Planes,
trains, & automobiles

Traveling by plane, train, automobile, bus, or motorbike: Which is the least climate-friendly? According to recent research, it is either the car or the plane, but it depends on what air pollutants are considered and the length of time since the journey.

Carbon dioxide emissions, which remain in the atmosphere for over 100 years, are responsible for warming much of the atmosphere. Yet different modes of transport emit a range of other air pollutants that can dramatically enhance or diminish the overall climate impact from a journey’s CO2 emissions. The lifetime of many of these substances ranges from hours to decades, which means that the magnitude of the climate impact also varies substantially over time.

In comparisons of air and car travel for the same distance and average occupancy level, both modes of transport have an equally adverse climate impact when only CO2 is considered. However, include the other emissions, and air travel increases global temperatures four times more than car travel in the first years after the journey. Flights at high altitudes have a disproportionately high impact on ozone and clouds which, in turn, has a significant effect on global warming. Yet the larger impact is short-lived. Twenty years after the journey by air is made, the climate impact of flying has fallen to almost half that of driving a car (see chart).

The research was conducted as part of an EU-funded project known as called QUANTIFY and brought together leading scientists from more than 20 countries, including the USA, India, and China. IIASA researchers developed a new, differentiated emission inventory for the global transport sector for QUANTIFY, and have recently published results with Norwegian research partners in the journal Environmental Science and Technology.

Traveling by bus or train are the most climate-friendly modes of transport taking into account the multiple effects of all air pollutants and greenhouse gases. The researchers calculate that passenger trains and buses cause four to five times less impact than automobile travel for every kilometer a passenger travels.

Which Journey Warms the Most. A comparison of how passenger travel by different modes affects mean global temperatures 5, 20, and 50 years after the journey. The bars also compare the warming effect from only carbon dioxide and the combined effect of all air pollutants and greenhouse gas emissions. Negative values mean cooling, positive values warming.

According to Dr. Jens Borken-Kleefeld, lead author of the study: “When it comes to freight transport, moving goods by planes will increase global temperatures between 7 and 35 times more than moving the same goods the same distance in an average truck. On the other hand, shipping exerts 25 times less warming in the long run, and even cools on shorter time scales.”

Ships contribute to global warming by emitting carbon dioxide, ozone, and soot. Currently they also emit relatively large amounts of sulfur dioxide which form sulfate particles in the atmosphere that reflect solar radiation back into space and so help cool the planet. This effect is so great that global shipping actually counteracts some of the temperature increases caused by global passenger travel. However, while the effects from the air pollutants will decline over time, the warming from the long-lived CO2 will accumulate and prevail sooner or later.

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

Last edited: 28 August 2012