Driving a car increases global temperatures in the long term more than making the same long-distance journey by air, according to a new study.
However, in the short run travelling by air has a larger adverse climate impact because airplanes strongly affect short-lived warming processes at high altitudes.
In the study, Jens Borken-Kleefeld and colleagues at the International Institute for Applied Systems Analysis in Laxenburg, Austria, compare 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.
They concluded that in the long run the global temperature increase from a car trip will be on average 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 mile 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 Dr. Jens Borken-Kleefeld, lead author of the study.
"Car travel emits more carbon dioxide than air travel per passenger mile. 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 report states: “The passenger transport volume was about 30 trillion passenger-kilometres globally in the year 2000. Car travel accounted for 51% of the total volume, buses and coaches for 20%, air travel for 16%, rail for 7%, and motorized 2- and 3-wheelers for about 6%.
“The travel is powered to 98% by fossil fuels; regenerative fuels, mostly ethanol, nuclear and hydro power, contribute the remainder. The emissions from the travel in this year alone will lead to an average increase of the surface temperature of 1.5 mK 50 years later.
“The relative contribution of the modes is almost equal to their share in fuel consumption on this long time horizon as the temperature response is primarily due to the forcing from the CO2. The shorter the time horizon the larger becomes the role of short-lived compounds. Only 5 years after the emission, the global travel of the year 2000 with car, planes, bus, 2- and 3-wheelers, and rail have respectively contributed 1.75 mK, 2.1 mK, 0.35 mK, 0.2 mK, and −0.1 mK to a total surface temperature change of 3.9 mK.
“Thus, the short-term temperature increase from one year of global air travel is higher than that from one year of road passenger travel, although passenger aviation is more than a factor of 3 and 4.5 lower in terms of transport volume and fuel consumption, respectively.
“The short-term aviation impact gets strongly enhanced by induced cirrus clouds, ozone, and contrails. Their combined warming in terms of GTP5 is more than eight times bigger than the warming from aviation emitted CO2 alone.
“The impact from car travel is increased by the warming due to ozone and black aerosols (BC) that more than outweigh cooling effects from sulfate aerosols and methane destroyed as a consequence of NOx emissions.
“These same effects also enhance the shorter-term warming from bus and coach travel. As both are essentially diesel powered, the contribution from both black carbon and sulfate aerosols are proportionally higher.
“With a high share of two-stroke engines notably in Asia, the global fleet of motorized two- and three-wheelers emitted relatively high amounts of CO and unburnt HC. Therefore their short-term climate impact is strongly enhanced by a high warming contribution from ozone.
“For rail travel however, the warming due to carbon emissions, ozone, and aerosols is more than offset by cooling from sulfate aerosols on short-time horizons. High SO2 emissions notably from the electricity produced in coal fired power plants lead to a strong cooling from sulfate aerosols.”
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