There is now little doubt about global climate change and its consequences. Anthropogenic greenhouse gases are associated with most of the observed temperature increase since the middle of the 20th century, and natural phenomena such as volcanoes and solar variation have had only a small warming effect anterior and a small cooling effect posterior to 1950 (Intergovernmental Panel on Climate Change, 2007b). Climate change has become an issue that has aroused much public concern around the world in recent years. In 2006 the world transport sector emitted 6.5 Giga tones of carbon dioxide (CO_2), approximately 23 of CO_2 emissions. According to the Organisation for Economic Cooperation and Development (2008) air transport could, based on a "business as usual scenario," grow from 14.8 in 2010 to 23.0 of transportation CO_2 emissions in 2050. This is partially due to more readily available environmentally friendly energy in the future for ground-based transportation compared to air transport, as well as faster growth rate of air transport compared to other modes.
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