Mikhail Chester and colleagues calculated the amount of energy required, and the quantity of emissions produced, over the entire lifetimes of automobiles (both buses and cars), trains and aircraft. The analysis included the energy, greenhouse-gas emissions and the production of air pollutants – such as sulphur dioxide, nitrogen oxides and carbon monoxide – associated with vehicle manufacture, the transport infrastructure required, fuel production and the supply chain, as well as actual operation of the vehicle itself.

"Including life cycle-component inventories results in around a 40% energy and greenhouse-gas increase over direct vehicle operation for autos, while for rail there is about a doubling," Chester told environmentalresearchweb. For aircraft the increase is around 30%.

Chester says that it is all too common to evaluate transport emissions based simply on the amount of fuel that vehicles consume. Often we see rankings based on these numbers, and global-warming mitigation schemes are subsequently based on such figures. But we need to analyse a vehicle's life-cycle components to evaluate properly how much energy it consumes and thus the amount of emissions that it produces, he explained.

The researchers say that the amount of occupancy can easily change the relative performance of the transport modes.

The work will be critical for policy and decision makers because the researchers have also analysed which life-cycle components have the most impact on the environment. "While policy has often focused on the vehicle's tailpipe emissions (for example, in the US Corporate Average Fuel Economy Standards, and removal of lead from petrol), our study shows that you may not want to focus on this component but somewhere else in the mode's life cycle," said Chester.

One of the best examples of such a strategic error is that of sulphur emissions, he stressed. Much has been done to remove sulphur from petrol and diesel fuels in recent years but, according to the new study, the bulk of sulphur emissions for transport actually comes from the electricity needed to manufacture a vehicle. This is particularly true for vehicles that are fabricated using electricity that is produced in coal-powered plants.

The team has already applied its inventory to several major metropolitan regions in the US. "We have also gone on to evaluate the life-cycle environmental impacts of the proposed California high-speed rail," revealed Chester.

The researchers used models that calculate the amount of electricity needed to produce the components of the three modes of transport. From this, they were able to determine the amount of polluting emissions created during different manufacturing processes. They then compared these values with the emissions produced by the same vehicles when they are on the road – the classical tailpipe scenario. The data employed were taken from previous literature, such as government reports, and more detailed modelling software like the US Environmental Protection Agency's Mobile Software for vehicle-operation emissions and the US Federal Aviation Administration's Emission Data Modelling Software for aircraft emissions.

The work is published in Environmental Research Letters.