Tuesday, December 31, 2019


The Carbon Footprint of Personal Travel

News item. Tuesday 14th January. The government has decided to use public money to support Flybe. More disturbing than casual use of public funds to support failing business is the announcement of intention to review air passenger duty. As we argue below failure to tax aviation fuel is a distortion of the market in favour of the highest emissions forms of transport - another subsidy from an environmentally challenged future to a self-indulgent present. Air passenger duty represented a small step towards redressing the balance and encouraging transport modes with lower emissions. Flybe is a major operator of UK domestic routes where there are more rail and road alternatives.

Most readers of this blog are increasingly environmentally conscious, particularly in relation to climate change. 
Transport may not be the largest source of greenhouse gas emissions, but it is still a major contributor and in some respects one of the most difficult sectors to convert to low carbon alternatives. It also, in many instances, offers a lot of personal choice. So it is a worthwhile exercise to compare the environmental costs of the choices we, individually, make. The calculation can be complicated, but some very interesting ideas arise in the process of making the comparisons. My example will be the choice that a traveller to the South of France might face, assuming a 1250 mile or 2000 km return journey. These may represent very different experiences but for many people this is a real choice. The BBC recently published comparative tables, as shown below.

These numbers are realistic, at least as broad averages, and align quite closely to statistics quoted by other sources. But there are a number of qualifications and observations that can be made.

·         Numbers are invariably quoted for “average” or typical flights, average aircraft or cars, and car journeys including urban driving, and are not necessarily accurate for any particular journey, aircraft, or road vehicle. This analysis also ignores the carbon footprint associated with the manufacture of the cars, trains, buses or planes.

·         For the purposes of a simple comparison, we should disregard the “marginality” argument, that if there are spare seats on any form of public transport, then an individual decision to use that mode of transport has near zero energy and environmental impact. This is technically correct but the argument tells us little about the reality of transport investment choices or what makes sense in policy terms.

·         Airline flights carry an additional penalty in non-CO2 emissions, according to the BEIS data. However the weighting of different GHG in terms of impact is itself a complex issue. But this clearly a major factor.

·         The occupancy rate is crucial for all modes of transport. The implicit and broadly justified assumption for air travel is that typical flights are usually full.  Trains and buses tend to be more lightly loaded. The carbon footprint per passenger mile for the car depends on the number of passengers in addition to the driver. Two in the car halves the footprint as a first approximation.

·         For the car the assumed average is the overall driving average in all conditions including urban and secondary roads. For a typical long journey, on trunk roads and French autoroutes with minimal congestion, and observing or staying a little below the speed limits, one should expect a typical modern diesel car to produce emissions much closer to 130g per km, as compared to the quoted 171g.

·         The other critical assumption is the fuel source for electricity generation (in the case of the train). However the calculation should relate to the incremental power generation induced by the mode of transport, not necessarily the average fuel input per kWh. This would considerably reduce the apparent advantage of the French railways, if incremental load is generated from fossil fuels.

·         The table does not include electric vehicles. If it did  the same question would apply – of dependence on how the electricity is generated. On the basis of current UK sources of generation this suggests a CO2 footprint of about 80g per km for small EVs, as compared to 171g per km for average petrol/diesel vehicles (driver only in each case).[1]

If electricity production can be made carbon-free then the ideal is any mode of transport that is electricity based. This simply reflects the now widely accepted view that the path to decarbonising transport rests on the decarbonisation of electricity followed by the penetration of electricity into the transport market.

And the environment friendly choice between flight and a conventional car journey?

If the individual has a petrol or diesel car, and the choice is between car and air travel, then, purely in terms of CO2, the choice is finely balanced for a single occupancy car as against a flight. However the inclusion of non-CO2 impacts shifts the balance decisively in favour of the car. Additional passengers have an even greater  effect, with two people in the car halving its emissions[2]. The flight is associated with emissions equivalent to 254g per passenger mile, and the car journey with only 65g per passenger mile. For a family of four on my hypothetical return journey of 2000 km, the carbon footprint is approximately 2.0 tonnes for the flight, but only 0.5 tonnes for the car journey.

So purely on an environmental perspective, the preference should be clear.

Current policies discriminate in favour of air travel and against the car

Of course there are several factors that can determine choice of travel mode, including convenience – which strongly favours flying, but in a world that clearly requires progress towards low or zero emissions, one might expect that the costs and prices of the alternatives would at least  reflect the huge additional environmental cost of flying. But this is not the case.

Car drivers pay very substantial fuel taxes, while aviation fuel is tax-free, and it takes little effort to work out that in many instances flying will be cheaper than driving. Advance booking for a flight from London to Nice, for example, can cost as little as £100 return fare. This is a perverse incentive, and continues to be responsible for a very substantial environmental cost, which I have previously argued could be valued at a sequestration cost [3]  of at least £75, will fall heavily on an increasingly near future.

The cost of cheap air travel

Cheap air travel has brought great and welcome opportunities for mass market leisure travel, but its cost has been so low that it has also hugely expanded the volume of travel and significantly changed lifestyles. Wealthy British families can routinely travel to their second homes in France for a weekend. Others routinely commute weekly by air between work in one European city and home in another. And young men from London organise stag weekends in Las Vegas.

Failure to reach international agreement on the taxation of aviation fuel has in consequence resulted in a subsidy from an environmentally challenging future to what some might see as a self-indulgent present. 
Time to find a sustainable solution? Electric aircraft are starting to look like a credible option, at least for short-haul. Otherwise it will have to be synthetic fuels (from an electric route) with a limited amount of sustainable biomass.

[1] The difference is largely attributable to the substantial proportion of low carbon generation in the UK.
[2] In fact extra passengers have a small effect on fuel consumption, so this is a small but not very significant exaggeration.
[3] I have previously suggested a figure of $200 per tonne to represent a minimal realistic cost of future carbon extraction from the atmosphere.

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