<P> Aviation emissions vary based on length of flight . For covering long distances, longer flights are a better investment of the high energy costs of take - off and landing than very short flights, yet by nature of their length inevitably use much more energy . CO emissions from air travel range from . 24 kg CO per passenger mile (. 15 kg / km per passenger) for short flights down to . 18 kg CO per passenger mile (. 11 kg / km per passenger) for long flights . Researchers have been raising concern about the globally increasing hypermobility of society, involving frequent and often long distance air travel and the resulting environmental and climate impacts . This threatens to overcome gains made in the efficiency of aircraft and their operations . Climate scientist Kevin Anderson raised concern about the growing effect of air transport on the climate in a paper (13) and a presentation (14) in 2008 . He has pointed out that even at a reduced annual rate of increase in UK passenger air travel and with the government's targeted emissions reductions in other energy use sectors, by 2030 aviation would be causing 70% of the UK's allowable CO emissions . </P> <P> Worse, aircraft emissions at stratospheric altitudes have a greater contribution to radiative forcing than do emissions at sea level, due to effects several greenhouses gases in the emissions, apart from CO . The other GHGs include methane (CH), NO which leads to ozone (O), and water vapor . Overall, in 2005 the radiative forcing caused by aviation amounted to 4.9% of all human - caused radiative forcing on Earth's heat balance . </P> <P> Unleaded gasoline has 8.91 kg and diesel has 10.15 kg of CO per gallon . CO emissions originating from ethanol are disregarded by international agreements however so gasoline containing 10% ethanol would only be considered to produce 8.02 kg of CO per gallon . The average fuel economy for cars sold in the US 2005 was about 25.2 MPG giving around 0.35 kg of CO per mile . The Department of Transportation's MOBILE 6.2 model, used by regional governments to model air quality, uses a fleet average (all cars, old and new) of 20.3 mpg giving around 0.44 kg of CO per mile . </P> <P> In Europe, the European Commission enforced that from 2015 all new cars registered shall not emit more than an average of 0.130 kg of CO2 per kilometer (kg CO2 / km). The target is that by 2021 the average emissions for all new cars is 0.095 kg of CO2 per kilometre . </P>

Compare and contrast the environmental impacts of different modes of transport