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TCRP 93 - Travel Matters: Mitigating Climate Change with Sustainable Surface Transportation

A majority of scientists now agree that the Earth’s climate is warming, as indicated by a rise in the average surface temperature of the earth. Positive climate change (warming) is thought to be the result of human-generated emissions, principally of carbon dioxide (CO2). Carbon dioxide, like the greenhouse gases methane (CH4) and nitrous oxide (N2O), allows solar radiation to pass through the atmosphere, but prevents surface radiation from escaping to outer space, effectively “trapping” it. This process leads to an overall increase in surface temperature. The observational evidence for positive climate change is circumstantial but extensive: direct measurement has established that atmospheric carbon dioxide levels have increased since the industrial revolution and the related surge in fossil fuel consumption. The gas physics behind the “heat-trapping” greenhouse effect is not disputed, and the man-made exacerbation of the greenhouse effect is considered to be very likely. The ultimate effects, however, remain uncertain.

The premise of the report, based on a review of climate change science summarized in Chapter 2, is that enough is now known, despite the uncertainties of measurement and forecasting, to warrant prudent actions to moderate or reduce emissions of greenhouse gases (GHGs). Much of what can be done in this regard will have the multiple effects of improving air quality and human physical health, as well as increasing fuel efficiency. Although improving personal and transit vehicle fuel efficiency is one tactic in any future GHG reduction strategy, another equally important tactic involves expanding the overall share of transit in U.S. transportation. This report concentrates on such transit-related strategies.