Study finds climate impact of long distance trip can vary by factor of 10 depending upon mode, efficiency and occupancy
A team from Austria and Norway has found that the climate impact from a long-distance trip (500–1,000 km, or 310–621 miles) can easily vary by a factor of 10 per passenger depending on mode choice, vehicle efficiency, and occupancy. Among the findings of the study, published in the ACS journal Environmental Science & Technology, is that a car’s fuel efficiency and occupancy are central to whether the impact from a trip is as high as from air travel or as low as from train travel.
With only one passenger in a car, corresponding to 20−25% occupancy, the climate impact is at the level of an average air trip, whereas a car with three or more passengers, 60% occupancy or more, it is at the low level of average trains or coaches. A notable exception is for the small diesel car; with two passengers ( i.e., 50% occupancy), the specific climate impact is lower than for an average train or bus trip.
In their study, Borken-Kleefeld et al. compared the specific climate impact of long-distance car travel with bus, train, or air trips. They accounted for both CO2 emissions and emissions of ozone precursors (NOx, VOC, CO) and aerosols (BC or black carbon, OC, SO2) as well as cloud effects (aviation-induced cirrus clouds and contrails)—i.e., shorter-lived climate forcers (SLCFs). This particularly affects the ranking of aircraft’s climate impact relative to other modes, they noted.
They used vehicle technical data, occupancy rates, and the electricity production mix from Germany, representing about one-quarter of European travel, and calculated the specific impact for the Global Warming Potential (GWP) and the Global Temperature Change Potential (GTP), considering time horizons between 20 and 100 years. They then compared these results with results accounting only for CO2 emissions.
Among the other findings of the study:
The impact from short-lived climate forcers (SLCFs) and notably of contrails and cirrus clouds is particularly strong for aircraft; as a result, their specific climate impacts are strongly influenced by the choice of metric and time horizon. Non-CO2 climate forcers increase impacts by 160% over the CO2-only case when using GWP20, and by 40% even at the longer time scale covered by GWP100. Cirrus clouds and contrails contribute more than four-fifths of extra warming.
For modern cars, SLCFs slightly reduce the CO2-only impact. However, in the end it is the fuel efficiency (and the carbon intensity of the fuel) that determine the climate impact of modern gasoline and diesel cars as exhaust after-treatment eliminates much of the SLCFs.
For the next generation of private cars the emissions of SLCFs are cut to such low levels that their climate impacts is completely dominated by the CO2 emissions—i.e., fuel consumption.
SLCFs have a substantial influence on the specific climate impact of diesel trains: CO2-only impacts are reduced by as much as 34% or increased by up to 22% for GTP20 and GWP20, respectively.
Similar effects apply for diesel buses, but because of their higher unit emissions of BC and NOx, the contribution of SLCFs is more pronounced
Trains and buses, with average occupancies around 40%, have the potential to increase loads without adding units or increasing emissions, and hence lowering the impact per passenger.
Aircraft have, in general, the highest specific climate impact, when all SLCFs are accounted for. Even for fully loaded aircraft typical for holiday charter flights, the specific climate impact is between 160 and 215 g CO2-eq per passenger-kilometer (pkm) for GWP100, with uncertainty ranging from 80 to 330 g CO2-eq per pkm). This is still higher than for a medium-sized car at average occupancy, and 3−5 times higher than average coach or train travel over the same distance.
The inclusion of short-lived climate forcers strongly affects the assessment, particularly for aircraft. Results are dependent on the metric used to translate effects to a common scale (i.e., CO2-equivalent emissions) and on the time horizon: the shorter the horizon, the more sensitive the specific climate impact is to SLCFs. Fuel efficiency and vehicle occupancy are the most important parameters in determining specific climate impact. In general, air travel causes significantly higher climate impacts than car travel, which in turn results in higher impacts than traveling by train or coach.
Our study quantifies how vehicle occupancy, fuel efficiency, and evaluation of climate impact over time affect comparisons across transport modes. It reconfirms the potential climate benefits of increased occupancies—in particular of reducing car trips that involve only 1−2 persons—and of transitioning from aircraft and cars to trains and coaches. Results could be used directly for calculating carbon credits or offsets required for single trips. For longer-term transportation planning our single trip results can be combined to an ensemble of trips or scenarios, duly accounting for the effects of continuous replenishment of SLCFs and the accumulation of the long-lived CO2.—Borken-Kleefeld et al.
Jens Borken-Kleefeld, Jan Fuglestvedt, and Terje Berntsen (2013) Mode, Load, And Specific Climate Impact from Passenger Trips. Environmental Science & Technology doi: 10.1021/es4003718