Stuber, N., P. Forster, G. Rädel, and K. Shine. 2006. The Importance of the Diurnal and Annual Cycle of Air Traffic for Contrail Radiative Forcing. Nature, Vol. 441, No. 7095, June 15, pp. 864–867.
UN. 1999. The World at Six Billion. Population Division, Department of Economic and Social Affairs, Oct. 12. www.un.org/esa/population/publications.sixbillion/sixbillion.htm. Accessed Jan. 30, 2008.
UN. 2003. World Urbanization Prospects: The 2003 Revision. Population Division, Department of Economic and Social Affairs. www.un.org/esa/population/publications/wup2003/2003wup.htm. Accessed Jan. 30, 2008.
UN. 2005. World Population Prospects: The 2004 Projection. Population Division, Department of Economic and Social Affairs, Feb.
UNCTAD. 2005. Review of Maritime Transport 2005. www.unctad.org/Templates/WebFlyer.asp?intItemID=3588&lang=1. Accessed Jan. 30, 2008.
U.S. Energy Information Administration. 2004. Emissions of Greenhouse Gases in the United States. DOE/EIA-0573. U.S. Department of Energy. www.eia.doe.gov/oiaf/1605/archive/gg05rpt/index.html. Accessed Jan. 30, 2008.
USEPA. 2005. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2003. EPA-430-R-05-003. Washington, D.C., April 15.
Weiss, M. A., J. B. Heywood, E. M. Drake, A. Schafer, and F. F. AuYeung. 2000. On the Road in 2020: A Life-Cycle Analysis of New Automobile Technologies. Energy Laboratory Report MIT EL 00-003. Cambridge, Mass., Oct.
World Business Council for Sustainable Development. 2004. Mobility 2030: Meeting the Challenges to Sustainability. Sustainable Mobility Project, Geneva, Switzerland. www.wbcsd.org/web/mobilitypubs.htm. Accessed Jan. 30, 2008.