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Study finds climate impact of long distance trip can vary by factor of 10 depending upon mode, efficiency and occupancy

GWP100 weighted specific climate impact (g CO2-eq per pkm) as a function of vehicle occupancy. Bold parts of curves indicate typical occupancy ranges. The envelope around the medium aircraft indicates one standard deviation uncertainty; otherwise uncertainties are indicated by selected bars at medium occupancy. Insert: Zoom in for bus, trains, and cars. Credit: ACS, Borken-Kleefeld et al. Click to enlarge.

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.

Specific climate impact across metrics for long-distance travel at 100% occupancy (left) and average occupancy (right) with various transport modes. For cars the Euro 4 emissions standard is used. The left and right ends of the bars represent a range of vehicles: coaches span occupancies in charter and regular service; trains cover electric and diesel trains; diesel cars span small to large diesel cars; gasoline cars span medium to large cars; aircraft span higher, medium (central line) or lower fuel efficiency, either with no contrails and cirrus clouds (c/c) or with 50% the global average impact value. The left/right whisker indicates one standard deviation uncertainty for the lower/upper variant. Credit: ACS, Borken-Kleefeld et al. Click to enlarge.

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



'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.'

That's easier said than done.
The present 40% includes them being packed to screaming point and beyond in rush hour, and the frequency of service dropping to often unacceptable levels outside it to retain a reasonable occupancy rate.

If small diesels can break even for emissions at a 50% occupancy rate with a bus or train at average loading, then a small fuel cell car, if we can ever sort out the economics of such a thing, should be able to break even in regard to CO2 emissions assuming all its hydrogen comes from natural gas even after reforming with just one person in the car.
All other emissions would of course drop to negligible levels.


One (capital intensive) way to solve the bus problem would be to have buses of different sizes for use at different times of day/week.
Thus, you would have an 80 seater, 40 seater, 20 seater and MPV (6-8 seater) bus all on the same route, driven by the same guys, at different times.
So you need 3 buses to do the work of one, but they should last the same time (each), and as long as you have the space to park them, and a taxation regime that does not punish "partial buses", it could work out OK.

I live on a bus route and I see buses zooming around with almost no-one on them at 9 am on Saturday mornings etc. You have to run buses on the route to provide a public service, but you don't need to run 80 seater buses all the time.
(If you want to move a football team at 9 on a Saturday, you should be able to ring up and tell them that you would need it.)

Trevor Carlson

To solve the public transport occupancy dilemma:
Wouldn't it be great if there were driverless electric vans that could be "called" by simply standing at a bus stop or by smartphone with a citybus app?

The buses could use something like a Chevy Volt powertrain and chassis with a van upper structure. There would be enough standing room for 8-10 people and maybe 5 to sit around the edges. If you were handicapped a different vehicle could be called.

There wouldn't necessarily need to be any pre-programed routes to follow as they could sit at distributed charging stations until called upon. In the event of a power failure or emergency they could serve as quickly deploy-able back-up generators. If wind power is plentiful a large fleet of these would help buffer the power-grid. The downside is that a large fleet would also put taxis out of business. They could be networked to reduce congestion and optimize traffic flow.

The technology to implement something like this is here now. Googles' Driverless cars have proven they're safer by driving more accident free miles than they would have if humans were driving them.

Trevor Carlson

The citybus app would work by having you input your destination and time you wanted to be picked up. It would then track your location and text you as "your" ride was ready to pick you up. It would need some sort of map or compass with a distance to help you find your ride in case there was something ambiguous about your pick-up location.

For instance if a concert or football game just let out nearly all the buses of the city would converge on the stadium and ferry their passengers to their end destinations automatically with no stops at a bus station and changes to other buses.

As your bus closed in on the stadium it would ping all the people going to a similar area it's own location and the rally point. If anyone could not make it to the rally point you could have the option to pass.

Occupancy rates would be nearly 80-100% every trip. It may even be more effective than owning your own vehicle and more people would move into the city to save money from the costs associated with transportation.

There would need to be some accommodation for bicyclists - maybe they would have to ride the vehicle designed for wheel-chair access.

Trevor Carlson

The system could be funded largely from subscriptions and a mile rate based on the type of bus used and occupancy. No money would even need to be carried as you could just use your phone.
If you didn't have a phone you could buy a bus pass with a keychain tag that you'd wave at a bus-stop scanner when you wanted to get picked up. You could pay for the pass on a monthly or yearly basis with cash.

Such a system could be built up slowly to match the user base and would not need large fleets to be purchased at once. As public transport it wouldn't need to make a profit but if the taxi companies implemented it first they could make public transportation less efficient, more expensive and essentially obsolete.


'IGT operates using a fleet of minibus vehicles called taxibuses which travel on the road networks. Typically a taxibus will carry around six to eight passengers aboard, with the taxibus driver guided by street navigation instructions received from a computer system which automatically controls the routing of each taxibus vehicle in the fleet.

Journeying by taxibus is delightfully easy. Prospective passengers request a taxibus ride simply by submitting their current location and desired destination addresses to the IGT computer system, typically using an ordinary cellular telephone. Regularly-used addresses would be pre-programmed on the passenger's phone, so this address submission is very straightforward. As soon as a taxibus journey is requested in this way, the IGT computer system searches its database for a nearby taxibus vehicle whose current itinerary is compatible with the passenger's submitted itinerary. Once a suitable taxibus is found, it is immediately diverted to pick up and convey the new passenger.

New passengers are collected extremely quickly, generally within three minutes of submitting their journey request. Such rapid pick-up is feasible because IGT operates with a large fleet of taxibus vehicles spread across the city, continually travelling the road networks, constantly conveying passengers. A new passenger is allocated to a nearby taxibus vehicle already on the roads carrying commuters, this vehicle's itinerary being modified on-the-fly to incorporate the new passenger.

The three minute pick-up response that IGT achieves using this on-the-fly approach makes taxibus travel vastly superior to existing door-to-door public transport concepts, many of which require prospective passengers to place an order for their journey hours in advance of travel. With IGT, a passenger can spontaneously order a taxibus, even when waiting on the high street, for example, knowing that a taxibus will arrive to collect him usually within one or two minutes. This is an incredibly fast response (the feasibility of which is analysed later). '

Electric roboticised cars would work even better.
The two biggest costs in public transport are wages and fuel.
Next biggest cost is maintenance.
Electric cars have around half the maintenance costs of ICE.

I haven't heard that Taxibus is off the ground yet.
I would imagine the taxi drivers would be rather against it.

HarveyD many places, the major operation cost for city buses are the drivers' cost (a grand total of $122,000+/year per driver in our city). Up to 4 drivers per bus are often required, costing more than $488,000/year or the full purchase price of the bus in less than 2 years or about 8 times the price of the bus over 16 years.

That being the case, 100+ seats articulated city buses, using (2.5 to 3.0) fewer drivers, are progressively replacing the standard 40-ft units.

Very small buses are too expensive to operate unless you have access to low cost drivers?

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