Transportation is responsible for 25% of global greenhouse gas emissions, not including emissions from the associated petroleum refining, automotive manufacturing, or road-building. Road transport is the dominant source of transportation-related greenhouse gas emissions, accounting for 80% of the total (UNEP, 2005). Airplanes are responsible for an additional 13%, with the remainder from ship and rail transport (UNEP, 2002). The contributions from transportation are also rising faster than in any other sector, and rapid motorization in the form of more cars and trucks is the principal cause (Sperling & Salon, 2002). Housing and workplaces moving to disperse suburban areas, increased affordability of car usage, large public subsidization of auto infrastructure, and declining familiarity with other forms of transport have resulted, in the U.S., in (Forman, 2003):
• decreasing vehicle occupancy: from 1.18 occupants per vehicle in 1970 to 1.09 in 1990
• lengthening of the average commute: from 13.7km in 1983 to 18.7km in 1995
• increasing numbers of vehicles per household: 1.5 vehicles per household in the U.S.
In Europe, which is generally ahead of North America in sustainable transportation, total distance travelled by bicycle and foot declined by some 75% from 1951 to 1993 (OECD, 2002). It is fair to say that during this time, private automobiles gained complete dominance of surface transport in the industrialized world. In Canada during the mid 1990's for example, people travelled 392 billion passenger-kilometres per year by car, but only 14 by bus and 1 by rail, while only in two OECD countries (Japan and Korea) did rail or bus travel come close to car transport for passenger-kilometres travelled. (OECD, 1999). It is not surprising then 
that transportation-related greenhouse gas emissions are on the rise: in the U.S. they increased 23% between 1990 and 2004, inspite of technological advances that have improved the efficiency of automobile engines.
An overall trend towards larger and more powerful vehicles including SUVs and minivans has also increased transportation emissions. First gaining widespread popularity in the 80's, sales of light-duty trucks (minivans, SUVs and pickups less than 2.7 tonnes) rapidly grew to rival car sales in the U.S. by 2002 (Forman, 2003). There are however indications of a potential reversal of the trend towards more fuel inefficient vehicles, as evidenced by hybrid vehicles which improve fuel efficiency without sacrificing vehicle size, engine power, or personal luxury (see for example the Ford Escape hybrid SUV). The Smart Car, in addition to being fuel efficient, also reduces the physical space required per vehicle so that more cars could be accommodated by existing infrastructure. Although these do not address the social and infrastructure challenges of urban sprawl and affordable transit, they may prove to be useful transition technologies.
Most of this discussion has focused on North America, because the global distribution of greenhouse gas emissions is extremely skewed: approximately 24% is from the United States, and 50% from the "industrialized" world, North America, Western Europe, Japan, Australia and New Zealand which represent less than 20% of the world's population. However, emissions from developing countries in South America and Asia are growing significantly faster than in these countries (EIA, 2005).
The Potential for Change
Alternatives for reducing the transportation impact on global warming can be broadly classified in two categories: mitigating the impacts of the present car and truck-dominated system, and reducing automobile dependence. Opportunities for mitigation rest primarily on improving vehicle efficiency and the use of alternative fuels such as hydrogen, ethanol, and biodiesel. However, there are significant technical and economic challenges behind the adoption of alternative fuels.
Unless there is widespread demand for a new fuel, inadequate distribution systems will limit its availability and thus limit its practicability for vehicles designed for its use. This is a real problem for biofuels when one considers that producing enough woody or herbaceous-based ethanol to power all the light-duty gasoline engines in the U.S. would require a land base equivalent to 70% of all the cropland in the U.S., and soybean biodiesel production would require 4 times the available cropland, even assuming fossil-fuel based agriculture (MacLean, Lave, Lankey, & Joshi, 2000). Smaller quantities of biofuels could be produced from agricultural waste and blended with conventional fuels, or used in pure form on a much smaller scale than current gasoline usage. Although biofuels reduce net greenhouse gas emissions compared with gasoline and diesel, it yields little to no improvement for other types of air pollution without specially-designed engines (Culaba & Tan, 2003).
The environmental impacts of conversion to hydrogen fuel vary between significant benefit and significant detriment, depending on how the hydrogen is obtained. Excepting nuclear fusion, hydrogen is not a primary energy source like oil or natural gas; it is an energy carrier like electricity which can be produced by splitting water molecules or by reforming other hydrocarbons. Almost all industrially-produced hydrogen today comes from steam reforming of natural gas, the least expensive option. Used in conjunction with hydrogen fuel cells, this method has the potential to reduce greenhouse gas emissions by up to 70% (Pembina 2000). 
However, conversion of all U.S. vehicles, for example, to hydrogen produced from natural gas, would increase the countries natural gas demand by 66% despite already decreasing production in North America (Anthrop, 12/2004). Splitting of water molecules using renewable energy could provide a very low-emission, sustainable option, but with the same problem of scale as biofuels. Hydroelectric is the dominant source of renewable energy in North America, but it would take 15 times the present U.S. capacity of hydroelectric energy to power its automobile fleet (Anthrop, 12/2004). A worst-case environmental scenario for hydrogen production would be electrolysis of water by fossil fuel energy. In the best-case scenario, with high-efficiency natural gas power plants, the global warming effect would be the same as with internal combustion gasoline engines (Pembina 2000). The worst-case scenario, electrolysis powered by burning coal, which is cheap and much more plentiful than oil, could result in a 2.7-fold increase in greenhouse gas emissions over the standard gasoline engine (Anthrop, 12/2004). the U.S. Department of Energy and other critics have dismissed Anthrop's analysis as not relevant to the hydrogen production pathways being pursued by the Department of Energy (DOE, 2006). However, are there any assurances that highly environmentally destructive hydrogen production methods will not be used even if they are cheaper?
The Future
While there are some promising options for sustainable transportation based on alternative fuels, it is clear that their practical adoption will rely on our willingness to pay significantly higher prices for energy, improvements in vehicle efficiency, and a significant shift towards less consumptive modes of transportation. What are the critical transition strategies to move to a more sustainable and diverse transportation system? How feasible is a shift away from the automobile, given our dependency on automobile infrastructure and its cultural stronghold in places? Are there links between automobile use and health? What are the government policies that can move us to a more diverse system? Join us for a fascinating discussion and e-Dialogues over the coming year.
References
Anthrop, D. (12/2004). Hydrogen's Empty Environmental Promise. CATO Institute.
Pembina. (2000). Climate-Friendly Hydrogen Fuel: A Comparison of the Life-Cycle Greenhouse Gas Emissions for Selected Fuel Cell Vehicle Hydrogen Production Systems. (2000). The Pembina Institute for Appropriate Development.
Culaba, A. B., & Tan, R. R. (2003). Life Cycle Assessment of Conventional and Alternative Fuels for Road Vehicles. The American Centre for Life Cycle Assessment.
DOE. (2006). U.S. department of energy comments on CATO institute briefing paper. Retrieved Sept. 25, 2006 from http://www.hydrogen.energy.gov/cato_comments.html
EIA. (2005). Emissions of greenhouse gases in the united states 2004 No. DOE/EIA-0573(2004)) Energy Information Administration.
Forman, R. T. T. (2003). Road ecology: Science and solutions. Washington, D.C.: Island Press.
MacLean, H. L., Lave, L. B., Lankey, R., & Joshi, S. (2000). A life-cycle comparison of alternative automobile fuels. [Electronic version]. Journal of the Air & Waste Management Association, 50, 1769-1779.
OECD. (2002). Report on the OECD conference Environmentally Sustainable Transport (EST): Futures, Strategies, and Best Practice. No. ENV/EPOC/WPNEP/T(2001)8/FINAL). OECD.
OECD. (1999). Indicators for the integration of environmental concerns into transport policies. No. ENV/EPOC/SE(98)1/FINAL). Secretariat of the European Conference of Ministers of Transport (ECMT) and the OECD Working Group on Transport.
Sperling, D. and Salon, D. (2002). Transportation in developing countries: An overview of greenhouse gas reduction strategies. Prepared for the Pew Center on Global Climate Change, Arlington, VA.
UNEP. (2005). Transport. Retrieved Sept. 25, 2006.
UNEP. (2002). Industry as a partner for sustainable development: Aviation. United Nations Environment Programme.
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