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Advancing the Science of Climate Change (2010)

Chapter: 13 Transportation

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Suggested Citation:"13 Transportation." National Research Council. 2010. Advancing the Science of Climate Change. Washington, DC: The National Academies Press. doi: 10.17226/12782.
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CHAPTER THIRTEEN
Transportation

The transportation sector encompasses all movement of people and goods. Almost 28 percent of U.S. greenhouse gas (GHG) emissions can be attributed to this sector, and the overwhelming share of these emissions are from CO2 emitted as the result of burning transportation fuels derived from petroleum (EPA, 2009c). Between 1970 and 2007, U.S. transportation energy use and hence GHG emissions nearly doubled.1 Consequently, transportation is a major driver of climate change, and a sector with a potentially large role in limiting the magnitude of climate change.


Reducing transportation-related GHG emissions, and understanding the impacts of climate change on transportation systems, are concerns of many decision makers. Questions they are asking, or will be asking, about transportation and climate change include the following:

  • How much do various modes of transportation contribute to climate change?

  • What technologies and strategies can be used to reduce GHG emissions by the largest transportation contributors?

  • How will transportation systems in my area be affected by climate change?

  • What steps can be taken to make transportation systems less vulnerable to the impacts of climate change, and how can I apply them in current systems and incorporate them in the design and development of new infrastructure and policy?

This chapter summarizes how reducing the total amount of transportation activity, shifting some of the activity to less energy- and emissions-intensive modes, increasing energy efficiency, and reducing the GHG intensity of transportation fuels could help in lowering GHG emissions from this sector. Additionally, the chapter outlines how climate change will affect the transportation sector and describes the scientific and engineering knowledge regarding adaptation options. The last section of the chapter indicates research that is needed to better understand the impacts of climate change on transportation and ways to reduce GHG emissions in the transportation sector.

1

 The almost exclusive reliance on a single fuel source, petroleum, in the transportation sector means that relative energy expenditures can be interpreted as relative GHG emissions.

Suggested Citation:"13 Transportation." National Research Council. 2010. Advancing the Science of Climate Change. Washington, DC: The National Academies Press. doi: 10.17226/12782.
×

ROLE OF TRANSPORTATION IN DRIVING CLIMATE CHANGE

A large proportion of GHG emissions can be attributed to transportation, specifically from the burning of gasoline, diesel, and other fuels derived from petroleum. In fact, the transportation sector is responsible for 70 percent of U.S. petroleum use, which exceeds the percentage of oil that is imported (Davis et al., 2008). Reducing transportation’s dependence on petroleum, much of it imported from politically unstable regions of the world, is one of the most direct connections between the issues of climate change, energy security, and national security (see Chapter 16). Transportation’s use of petroleum fuels also leads to emissions of particulate matter, sulfur dioxide (which forms sulfate aerosols and ultimately leads to acid rain), and substances that are precursors to photochemical smog (nitrogen oxides [NOx] and carbon monoxide [CO]) and to various forms of pollution in freshwater and marine systems. Hence, efforts to reduce GHG emissions in the transportation sector will also confer other benefits to the environment and public health (see Chapter 11).


Transportation activity is typically divided into two categories: the movement of people and the movement of goods. The movement of people, usually expressed in passenger-miles, accounts for 70 percent of the transportation sector’s energy use and GHG emissions (Davis et al., 2008). The principal vehicles involved in the movement of people are light-duty personal vehicles—automobiles and light trucks—and commercial aircraft, which together account of almost 99 percent of passenger-miles (Davis et al., 2008). The movement of goods, usually expressed in ton-miles, is dominated by trucks, railroads, and ships. These freight modes account for the remaining 30 percent of transportation-related emissions (Davis et al., 2008). Table 13.1 shows the relative importance of different modes of personal and goods transport to total transport energy use and, by implication, its approximate contribution to GHG emissions.


In the United States between 1970 and 2007, energy intensity—the amount of energy required to produce a unit of transport activity—declined for nearly all transportation modes (for example, energy intensity declined by 0.3 percent per year on average for medium and heavy freight trucks, 0.8 percent per year for passenger cars, 1.5 percent per year for light trucks, 1.8 percent per year for freight rail, and 3.3 percent per year for domestic passenger air travel). However, these increases in efficiency were more than offset by an increase in total transportation activity (for example, the number of passenger-miles flown grew by 4.9 percent per year), leading to the overall growth in energy use and GHG emissions.

Suggested Citation:"13 Transportation." National Research Council. 2010. Advancing the Science of Climate Change. Washington, DC: The National Academies Press. doi: 10.17226/12782.
×

TABLE 13.1 Energy Use and Activity Characteristics of Various Transportation Modes in 2006

Mode

Energy Use

Passenger Transport Activity

Goods Transport Activity

Trillion BTU

%

Cum %

Passenger-miles (millions)

%

Cum %

Ton-miles (millions)

%

Cum %

Light-duty personal vehicles

16,824

65%

65%

4,546,618

87.5%

87.5%

 

 

 

Medium/heavy trucks

5,188

20%

85%

 

 

 

1,294,492

34.8%

34.8%

Domestic air transport

1,834

7%

92%

590,633

11.4%

98.9%

15,860

0.4%

35.2%

Pipeline

842

3%

95%

 

 

 

 

 

 

Freight rail

585

2%

98%

 

 

 

1,852,833

49.7%

84.9%

Domestic waterborne freight

304

1%

99%

 

 

 

561,629

15.1%

100%

Transit (all modes)

164

1%

99%

52,154

1.0%

99.9%

 

 

 

School bus

73

0%

100%

 

 

 

 

 

 

Intercity bus

30

0%

100%

 

 

 

 

 

 

Motorcycles

28

0%

100%

 

 

 

 

 

 

Intercity passenger rail

14

0%

100%

5,381

0.1%

100%

 

 

 

Total of above

25,886

 

 

5,194,786

 

 

3,724,814

 

 

SOURCE: Based on data from Davis et al. (2008) and DOT (2008).

Suggested Citation:"13 Transportation." National Research Council. 2010. Advancing the Science of Climate Change. Washington, DC: The National Academies Press. doi: 10.17226/12782.
×

REDUCING TRANSPORTATION-RELATED GREENHOUSE GAS EMISSIONS

There are four possible strategies that could be employed to reduce GHG emissions from the transportation sector:

  • Reduce the total volume of transportation activity;

  • Shift transportation activity to modes that emit fewer GHGs per passenger-mile or ton-mile;

  • Reduce the amount of energy required to produce a unit of transport activity (that is, increase the energy efficiency of each mode); or

  • Reduce the GHG emissions associated with the use of each unit of energy.

Each of these strategies is briefly discussed below. Additional details can be found in the companion report Limiting the Magnitude of Future Climate Change (NRC, 2010c), and the Transportation Research Board report Potential Energy Savings and Greenhouse Gas Reductions from Transportation (NRC, 2010f). The Limiting report concludes that “near-term opportunities exist to reduce GHGs from the transportation sector through increasing vehicle efficiency, supporting shifts to energy efficient modes of passenger and freight transport, and advancing low-GHG fuels.” Achieving large (that is, on the order of 50 to 80 percent) long-term reductions in GHG emissions in the transportation sector, however, would require major technological and behavioral changes (e.g., Fawcett et al., 2009); this in turn implies a need for additional research to support the development and deployment of new and improved transportation modalities.

Reducing the Volume of Transport Activity

The most basic—but perhaps most difficult—way to reduce transportation-related GHG emissions is to reduce the total amount of transportation activity. While there has been some attention devoted to reducing total freight transport volumes—by, for example, promoting consumption of locally produced food and goods—most of the attention in this area has focused on reducing personal transportation activity, especially activity by light-duty vehicles. Since 1980, the number of light-duty vehicle passenger-miles has grown at an average rate of 2.3 percent per year (FHA, 2008). This growth has been spurred by, among other factors, the suburbanization of America. As recently as the 1960s, the majority of daily commutes were from downtown to downtown or from close-in suburbs to downtown. Now, the majority of commutes are from suburb to suburb, with the attendant traffic and pollution issues (NRC, 2006a; see also Chapter 12). Suburbanization has also stimulated the increased use of light-duty vehicles for trips other than commuting—for example, according to the National House-

Suggested Citation:"13 Transportation." National Research Council. 2010. Advancing the Science of Climate Change. Washington, DC: The National Academies Press. doi: 10.17226/12782.
×

hold Travel Survey, in 2001 commuting accounted for 27 percent of all vehicle trips per household while “household-serving” travel (e.g., shopping errands, chauffeuring family members) accounted for most of the remainder (BTS, 2001).


Both logic and empirical evidence suggest that developing at higher population and employment densities results in trip origins and destinations that are closer to one another, on average, leading to shorter trips on average and less vehicle travel. Shorter trips can also reduce vehicle travel by making walking and bicycling more viable as alternatives to driving, while higher densities make it easier to support public transit. A recent National Research Council report, Driving and the Built Environment (NRC, 2009e), examined the relationships between land use patterns and vehicle miles traveled and concluded “[l]ooking forward to 2030 and, with less certainty, to 2050, it appears that housing preferences and travel patterns may change in ways that support higher-density development and reduced [vehicle miles traveled], although it is unclear by how much.” While the study concluded that significant increases in more compact, mixed-use development result in only modest short-term reductions in energy consumption and CO2 emissions, these reductions will grow over time. The implications of this and other findings for limiting GHG emissions from the transportation sector can be found in the companion report Limiting the Magnitude of Future Climate Change (NRC, 2010c).


Another trend that has led to increased travel activity has been the reduction over time in the average number of people traveling in each automobile and light truck. In 1977, the average vehicle carried 1.9 people; by 2001, this had declined by 14 percent, to 1.6 people. For travel to and from work, the average declined from 1.3 to 1.1 (Hu and Reuscher, 2005). Increasing the average vehicle occupancy could lead to reductions in total vehicle miles traveled and thus GHG emissions, even considering small offsets due to the need to pick up and drop off the additional passengers. Many municipalities have instituted policies to encourage carpooling; however, few of these policies were developed based on research on patterns and determinants of human behavior or effective mechanisms for informing such behavior, and there is a need for more evaluation of effectiveness.


Because commuting only accounts for about a quarter of passenger trips, carpooling strategies have limited potential for reducing transportation-related GHG emissions. However, it may be possible to increase the prevalence of ridesharing through more effective conveyance of information and the provision of incentives, both in monetary and convenience terms. New technologies could help in this regard; for instance, it is already possible to use personal telecommunications devices and computers to connect drivers with prospective riders to create casual forms of carpooling. Such op-

Suggested Citation:"13 Transportation." National Research Council. 2010. Advancing the Science of Climate Change. Washington, DC: The National Academies Press. doi: 10.17226/12782.
×

portunities will increase. Indeed, it is conceivable that in some locations public transit services will evolve away from the large fixed-route systems into smaller van-type vehicles that employ dynamic routing technologies to offer transportation services similar to that of private cars but with higher average occupancy (WBCSD, 2004). While such concepts are in limited use in Europe, they have not been explored in the United States.

Shifting Transportation Modes

Because there are significant differences in the energy expended per passenger-mile or ton-mile among the major modes of transportation, a second candidate strategy for reducing transportation-related GHG emissions is to shift people or freight to more energy efficient modes. The two most widely discussed options are (1) inducing people to substitute some of their driving with public transportation service, bicycling, and walking; and (2) shifting more freight from truck to rail.


The viability of public transportation (as well as walking and biking) as an alternative to driving hinges in part on there being favorable urban land use patterns, as discussed in the preceding subsection and in the recent report Driving and the Built Environment (NRC, 2009e). For public transportation to be an energy efficient alternative to the private vehicle, however, requires that the services be heavily used. At present, except in a few very dense urban areas such as New York City, public transportation load factors are not high enough to make these services more energy- and GHG-efficient than driving. Because demand is especially low outside of rush hours, transit systems often operate with very low levels of occupancy for much of the day (NRC, 2009c). As a consequence, buses—the most prevalent form of transit—used 24 percent more energy per passenger-mile than private cars in 2006 (Davis et al., 2008). Subways and commuter rail systems, in contrast, used about 20 percent less energy per passenger-mile than private cars, but these systems accounted for a minority of total public transportation ridership.


There is also significant geographic variability in the availability of public transportation: 97 percent of all subway and transit rail trips occurred in metropolitan areas with a population of over 5 million, and the New York metropolitan area alone was responsible for 38 percent of all national transit use for travel to and from work (NRC, 2006a). Bicycling and walking do not emit any GHGs and are associated with health co-benefits, but they currently constitute a very small share of all miles traveled by people when compared with motorized modes. Strategies designed to facilitate and promote these modalities could yield multiple benefits.

Suggested Citation:"13 Transportation." National Research Council. 2010. Advancing the Science of Climate Change. Washington, DC: The National Academies Press. doi: 10.17226/12782.
×

There has also been interest in using passenger rail for medium-distance (500 miles or less) intercity travel in the United States, which is currently dominated by automobiles and, to a lesser extent, air travel. In Europe and Japan, high-speed rail is succeeding in winning substantial market share away from automobiles and air transport for city-to-city travel at distances of up to 500 miles (FRA, 2009). There are many challenges, however, to duplicating such a system in the United States. While high gasoline and deisel fuel taxes and road tolls tend to discourage intercity travel by private car in Europe and Japan, the ease and low out-of-pocket cost for automobile travel in the United States favors their use. Automobiles also offer flexibility for local travel once at the final destination, which is particularly important for families and leisure travelers who make trips between suburbs rather than center cities. A large share of business travel also takes place in suburban areas, which are poor locations for high-speed rail terminals. Another challenge is that there are relatively few large U.S. metropolitan areas located within 500 miles of one another, especially when compared with Europe and Japan. Because of the long distances between cities, aviation is the only practical alternative for timely intercity travel in the United States. Moreover, U.S. airlines, operating in vast networks that funnel passengers through hubs, have the passenger volumes required to offer large numbers of flights between city pairs. This ability to offer a dense schedule of flights—which is highly valued by time-sensitive business travelers—cannot be matched by high-speed rail. The recent uptick in intercity bus travel in the United States, which has been attributed both to the recent economic downturn and to higher fuel prices, is another longer-distance travel option that could potentially be promoted to reduce overall energy use and GHG emissions, particularly among leisure travelers.


The practicality and benefits of shifting additional freight traffic from truck to rail has been studied and debated for years. In 1939, 64 percent of freight ton-miles moved by rail, while trucks carried only 9 percent, with most of the remainder moved on water-ways (Department of Commerce, 1975). In 2006, rail’s share had declined to 40 percent, dominated by heavy, bulk commodities such as coal, while trucking had increased its share to 28 percent (Margreta et al., 2009). Although moving freight by rail is generally more energy efficient than moving freight by truck, it is not clear that a significantly larger share of freight could be practically moved by rail. For example, because many rail sidings have been abandoned, most freight traffic, and especially manufactured goods, are moved by truck for at least a portion of the journey. On the other hand, the containerization of freight—especially for imported goods—increases the potential for movement by rail, and the recent sharp increases in the price of fuel seem to have shifted some containers from truck to rail and some truck trailers to rail (in “piggyback” service) for the line-haul segment of the trip. Observers who study freight movements

Suggested Citation:"13 Transportation." National Research Council. 2010. Advancing the Science of Climate Change. Washington, DC: The National Academies Press. doi: 10.17226/12782.
×

contend that rail container and trailer movements such as these are generally not economically viable until line-haul distances reach 700 miles and, with the exception of the longest moves (over 1,500 miles), between the most heavily traveled markets having lane traffic densities in excess of 400,000 tons annually (Wittwer, 2006).

Reducing Energy Intensity

Increasing the efficiency of transportation—especially light-duty vehicles—has been a major strategy for reducing U.S. petroleum consumption. The companion report Limiting the Magnitude of Future Climate Change (NRC, 2010c) includes a summary of changes in fuel economy standards over the past 30 years, the effectiveness of these standards, and their implications for climate policy. For example, the fuel economy potential of new passenger cars and light trucks (measured in terms of ton-miles per gallon) has improved at a rate of about between 1 and 2 percent per year since 1975 (EPA, 2009c), mainly through a series of technological advances in engines and aerodynamics. However, this potential has not been reflected in actual new vehicle fuel economy; since the mid-1980s, the fuel economy of new automobiles and light trucks as tested by the Environmental Protection Agency (EPA) has essentially been stable. Instead, vehicles have become heavier (by about 900 pounds on average [Davis et al., 2008]) and have improved their acceleration performance (average 0 to 60 mph times have declined from just over 14 seconds to about 9.5 seconds [Davis et al., 2008]). The EPA estimates that if the potential improvements in fuel economy had been realized, model year 2008 cars would have averaged 33 to 34 mpg instead of the 30 mpg they did average, and new light trucks would have averaged 27 to 28 mpg instead of 22 mpg.


Congress has called for a fleetwide combined fuel economy for cars and light trucks that reaches 35 mpg by model year 2020, representing a 30 percent increase over current levels (Energy Independence and Security Act of 2007, P.L. 110-140). In addition, new EPA GHG-performance standards for cars and light trucks will acclerate these fuel economy improvements by 3 or 4 years (EPA, 2009c). Tapping the reservoir of unrealized fuel economy potential with continued modest improvements in the efficiency of conventional gasoline and diesel engines would be the easiest way for motor vehicle manufacturers to meet these new efficiency standards. Doing so, however, would require consumers to sacrifice certain desired performance attributes such as acceleration capabilities. In order to meet the new standards under these constraints, manufacturers will need to increase the use of hybrid-electric propulsion systems, make cars and trucks lighter (typically through the use of materials such as fiberglass and carbon fibers), and develop next-generation propulsion systems—batteries and fuel cells being the two main candidates (see next subsection). It will be important with respect

Suggested Citation:"13 Transportation." National Research Council. 2010. Advancing the Science of Climate Change. Washington, DC: The National Academies Press. doi: 10.17226/12782.
×

to some of these vehicle technologies to consider the life-cycle energy costs associated with producing more efficient vehicles; for example, some of the materials used for lightweight and hybrid vehicles are associated with significant energy production costs, which may offset some fuel savings.


To advance the technologies required to enable the production of more fuel-efficient light vehicles, the federal government has over the years funded cooperative research and development programs such as the Program for a New Generation of Vehicles. In addition to such federal actions, some states, led by California, have set their own fuel economy standards and taken other actions, such as requirements to sell a certain number or fraction of low-emissions vehicles.


In addition to improving the efficiency of the vehicle fleet, there are behavioral changes that may be able to increase the energy efficiency of the operations of existing vehicles in the light-duty fleet, such as maintaining properly inflated tires, reducing time spent idling, and removing excess weight from trunks. Each of these alone is a minor factor for the individual driver, but small changes multiplied across the U.S. passenger vehicle fleet could have an impact (Dietz et al., 2009b). More information is needed on the prevalence and effectiveness of these behaviors as well as on how they might be further encouraged.


It merits noting that Congress has called for fuel efficiency standards for medium- and heavy-duty trucks (P.L. 110-140). EPA may also develop GHG performance standards for trucks and other transportation vehicles (EPA, 2010b). Developing efficiency standards for trucks presents a particular challenge, because these vehicles are used in so many different ways that a single metric for efficiency is impractical (e.g., using miles per gallon as a metric would encourage smaller trucks with less payload and would reduce ton-miles per gallon). A recent NRC report examines the issues surrounding the development of such standards (NRC, 2010i). As this report and others have pointed out, trucking and the other long-distance freight and passenger modes of transportation already have powerful economic incentives to care about energy efficiency, since they are highly competitive and cost-conscious industries in which fuel is a main operating cost.

Reducing the GHG Intensity of Transportation Fuels

A final strategy for reducing transportation GHG emissions is reducing the GHG emissions associated with the use of each unit of transport energy, typically through the development and deployment of vehicles that run on electricity or liquid or gaseous transportation fuels not based on petroleum, such as biofuels or hydrogen. In ad-

Suggested Citation:"13 Transportation." National Research Council. 2010. Advancing the Science of Climate Change. Washington, DC: The National Academies Press. doi: 10.17226/12782.
×

dition to propulsion and energy storage technologies themselves, this requires the development of ways to manufacture and distribute the new fuel or energy sources. While some of these vehicle and fuel combinations would significantly reduce or completely eliminate tailpipe GHG emissions, the GHG emissions generated as a result of fuel production and distribution could be significant and offset all or some of these benefits. Indeed, in some circumstances, the resulting “well-to-wheels” GHG emissions—emissions resulting from the extraction, production, and distribution of fuel plus the emissions resulting from its use by the vehicle—can exceed the well-to-wheels emissions generated by current transport vehicles using petroleum-based fuels. For example, some biofuels, especially corn-based ethanol but also certain forms of biodiesel, may not yield a net reduction in GHG emissions (Campbell et al., 2009; Searchinger et al., 2008).


In its analysis of the well-to-wheels impacts of alternative liquid transportation fuels, the America’s Energy Future panel on this topic found that CO2 emissions from corn grain ethanol are only slightly lower than those from gasoline (NRC, 2009b). In contrast, CO2 emissions from cellulosic ethanol (biochemical conversion) are much lower (NRC, 2009b). However, cellulosic processes are not yet economical and production of corn-based ethanol may be encouraged for other reasons, such as bolstering domestic agricultural markets and building the market for biofuels (see NRC, 2009b). Similar concerns have been raised about battery- and hydrogen-powered vehicles, especially if the feedstock used to make the hydrogen or electricity that charges the batteries comes from GHG-intensive energy sources. In addition, the production of alternative fuel sources may carry unintended negative consequences for other resources, environmental concerns, trade issues, and human security issues, and the trade-offs and life-cycle costs and benefits of these alternatives have to be evaluated (see Chapter 14).

IMPACTS OF CLIMATE CHANGE ON TRANSPORTATION

In 2008 the Transportation Research Board released a report titled Potential Impacts of Climate Change on US Transportation (NRC, 2008g). The report assesses some of the possible impacts of climate change on various transportation systems, with an emphasis on four categories of climate change impacts: increases in very hot days and heat waves, increases in arctic temperatures, rising sea levels, and increases in hurricane intensity. These impacts are summarized in Table 13.2. While not an exhaustive or quantitative list, this analysis provides an overview of the types of impacts that could be experienced in the transportation sector.

Suggested Citation:"13 Transportation." National Research Council. 2010. Advancing the Science of Climate Change. Washington, DC: The National Academies Press. doi: 10.17226/12782.
×

TABLE 13.2 Potential Climate Change Impacts on Transportation

Potential Change in Climate

Impact on Operations

Impact on Infrastructure

Increases in very hot days and heat waves

Impact on liftoff load limits at high-altitude or hot-weather airports, resulting in flight cancellations or limits on payload or both

Limits on periods of construction activity due to health and safety concerns

Thermal expansion on bridge joints and paved surfaces

Concerns regarding pavement integrity, traffic-related rutting, and migration of liquid asphalt

Rail-track deformities

Increases in Arctic temperatures

Longer ocean transport season and more ice-free ports in northern regions

Possible availability of a northern sea route or a northwest passage

Thawing of permafrost, causing subsistence of roads, railbeds, bridge supports, pipelines, and runway foundations

Shorter season for ice roads

Rising sea levels, combined with storm surges

More frequent interruptions to coastal and low-lying roadway travel rail service due to storm surges

More severe storm surges, requiring evacuation or changes in development patterns

Potential closure or restrictions at airports that lie in coastal zones, affecting service to the highest-density U.S. population centers

Inundation of roads, rail lines, and airport runways in coastal areas

More frequent or severe flooding of underground tunnels and low-lying infrastructure

Erosion of road base supports

Reduced clearance under bridges

Change in harbor and port facilities to accommodate higher tides and storm surges

Increases in intense precipitation events

Increase in weather-related delays and traffic disruptions

Increased flooding of evacuation routes

Increase in airline delays due to convective weather

Increase in flooding of roadways, rail lines, runways, and subterranean tunnels

Increase in road washout, damages to railbed support structures, and landslides and mudslides that damage roads and tracks

Increases in scouring of pipeline roadbeds and damage to pipelines

More intense or more frequent hurricanes

More frequent interruptions in air service

More frequent and potentially more extensive emergency evacuations

More debris on roads and rail lines, interrupting travel and shipping

Greater probability of infrastructure failures

Increased threat to stability of bridge decks

Impacts on harbor infrastructure from wave damage and storm surges

SOURCE: NRC (2008g).

Suggested Citation:"13 Transportation." National Research Council. 2010. Advancing the Science of Climate Change. Washington, DC: The National Academies Press. doi: 10.17226/12782.
×

SCIENCE TO SUPPORT ADAPTING TO CLIMATE CHANGE IN THE TRANSPORTATION SECTOR

The report Potential Impacts of Climate Change on US Transportation (NRC, 2008g) identifies a number of potential engineering options for strengthening and protecting transportation facilities such as bridges, ports, roads, and railroads from coastal storms and flooding as a short-term adaptation measure. The report also identifies a number of research needs and potential actions that will be necessary to support climate-related decision making in the transportation sector, including improved communication processes among transportation professionals, climate scientists, and other relevant scientific disciplines; a clearinghouse for transportation-relevant information on climate change; developing climate data and decision-support tools that incorporate the needs of transportation decision makers; developing and implementing monitoring technologies for major transportation facilities; developing mechanisms for sharing best practices; reevaluation of existing and development of new design standards; and creating a federal-level interagency working group focused on adaptation. Many of these initiatives would require federal action, while others would require action by professional organizations and university researchers.


Potential options and considerations for adaptation to climate change in the transportation sector are discussed in the companion report Adapting to the Impacts of Climate Change (NRC, 2010a). The report also notes that planning for adaptation in the transportation sector will require new modeling tools, the establishment of standards consistent with future climate risks (as opposed to those based on historical conditions), and improved communication between the climate science and transportation decision making communities.

RESEARCH NEEDS

Improve understanding of how transportation contributes to climate change. As society moves from vehicles propelled by internal combustion engines using petroleum-based fuels to vehicles using more varied types of propulsion systems and fuels, it will be increasingly important to understand the full life cycle of GHG emissions generated by various vehicle and fuel combinations, including the emissions and energy implications associated with vehicle production. The move from tank-to-wheels to well-to-wheels emissions analyses represents an important step in this understanding. For example, our understanding of the true life-cycle emissions from various biofuels is still incomplete, as is understanding of trade-offs and consequences for other resources and environmental issues. Also, the construction and maintenance of trans-

Suggested Citation:"13 Transportation." National Research Council. 2010. Advancing the Science of Climate Change. Washington, DC: The National Academies Press. doi: 10.17226/12782.
×

portation infrastructure is an additional source of GHG emissions, but little is known of the relative emissions associated with different transportation modes or infrastructure types even as large investments are being planned for constructing new systems such as high-speed rail.


Improve understanding of what controls the volume of transportation activity. While there is potential for tempering growth in vehicle miles traveled by increasing land development densities, a recent NRC report (NRC, 2009e) found a lack of sound research on the potential for increasing metropolitan densities to affect travel, energy use, and emissions. Further research is needed on the relationships among household location, workplace location, trip-making activity, and light-duty vehicle travel, and on the effectiveness of various policy mechanisms to influence these relationships. Technological improvements such as online shopping, telecommuting, and virtual conferencing also have the potential to significantly reduce total transportation activity, but further research is needed on how to facilitate and promote expanded use of these technologies (and this research will require data on current levels of usage of these technologies—an example of a climate-relevant observation that falls outside the rubric of traditional climate observations).


Conduct research on the most promising strategies for encouraging the use of less fuel-intensive modes of transportation. Any increase in fuel prices, whether a result of climate or energy policy or other factors, can be expected to promote a shift toward more fuel-efficient modes of transportation, both at the personal level and through major private-sector transportation providers. However, as noted earlier in this chapter, there are a variety of strategies that might be employed to encourage less energy-intensive modes. As with overall reductions in travel volume, additional research is needed on the factors that influence travel mode choice—understanding how, for example, intermodal service can be made more attractive to shippers or public transit more attractive to passengers. Research is also needed on potential large-scale changes in the built environment and infrastructure that would encourage less energy-intensive modes, and the policy mechanisms that might be used to facilitate these changes.


Continue efforts to improve energy efficiency. In addition to the continued improvement of more efficient vehicle designs and propulsion systems, there could potentially be major energy efficiency gains in other transportation modes. For example, there is room for improvement in medium- and heavy-duty truck aerodynamics and means of reducing idling (NRC, 2010i). Ultralight materials such as carbon fiber are already beginning to see widespread application in new commercial aircraft (e.g., the Boeing 787), and additional research by both public and private sectors may help ac-

Suggested Citation:"13 Transportation." National Research Council. 2010. Advancing the Science of Climate Change. Washington, DC: The National Academies Press. doi: 10.17226/12782.
×

celerate this and other efficiency improvements, such as “blended wings” and open fan propulsion systems.


In addition to technology development and deployment, there is a wide range of research needed on human behavior as it relates to transportation use and on the best policies for influencing both technology development and human behavior. For example, there are behavioral changes that increase the efficiency of existing vehicles, such as maintaining properly inflated tires, but we lack basic data on the prevalence of these behaviors as well as on how they might be effectively encouraged. Further research is also needed on factors that encourage the purchase of more efficient vehicles—fuel prices are certainly one factor, but, as with the adoption of any new technology, prices are only part of the explanation and a more nuanced understanding might lead to the design of effective policies. There may actually be substantial proprietary information on what influences consumer choice and technology adoption, but there is little open literature on this subject or on how policies, programs, and institutions might influence vehicle or mode choice. Finally, the history of U.S. fuel economy over the last 35 years, where efficiency improvements were offset by consumer demands for larger, more powerful vehicles (with little resulting fuel consumption penalty, because efficiency had increased), suggests a need for better understanding of how to design regulatory policies that have the intended results.


Accelerate the development and deployment of alternative propulsion systems, fuels, and supporting infrastructure. New, less carbon-intensive fuels and alternative propulsion systems will ultimately be needed to make major reductions in GHG emissions from the transportation sector. The two primary candidates for replacing internal combusion engines are batteries and hydrogen fuel cells, and major technological advances are still needed to make these methods competitive with current propulsion systems. Moreover, while these alternative propulsion systems would reduce petroleum consumption, they will only reduce GHG emissions significantly if the needed electricity or hydrogen is produced using low-emissions fuels and processes. As discussed in the companion report Limiting the Magnitude of Climate Change (NRC, 2010c) and elsewhere, widespread adoption of these technologies also implies a major restructuring of the nation’s transportation infrastructure, and reasearch will play an important role in optimizing that design.


Advance understanding of how climate change will affect transportation systems and how to reduce the magnitude of these impacts. One of the most difficult tasks for transportation planners in addressing climate change is obtaining relevant information in the form they need for planning and design (NRC, 2008g). Improved regional-scale climate information is needed, but so is a better understanding of how

Suggested Citation:"13 Transportation." National Research Council. 2010. Advancing the Science of Climate Change. Washington, DC: The National Academies Press. doi: 10.17226/12782.
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projected climate changes, such as changes in temperature and precipitation, will affect different kinds of infrastructure in different regions, and improved methods of providing information to transportation decision makers. Practical research on adaptation measures, both for current transportation systems and for the design of new systems and infrastructure, is needed to better inform all kinds of transportation-related decisions as climatic conditions continue to exit the range of past experience.

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Climate change is occurring, is caused largely by human activities, and poses significant risks for--and in many cases is already affecting--a broad range of human and natural systems. The compelling case for these conclusions is provided in Advancing the Science of Climate Change, part of a congressionally requested suite of studies known as America's Climate Choices. While noting that there is always more to learn and that the scientific process is never closed, the book shows that hypotheses about climate change are supported by multiple lines of evidence and have stood firm in the face of serious debate and careful evaluation of alternative explanations.

As decision makers respond to these risks, the nation's scientific enterprise can contribute through research that improves understanding of the causes and consequences of climate change and also is useful to decision makers at the local, regional, national, and international levels. The book identifies decisions being made in 12 sectors, ranging from agriculture to transportation, to identify decisions being made in response to climate change.

Advancing the Science of Climate Change calls for a single federal entity or program to coordinate a national, multidisciplinary research effort aimed at improving both understanding and responses to climate change. Seven cross-cutting research themes are identified to support this scientific enterprise. In addition, leaders of federal climate research should redouble efforts to deploy a comprehensive climate observing system, improve climate models and other analytical tools, invest in human capital, and improve linkages between research and decisions by forming partnerships with action-oriented programs.

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