Scientists generally agree that increasing concentrations of greenhouse gases in the atmosphere are causing changes in the earth's climate. Greenhouse gases include water vapor, carbon dioxide, methane, nitrous oxide, and halocarbons. The timing, magnitude, and consequences of this temperature increase are not fully understood or agreed on, but it could lead to marked changes in sea level, precipitation, and storm patterns.

In response to the potential threat of global warming, a series of international conferences have been held to develop a plan of action. The 1997 Kyoto Protocol illustrates the kinds of actions many scientists believe are necessary to significantly slow the rate of climate change. The Protocol, if ratified, would set a U.S. target of 7 percent reduction in greenhouse gas emissions from 1990 emission levels.

Finding ways to reduce emissions is a major challenge, particularly because emissions are projected to increase substantially over the next decades. Atmospheric carbon dioxide is reportedly increasing by about 1/2 percent per year and will increase faster as automobile use continues to increase worldwide.

Emissions from surface transportation in the United States contribute significantly to global emissions of atmospheric carbon dioxide, the most common of the greenhouse gases except for water vapor. The United States accounts for about 25 percent of the world's carbon dioxide emissions, and U.S. surface transportation accounts for about 25 percent of that, or roughly 6 percent of world carbon dioxide emissions.

Three basic transportation strategies are available for reducing emissions of greenhouse gases: technological changes to vehicles and fuels, road and vehicle operational improvements, and reductions in vehicular travel. Public transportation can contribute to reducing emissions technologically by introducing low-emission technologies. It can also help reduce vehicle emissions if public transportation vehicle scheduling and routing are optimized and given priority in traffic. Most importantly, transit and alternatives to single-occupant driving can contribute to reducing greenhouse gas emissions by providing improved alternatives to automobile travel. It is critical, therefore, that Americans, beginning at a very young age, understand the air-quality and associated environmental impacts of their trip-making choices.

There is a growing belief that something must be done to mitigate global warming. This is playing out locally as support for sustainable communities. Examples include the Maryland Smart Growth Initiatives, the Portland 2040 Plan, Sustainable San Francisco, Sustainable Toronto, Sustainable Seattle, and the Bay Area Alliance for Sustainable Development. The President's Council for Sustainable Development issued a report, "Sustainable America-- A New Consensus," which argues that sustainable development can only be achieved by creating sustainable communities.

Plans for sustainable development generally address transportation, housing, and employment and their effects on air quality, energy use, economic prosperity, and social equity. It is unclear how (or whether) communities can address climate change in this context. Recommended policies often include transit-oriented development; mixed-use developments; urban infill; brownfields redevelopment; encouragement of more transit use, biking, and walking; and better transportation information. These policies have other direct and indirect benefits for the community, many of which are more easily explained and quantified than is climate change.

While there is no agreement on how to reduce greenhouse gas emissions, there is agreement that multiple strategies and coordinated policies will be required to achieve measurable results. Public transit agencies have an opportunity to support sustainable communities and address greenhouse gas emissions.

The objective of this project is to identify opportunities for public transportation services, and related sustainable transportation strategies, to aid in the reduction of greenhouse gas emissions from the transportation sector. The project will develop an easy to understand method to measure the effects of personal travel decisions on global warming and will identify strategies to increase public awareness of the impacts of travel decisions on global warming.

Status: The research project for the initial contract is complete and the final report was published in October 2003. Additional funding of $100,000 was allocated to this project in October 2002 by the TOPS Committee to conduct additional research under TCRP Project H-21A. The final report from Project H-21A was published as well.

Product Availability: TCRP Report 93 presents the findings Project H-21. The Travel Matters website also presents the findings of Project H-21, but it includes two online calculators that track greenhouse gas emissions for individuals or transit fleets and a series of geographic information system maps illustrating the correlation between land use, auto use, and carbon dioxide emissions. TCRP Web Document 26 presents the findings of Project H-21A and includes teaching materials.