"Transport accounts for about 14 percent of global GHG emissions, making it a major contributor to global climate change. This is equivalent to 18 percent of global CO2 emissions from energy-related sources."
Source: Baumert, Kevin A., Timothy Herzog, and Jonathan Pershing. "Navigating the Numbers: Greenhouse Gas Data and International Climate Policy." World Resources Institute (2005): 63. http://www.wri.org/publication/navigating-numbers