from the perspective of transportation’s contribution to global warming through the burning of fossil fuels, which releases carbon dioxide (CO2) and other greenhouse gases (GHGs) into the atmosphere.3 CO2 from combustion of fossil fuels is the largest source of U.S. GHG emissions. In 2005, the most recent year for which data are available, the transportation sector accounted for 33 percent of U.S. CO2 emissions from fossil fuel combustion,4 exceeded only by electricity generation by the electric power industry at 41 percent (USEPA 2007, Table 3-7).5 CO2 emissions from U.S. transportation activities are expected to increase over the next several decades, primarily as a result of growth in road travel, fueled by population and economic growth (World Business Council for Sustainable Development 2004). However, these emissions are likely to be regulated. In a landmark decision in April 2007 (Massachusetts et al., Petitioners, v. Environmental Protection Agency et al.), the U.S. Supreme Court ruled that the U.S. Environmental Protection Agency has the authority under the Clean Air Act to regulate GHG emissions and that CO2 can be construed as an air pollutant under the statute.

Far less attention has been paid to the consequences of potential climate changes for U.S. transportation infrastructure and operations.6,7 For example, projected rising sea levels, flooding, and storm surges could swamp marine terminal facilities, airport runways near coastlines, subway and railroad tunnel entrances, and roads and bridges in low-lying coastal areas.


 CO2 and other GHGs allow sunlight to enter and prevent heat from leaving the earth’s atmosphere—the so-called greenhouse effect, loosely analogous to the operation of a greenhouse window. Higher concentrations of CO2 and other GHGs than occur naturally trap excess heat in the atmosphere and warm the earth’s surface (Staudt et al. 2005).


 Emissions from combustion of both aviation and marine international bunker fuels (i.e., fuel loaded on transport vehicles in the United States but consumed in international operations) are excluded from this total. See Appendix B for a more detailed discussion of the transportation sector’s contribution in general, and the U.S. contribution in particular, to worldwide GHG emissions, particularly emissions of CO2 from fuel combustion.


 The total is larger if emissions from the extraction, production, and distribution of transport fuels and from the manufacture, distribution, and disposal of transportation vehicles are summed to produce a total life-cycle emissions estimate (see the discussion in Appendix B).


 In this report, infrastructure refers to both transportation networks (e.g., road and rail systems) and facilities (e.g., bridges, tunnels, ports).


 In fact, a recent assessment of the U.S. Climate Change Science Program (CCSP) found that the scientific community is not well structured to develop information that would enable adaptive response for any sector in the United States (NRC 2007). The CCSP integrates federal research on climate and global change, as sponsored by 13 federal agencies.

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