The study, published in the journal Environmental Science & Technology by a team of researchers from Austria and Norway, compares the impacts on global warming of different means of transport. The researchers use, for the first time, a suite of climate chemistry models to consider the climate effects of all long- and short-lived gases, aerosols and cloud effects, not just carbon dioxide, resulting from transport worldwide.

In the long run the global temperature increase from a car trip will on average be higher than from a plane journey of the same distance. However, in the first years after the journey, air travel increases global temperatures four times more than car travel. Passenger trains and buses cause four to five times less impact than automobile travel for every kilometer a passenger travels. The findings prove robust despite the scientific uncertainties in understanding the earth's climate system.

"As planes fly at high altitudes, their impact on ozone and clouds is disproportionately high, though short lived. Although the exact magnitude is uncertain, the net effect is a strong, short-term, temperature increase," explains IIASA's Dr Jens Borken-Kleefeld, lead author of the study. "Car travel emits more carbon dioxide than air travel per passenger kilometer. As carbon dioxide remains in the atmosphere longer than the other gases, cars have a more harmful impact on climate change in the long term."

The research also showed that when it comes to freight transport, moving goods by planes will increase global temperatures between 7 to 35 times more than moving the same goods the same distance in an average truck. Shipping on the contrary exerts 25 times less warming in the long run, and even cools on shorter time scales.

"Ships contribute to global warming through carbon dioxide, ozone and soot. Currently they also emit relatively large amounts of sulfur dioxide which forms sulfate particles in the atmosphere. Those particles cool the planet by reflecting solar radiation back into space," says co-author Dr Jan Fuglestvedt from CICERO. "In the first decades after a shipment, the cooling effect more than offsets the warming. And because of the large volumes of goods traded by ship, global trade actually counteracts some of the temperature increases caused by global passenger travel. However, in the long term all means of motorized transport add to global warming."

The study concluded that as climate change acts at various time scales, it is important to have policies to reduce both the air pollutants that have strong, short-term impacts and the long-lived gases that lead to long-term warming. In addition, Dr Borken-Kleefeld argues: "A comprehensive strategy to tackle climate change caused by the transport sector is actually to minimize the demand for transport."