Study seeks to unravel the complexity of how air travel compares with other transport modes on climate impact
Thu 7 Oct 2010 – Although CO2 emissions, which remain in the atmosphere for over 100 years, are considered the main culprit for global warming, other short-lived components and compounds contribute significantly to the climate impact of transportation, with the magnitude varying over time. A team of researchers led by Dr Jens Borken-Kleefeld of the International Institute for Applied Systems Analysis in Austria has quantified and compared for the first time the climate impacts of various passenger and freight transportation modes over five, 20 and 50-year periods. Because of the high contribution from contrails and cirrus clouds, aviation has a far higher climate impact in the short term than all other forms of transport but over the longer term car travel has an equal or higher impact per passenger kilometre, reports the study.
The findings are laid out in a paper ‘Specific Climate Impact of Passenger and Freight Transport’ published in the American Chemical Society’s journal Environmental Science & Technology, and co-authored with Terje Berntsen and Jan Fuglestvedt of the Center for International Climate and Environmental Research in Oslo (CICERO).
Scientists have long suspected that aircraft contrails at high altitude have a significant global warming impact in addition to that caused by carbon emissions. Although there is still a high degree of uncertainty over the level of that impact, scientific understanding has considerably improved over the past few years and recent research has suggested the non-CO2 impact is higher than thought previously.
However, each transport sector has non-CO2 emissions that contribute differently to the global climate and the magnitude of the effects varies over time for each mode of transport, says Borken-Kleefeld. Some components emitted, in fact, have a negative forcing – they cool the atmosphere. In maritime shipping, for decades after the emissions, the warming due to CO2 is more than compensated by strong cooling from sulphate aerosols and by increased destruction of the greenhouse gas methane from the effects of NOx emissions. Similarly, for rail transport, sulphur emissions from both electricity generation as well as diesel traction lead to a cooling that outweighs warming for decades. On the other hand, sulphur emissions from ships have harmful effects on human health and are in the process of being regulated, so the cooling effects of shipping will be replaced over the long-term by warming, thereby increasing that sector’s climate impact.
Knowing how to compare the impact of different emissions over time in the various transport sectors is, say the researchers, necessary for a rational approach to evaluating and prioritizing efficient mitigation actions by policy makers.
The researchers have used 2000 as the base year and taken global emissions and transport data for passenger and freight road, rail, air and ship transport for that year. Three distinct time horizons – five, 20 and 50 years – were chosen and the global mean temperature change used as the metric for climate change.
The short time horizon is adequate to capture the impact from clouds and aerosols, and the trade-off between warming from ozone produced initially and subsequent cooling as more methane is oxidized is captured at the intermediate time scale. Finally, the impact from the long-lived gases, essentially CO2, is captured on the scale of several decades.
“A shorter time horizon is relevant to assess the impact in terms of rate of change, while the longer-time horizons relate to the absolute level of change,” says the study. “However, which time horizon is chosen is a political decision, and resulting values depend (sensitively) on it.”
The short-term temperature increase from one year of global air travel is higher than that from one year of road passenger travel, says the study, 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 is strongly enhanced by induced cirrus clouds, ozone and contrails. “Their combined warming in terms of global temperature potential is more than eight times bigger than the warming from aviation emitted CO2 alone,” finds the study.
On a per passenger-km basis, air travel’s specific climate impact becomes four times higher than the impact from car travel at the five-year time horizon. On long-term horizons, when the impact of CO2 prevails, the transport specific climate impact of car travel is larger than air travel on global average. Both are then about three times higher than the impact from bus and rail travel.
For freight transportation, the differences between modes are even more pronounced. Per tonne-km, the transport specific climate impact is by far the lowest for rail and shipping and highest for light trucks and air transport. At the 50-year time horizon, the warming from rail and ship transport is eight and 25 times lower respectively compared to average truck transportation, while transportation in delivery vans or planes resulted in four to seven times higher warming per tonne-km.
The researchers acknowledge the aviation industry’s efforts to increase fuel efficiency and reduce NOx emissions. The study’s calculations are based on an assumption that such measures would reduce CO2 and NOx emissions by 20% each per passenger-km on fleet average.
According to Borken-Kleefeld, the fuel efficiency of air travel would therefore improve from 34 g fuel per passenger-km (pkm), or 4 litres per 100 pkm, as the global average in the year 2000, to 27 g fuel per pkm, or 3.2 litres per 100 pkm. The emission factor for NOx would improve from 13 g NOx per kg fuel to 10.4 g NOx per kg fuel consumed.
However, as long as aviation induced cloud effects remain as high, aviation’s specific climate impact per passenger-km on the shorter time scales would still be two to three times higher than car travel’s impact. If road vehicles were not to reduce their climate impact in the future as well as aircraft, then future air travel could have a somewhat lower impact than average car travel, but still two times higher than travel by bus or rail.
“This once more underlines the importance to address aviation induced cloud effects as the single biggest warming agent from aviation,” say the researchers, who call for more studies into non-CO2 climate warming impacts.