It is important to take all aspects of production, use, and disposal into consideration when calculating the sustainability of transportation options. Some sustainable transportation options may require energy from heavily polluting sources, even though the option itself emits very small amounts of pollutants. Similarly, some options may emit fewer greenhouse gasses during their usable life, but then have serious environmental implications for disposal.
Estimates of greenhouse gas emissions from vehicle exhaust do not include additional "lifecycle" emissions related to transportation, such as the extraction and refining of fuel and the manufacture of vehicles, which are also a significant source of domestic and international GHG emissions. http://climate.dot.gov/about/transportations-role/overview.html
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