Today, passenger transport worldwide is responsible for almost 15% of anthropogenic energy-related emissions of carbon dioxide (CO2), the most abundant greenhouse gas. If the strong forces that generate travel demand and concomitant greenhouse gas emissions continue, world passenger traffic volume may rise more than fourfold over the 1990 level by 2050. During the same period, carbon dioxide emissions due to passenger transport are expected to multiply by a factor of more than 3, ultimately accounting for 2.7 billion tons of carbon in 2050. Based on these projections, the present study evaluates a range of emission-reduction options. Among these, technological measures offer the greatest potential and are key to drastically reducing carbon dioxide emissions. Radical fuel efficiency improvements in the world's automobile fleet--along with continuations of past trends in the energy intensity of other passenger transport modes--could curtail the projected 2050 baseline emissions level by about 40%. Simultaneous substitution of oil products by natural gas could reduce CO2 emissions by another 25% and ultimately lead to emission stabilization at 1.2 billion tons of carbon in 2050; any further significant reduction in CO2 emissions would require the large-scale introduction of zero-carbon fuels. Although the CO2-reduction potential of transportation systems management measures is comparatively limited, such measures are needed to abate other transport sector externalities such as accidents, noise, and traffic congestion.

Copyright © 2008. National Academy of Sciences