"Full life-cycle" carbon emissions
by A Person | Published on June 7th, 2009, 10:32 pm | Science
One of the problems facing a consumer trying to make good environmental choices is that the necessary information just isn't available.Often the emissions we see are only part of the problem. A recent study compares full life cycle emissions and comes up with some surprising results
True or false: taking the commuter train across Boston results in lower greenhouse gas emissions than travelling the same distance in a jumbo jet. Perhaps surprisingly, the answer is false.
A new study compares the "full life-cycle" emissions generated by 11 different modes of transportation in the US. Unlike previous studies on transport emissions, this one looks beyond what is emitted by different types of car, train, bus or plane while their engines are running and includes emissions from building and maintaining the vehicles and their infrastructure, as well as generating the fuel to run them.
Including these additional sources of pollution more than doubles the greenhouse gas emissions of train travel. The emissions generated by car travel increase by nearly one third when manufacturing and infrastructure are taken into account. In comparison to cars on roads and trains on tracks, air travel requires little infrastructure. As a result, full life-cycle emissions are between 10 and 20 per cent higher than "tailpipe" emissions.
Mikhail Chester and Arpad Horvath of the University of California, Berkeley, included in their calculations data on the "life expectancy" of each component of each mode of transportation, such as the tracks used by a train and the airports used by aircraft.
They calculated the total "travel kilometres" each component allows and how many tonnes of greenhouse gases were emitted to build and maintain each component. This allowed them to calculate the component's emissions per kilometre travelled, for each mode of transport per kilometre for each traveller on board.
Empty seats
Cars emitted more than any other form of transport with the notable exception of off-peak buses, which often carry few passengers. Passengers on the Boston light rail, an electric commuter train, were found to emit as much or marginally more than those on mid-size and large aircraft. This is because 82 per cent of electricity in Massachusetts is generated by burning fossil fuels.
The researchers found that travelling 1 kilometre on a nearly empty bus during off-peak hours emits eight times more per person than taking the same bus at rush hour – suggesting peak-time commuters may suffer, but they do less harm to the environment.
The occupation level of a vehicle is an important but often-overlooked factor, says Chester. "Although mass transit is often touted as more energy efficient than cars, this is not always the case." Buses turned out to be the most sensitive to how full they were – those with only five passengers were less efficient than cars; even large SUVs and pick-up trucks.
