometers in 2050, depending on the rate of economic growth. As is the case with mobility demand trends, the types of transportation people migrate to will likely remain consistent with historical patterns, in that people who grow more affluent will IPVSTQFSDBQJUB WJMMBHFT shift toward faster transport modes. If DJUJFT per-person travel time remains close to BWFSBHFEBJMZUSBWFMUJNF DPVOUSJFT its current level of about 1.2 hours per day in 2050, high-speed transportation modes could claim about two-thirds of (%1QFSDBQJUB EPMMBST 64 ZFBSWBMVF total PKT in the industrialized world. Figure 2. The amount of time people around the world spend traveling is startlingly consistent, Automobile use would account for no matter whether they live in African villages, Europe or the most affluent regions of Asia. nearly the entire remaining traffic vol- People in the industrialized world spend a larger share of their traveling time for leisure activi- ume. In contrast, light-duty vehicle ties—about half—but they travel similar amounts of time on average each day nonetheless. travel would grow most in developing regions, making up 40 to 50 percent of mobiles, but include minivans, vans, average travel costs do not change dra- total PKT. Globally, automobiles could pick-up trucks and sport utility vehicles. matically, rising income will continue account for more than 40 percent, high- Finally, in the third stage of develop- to push up travel demand. At the same speed transport for almost 40 percent ment, mobility levels of 10,000 kilome- time, the fixed travel-time budget will and low-speed public transport for ters per person and higher require a sub- induce travelers to use faster transport about 20 percent of worldwide PKT. stantial share of air traffic or high-speed modes. According to the United Na- Despite the increasing dependence rail-based ground transportation. (We tions’ “medium variant” projections, on high-speed transportation modes, aggregate these as “high-speed trans- world population is expected to in- especially within the industrialized port,” although air travel is the largest crease by 44 percent by 2050. During world, day-to-day travel would not be single component now in use.) In North that same period, per capita world significantly different than it is today. America, airline travel market share has gross product (with nations aggregat- Because of the comparatively low trav- grown at the expense of passenger cars ed by market exchange-rate equiva- el speed of automobiles, North Ameri- over the past four decades. Nearly all lents) is projected to multiply by 2.2 to cans would still spend most of their passenger traffic is split between au- 2.6. Using simple statistical models, we travel time in cars, roughly 45 minutes tomobiles (81 percent) and aircraft (15 project that gain could triple or qua- per day. The remainder of their travel percent); rail and bus services are active druple world travel by midcentury. In time budget would be spent in low- mainly in niche, urban areas. the developing world, passenger mo- speed public transportation modes These two characteristics of aggre- bility could increase to levels three to and increasingly high-speed transpor- gate human behavior may tell us some- six times those in 2005. Over the same tation systems. In regions dependent thing important about the future: If period, travel demand in the indus- on high-speed transportation, rapid

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JOQBTTFOHFSLJMPNFUFSTUSBWFMFE QFSDFOU QBTTFOHFSLJMPNFUFSTUSBWFMFE QFSDBQJUB access to airplanes would become criti- cal. Already, entrepreneurs are imple- JOEVTUSJBMJ[FESFHJPOT menting business models to serve that /PSUI"NFSJDB demand. One model employs very 1BDJGJD0&$% light jets with four- to six-passenger ca- 8FTUFSO&VSPQF pacity and low-traffic airfields to help SFGPSNJOHFDPOPNJFT travelers avoid long check-in times and &BTUFSO&VSPQF congestion at primary airports. GPSNFS4PWJFU6OJPO More Size and Comfort, More CO2 EFWFMPQJOHSFHJPOT It isn’t simply growth in travel de- 4VC4BIBSBO"GSJDB mand that has expanded the amount of energy each passenger uses. Societal -BUJO"NFSJDB changes have played a role, as have .JEEMF&BTU/PSUI"GSJDB consumer preferences. In the United 4PVUI"TJB

States, for instance, energy use per QBTTFOHFSLJMPNFUFSTUSBWFMFE QFSDBQJUB DFOUSBMMZQMBOOFE"TJB passenger kilometer, which in 1950 PUIFS1BDJGJD"TJB was 2.0 megajoules, is now 2.2 megajoules. This has occurred even though automobile engines are much more (%1QFSDBQJUB EPMMBST 64 ZFBS fuel efficient now. Similar trends are observed in other industrialized coun- Figure 4. Per-capita passenger-kilometers traveled (PKT) is correlated with economic develop- tries. One prominent explanation is the ment, shown as growth in per capita gross domestic product. The 11 regional trajectories rep- decline in the number of people using resent the years 1950 to 2005. The upper point of the shaded section is the theoretical highest the same car. The average number of mobility level, achieved when a population relies exclusively on the fastest mode of transport people sharing a light-duty vehicle in (aircraft) over the entire travel time budget (1.2 hours per person per day). Although development toward the endpoint seems possible through midcentury, the finite capacity of airspace the U.S. declined from 2.2 in 1969 to 1.6 will very likely limit that trend over the very long term. At some point, regional trajectories in 2001, yielding a nearly 40 percent in- may level off. Pacific OECD includes mainly Japan, Australia and New Zealand. Centrally crease in energy use per PKT of light- planned Asia includes mainly China, North Korea and Mongolia. duty vehicles. This trend has likely lev- B C eled off in industrialized countries, but per kilometer and per ton of vehicle (a clined significantly for air transporta- it is only beginning in many parts of metric that corrects for the increases in tion. Still, in air travel as well, trends to- the developing world. vehicle weight associated with rising ward comfort, convenience and speed Another important contributor to levels of safety and comfort) has been have stood in the way of further energy the rising energy intensity of personal as much as 80 percent. use reductions. Expanding business travel has been consumers’ appetite for Airlines have always sought fuel ef- and first-class sections in legacy carriers improved safety, comfort and engine ficiency, given that fuel expenditures reduces passenger density, for example. power. Compared to Henry Ford’s are frequently the largestMJHIUEVUZWFIJDMFT single fraction The trend toward more frequent flights Model T, the average new light-duty of their operating costs. In their attempt using very light jets also contributes, vehicle sold in the United States in to remain profitable, airlines have also by increasing energy-intensive takeoffs TIBSFPG

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© 2009 Sigma Xi, The Scientific Research Society. Reproduction www.americanscientist.org 2009 November–December 479 with permission only. Contact [email protected]. Technology and New Fuels Can Help Fortunately, emerging technologies and alternative fuels could substantially reduce CO2 emissions. But it’s important to be realistic about what can be accomplished. Sometimes, what looks like an obvious solution isn’t fea- sible without overcoming challenges that arise when implementing fuel- saving technology. For instance, one might assume that automobile engine efficiency could be improved significantly by increasing the amount of air per unit of fuel burned. But lean-burn engines would have an unwanted side effect. Three-way catalysts in today’s engines cannot reduce nitrogen oxide emissions in an oxygen-rich environment. In aircraft engines, higher combustion temperatures would increase engine efficiency but would also in- Figure 5. Although hybrid vehicles powered by battery and petroleum are only now gaining crease nitrogen oxide emissions. market share, efforts to combine an internal combustion engine and one or more electric mo- tors date back more than a century. Above is the electric Voiturette System Lohner-Porsche, Another key factor is time. Develop- displayed at the 1900 Parisian World Exhibition. It was designed by Ferdinand Porsche, a ing new road and air vehicles can take pioneer of automobile engineering. Within a year, he added an internal combustion engine, decades. An extreme example is the creating the first hybrid. Image courtesy of Porsche Cars North America. hybrid-electric vehicle, which was first designed by Ferdinand Porsche, more costs. Aggressive pricing policies and mained steady in the global transpor- than 90 years before the marketing of new business models, developed during tation fleet in 2050—and oil products modern hybrids. Even what are consid- battles for market share, also accelerated remained the primary fuel—energy use ered quick transitions in transportation cost reductions. The combination of re- per PKT would increase by 12 to 25 per- aren’t always swift. It took 20 years be- duced costs and rising income made cent globally. The energy cost per PKT tween the first workable design of a gas 6OJUFE4UBUFT improved safety, comfort and speed in- would rise from roughly 1.4 megajoules turbine engine and the implementation creasingly more affordable. In 1967, the per passenger kilometer to between(SFBU#SJUBJO 1.6 of commercial jet service in 1952 with average U.S. household needed to work and 1.8 megajoules. The extent of (FSNBOZthis the de Havilland Comet, the world’s 21 weeks to afford a new car. Today, that intensification depends on the rate of 'SBODFfirst jetliner. One reason is that devel- period has shrunk to 17 weeks, despite economic growth and its distribution oping new transportation technologies the significant enhancement of vehicles. across world regions. In this scenario,+BQBO often requires huge investments, which FOFSHZJOUFOTJUZ NFHBKPVMFT

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Figure 6. Despite continued improvements in technology, the amount of energy used per passenger kilometer travelled has not declined in much of the industrial world, as displayed at left. That’s due partly to consumers’ preferences for increased size, weight and power in new cars. The trendline in Germany drops after 1990, when West Germany and less affluent East Germany reunited. At right, the rate of decreasing energy use per passenger kilometer in aircraft outpaces that of automobiles in the United States, a development observed around the world. © 2009 Sigma Xi, The Scientific Research Society. Reproduction 480 American Scientist, Volume 97 with permission only. Contact [email protected].

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(Note that a GVFMDPOTVNQUJPO MJUFSTQFSLJMPNFUFST 50 percent reduction in fuel consump- Figure 7. Reducing fuel consumption in new cars costs money and cost increases accelerate at tion corresponds to a doubling, or a 100 higher fuel efficiencies. Above are the projected cost increases that four research studies pre- percent increase, in fuel efficiency.) dicted would occur when improving fuel efficiency in a so-called reference vehicle. That vehicle The expanded use of alternative fuels represents the average new automobile sold in the United States in 2000. It would sell for $20,000 could also reduce greenhouse gas emis- and consume about 9.8 liters of gasoline per 100 kilometers, or roughly 2.6 gallons per 62 miles. sions. However, given oil products’ ex- cellent fuel qualities, finding satisfactory ing fueled by cellulosic ethanol. Efforts 2050 could fall below the 2005 level, substitutes has been difficult. Achieving to develop synthetic oil products for due to lower growth in travel demand compatibility with the existing fuel in- surface and air transport also hold and quicker adoption of emerging frastructure can be a significant barrier. promise. In contrast, hydrogen is un- technologies. The recent growth of corn-derived etha- likely to gain significant market share Costs will influence whether techno- nol and biodiesel made from vegetable before 2050. That is because of the lack logical upgrades penetrate the world’s oils can be explained, in part, by their of a hydrogen production, distribution, vehicle fleets. New technologies can be blending flexibility with oil products. storage and fueling infrastructure, and expensive. The average car sold in the But development of these products was the high cost of fuel-cell vehicles. U.S. around the year 2000 consumed 9.8 spurred more by concerns over oil price When we incorporate what we label liters of gasoline per 100 kilometers. We volatility and energy security risks than “maximum technology” improvements project that reducing vehicle energy use concerns over greenhouse gas emis- into our world transportation projec- by 30 percent would increase the retail sions. Nearly all biofuel blends deliver tions, we predict that global average price of the car by $1,500. Because fur- only a few percent reduction in green- automobile energy use per PKT could ther fuel consumption reductions will house gas emissions over the course of decline substantially—by 35 percent by require still more sophisticated fuel-sav- the fuel’s life cycle compared to petro- 2050 compared to 2005. That number ing technology, cutting energy use in leum fuels. Synthetic fuels derived from represents a 50 percent reduction in the half would increase the price by $5,000. non-petroleum sources—whether coal, industrialized world and a 30 percent Given that, the average $20,000 cost of natural gas, or oil shale—can result in reduction in developing economies, a new automobile in the U.S. could in- fuels with even higher life-cycle green- where infrastructure and capital con- crease by 8 to 25 percent. With the luxu- house gas emissions than petroleum, straints will impede some change. We ry of relatively low fuel prices, U.S. con- unless CO2 released during their pro- also predict a 40 percent reduction in sumers so far have shown more interest duction is captured and sequestered. energy use per PKT for aircraft rela- in vehicle comfort and power than fuel Still, there is reason to be hopeful tive to a constant technology scenario. efficiency. These low fuel prices have that the next generation of biofuels, If these reductions are complemented also constrained research and develop- expected to be derived from cellulose by second-generation biofuels, which ment of liquid-fuel alternatives, such rather than from today’s feedstocks, we aggressively assume will provide as second-generation biofuels. These will emit substantially less CO2 over 20 percent of world passenger travel are two reasons why introducing such their life cycle. They should also in- energy by 2050, the level of life-cycle technologies and fuels on a large scale crease the fuel yield per hectare. Sci- greenhouse gas emissions from world will require public policy changes. ence hasn’t yet produced a commer- passenger travel could drop consid- cially viable means to break down erably by midcentury. This scenario Complicated Policy Choices cellulose into fermentable sugars, but would produce emission levels only To expand the use of fuel-saving tech- progress is being made. A particularly 40 to 140 percent above the 2005 lev- nologies and low-carbon fuels, govern- promising future technology in road el—despite the huge expansion in ments have a range of policy instru- transportation is the electric vehicle transportation demand that is coming. ments available. The ultimate objective with advanced batteries, perhaps in In the industrialized world, including is to induce consumers to change their hybrid mode, with an internal com- the United States, life-cycle greenhouse vehicle preferences to favor low fuel bustion engine for long-distance driv- gas emissions from passenger travel in consumption over size, comfort, en-

© 2009 Sigma Xi, The Scientific Research Society. Reproduction www.americanscientist.org 2009 November–December 481 with permission only. Contact [email protected]. XPSME which use costs to influence consumer DPOTUBOUUFDIOPMPHZ NBYJNVNUFDIOPMPHZ and industry behavior. This family of measures is more economically efficient than regulatory measures because it can impose the same penalty on all greenhouse gas emitters—whether they be vehicles, coal-fired power plants or other sources. Importantly, a FRVJWBMFOU surcharge in the transportation realm that raises fuel prices reduces energy use and emissions across an existing fleet, not just among new vehicles. Im- posing equal marginal costs across CO2 emission sources would be most easily accomplished with a national carbon 6OJUFE4UBUFT tax or a cap-and-trade system. Over long periods of time, either price or DPOTUBOUUFDIOPMPHZ NBYJNVNUFDIOPMPHZ regulatory approaches could achieve a target level of greenhouse gas emissions, although the rate at which each produces changes in the marketplace will differ. Complications regarding the likely impact of a market-based approach must be considered too, however. With income rising globally and the increasing reliance on the MJGFDZDMFHSFFOIPVTFHBTFNJTTJPOT NJMMJPOTPGUPOT $0 automobile and high-speed transport systems outlined above, an already small consumer response to fuel price increases and other market incentives &11"3FG 43&4# &11"3FG 43&4# will likely decline. The lack of alternatives to oil-based fuels does not help. As a result, any policy that imposes GVFMWFIJDMFDZDMF MPXTQFFEQVCMJDUSBOTQPSU emissions penalties evenly across the IJHITQFFEUSBOTQPSU MJHIUEVUZWFIJDMFT economy will result in a lower percentage reduction in transport emissions Figure 8. Depicted here are mobility-related greenhouse gas emissions in 1950 and 2005 world- compared to other sectors. This puts wide, in the upper box, and in the United States, below. Projections for 2050 are based on two in question whether the transportation separate scenarios of economic growth. EPPA-Ref represents per capita GDP growth predicted sector would carry an equitable share by the MIT Emission Prediction and Policy Analysis model. The SRES-B1 scenario comes of the burden under an equal-cost ap- from the Intergovernmental Panel on Climate Change and predicts higher average GDP per proach. Because of this, governments capita growth. Increases in greenhouse gas emissions from transport sources are much higher must decide whether to treat passen- in both scenarios when transportation technology does not change. But when aggressive steps ger transport separately and impose are taken to lower fuel consumption, emissions fall. different (and likely tougher) penalties compared to other economic sectors. gine power and other attributes. Ide- meet the least public resistance. How- A related question is whether it ally, new policies will induce the au- ever, because it takes several years for would be wise to emphasize a single- tomobile and aircraft industries to industry to design vehicles that meet policy approach in this sector—a price implement fuel-saving technology at new standards, and many more years penalty for CO2 emissions or a set of a faster pace and on a larger scale than for more fuel-efficient vehicles to be regulations, such as tailpipe emission they otherwise would. The potential sold in sufficiently large numbers, standards, for example—or to apply a payoffs can be significant, as indicated several decades may pass before a portfolio of measures. Neither a pure by the different emission levels in the regulation’s impact is felt. In addition, market-based nor a pure regulatory “constant technology” and “maximum experience in the United States follow- strategy is likely to succeed. One rea- technology” scenarios described ear- ing passage of the Corporate Average son is that both types of policies are lier. Either regulations or market-based Fuel Economy regulations suggests already in place. In the United States, measures, or both, could change con- that as automobile drivers observe the they include fuel taxes, fuel economy sumer and industry choices. Histori- reduced marginal costs of more fuel- and safety standards, research and de- cally, nearly all environmental legis- efficient vehicles, they travel more. velopment expenditures and subsidies lation has been based on regulatory This so-called rebound effect could for biofuels and hybrid-electric ve- measures. That’s true, in part, because weaken energy use reductions. hicles. Also, no single policy measure regulations tend to hide the extra costs More recently, governments have works equally well during all stages for fuel-saving technologies, and thus embraced market-based measures, of a technology’s life cycle, from the

© 2009 Sigma Xi, The Scientific Research Society. Reproduction 482 American Scientist, Volume 97 with permission only. Contact [email protected]. vehicle concept stage to its use on the sign, such as the blended wing-body portation. And there will be no changes road or in the air. Indeed, additional aircraft. Given the uncertainties regard- in transportation without coherent gov- policy measures would be needed just ing what will work, research and devel- ernment policies requiring them. to compensate for the undesired con- opment investments should be distrib- sequences of some interventions. For uted widely among promising options. Bibliography example, regulatory measures that aim Research on battery technology is criti- Lee, J. J., S. P. Lukachko, I. A. Waitz and A. to increase the fuel efficiency of new cal, but competing technologies such as Schäfer. 2001. Historical and future trends in aircraft performance, cost, and emissions. automobiles exclusively could produce the production and storage of hydrogen Annual Review of Energy and the Environment a rebound effect unless they are accom- should be supported as well. Because 26:167–200. panied by an increase in fuel tax. biomass-derived synthetic oil products Schäfer, A., J. B. Heywood, H. D. Jacoby and I. These complexities could be ad- can be readily used in automobiles and A. Waitz. 2009. Transportation in a Climate- dressed somewhat by an aggressive aircraft, research and development in- Constrained World. Cambridge: The MIT emission-reduction policy. An example vestments in these fuels could have an Press. Schäfer, A., J. B. Heywood and M. A. Weiss. would be a global carbon tax designed especially high payoff. 2006. Future fuel cell and internal com- to stabilize the accumulated atmo- Changing vehicles and fuels—along bustion engine automobile technologies: spheric concentration of greenhouse with altering consumer and industry A 25-year life-cycle and fleet impact as- gas emissions to, say, 550 parts per mil- behavior in the transportation sector— sessment. Energy–The International Journal lion. Many observers consider that level can reduce emissions. But this will take 31:1728–1751. Schäfer, A., and H. D. Jacoby. 2005. Technology unsafe, but it would be challenging to time, possibly decades. That means ef- detail in a multi-sector CGE model: trans- achieve nonetheless. Our analysis of forts to reduce mobility-related emis- port under climate policy. Energy Economics the United States under such a scenario sions, as part of a broader effort to con- 27:1–24. predicts that greenhouse gas emissions trol all greenhouse gas emissions, must Schäfer, A., and D. G. Victor. 2000. The future from passenger travel in 2050 would be a high priority now. Otherwise, we mobility of the world population. Transpor- tation Research A 34:171–205. fall significantly, to about the 2005 level. are confident, the past will repeat itself. That level would be considerably lower Demand for motorized travel will grow that what is expected with our “con- apace with global population and in- stant technology” scenario but well comes. And emissions will increase to above that of our “maximum technol- levels where transport emissions alone For relevant Web links, consult this ogy” scenario. At the same time, this could prevent the world from reaching issue of American Scientist Online: approach’s impact on the fuel efficiency the more ambitious targets for atmo- http://www.americanscientist.org/ of transportation technology and on re- spheric greenhouse gas concentration issues/id.81/past.aspx ducing travel demand would be rela- limits. There will be no solution to the tively small. That’s because reaching climate threat without changes in trans- that emission goal would not require extreme changes within transportation, such as expanded use of hybrid electric vehicles versus the traditional, mechanical drivetrain. Alternatively, passenger travel could be treated as a special category, with its own targets for emission reductions. Arguments for this approach include the fact that controls are more politically acceptable on passenger travel than in other economic sectors—such as power generation. Also, it’s argued that the combination of gains that would result could be more valuable than the extra costs imposed. The latter argument emerges when multiple policy concerns are pursued at once, such as reducing greenhouse gas emissions and reducing a country’s dependence on foreign oil. Whatever mitigation strategy governments adopt, advances in technology must also be encouraged by support of research and development. Points of focus should be improving propul- sion and energy storage systems and reducing the driving resistance of road vehicles through further weight reduction and downsizing. It is also time to explore radical changes in aircraft de-