A marginal victory for the well-meaning environmentalist

6th February, 2010

At the risk of turning this column into “The Undercover Environmentalist”, I need to return to that vexed question of carbon dioxide emissions. In my first column of the year, I vowed to reduce my carbon footprint from air travel – easy enough, given that it was 50 tonnes of CO2 last year. A kind reader wrote to reassure me that I needn’t lose any sleep, because the planes were making the journey anyway. Glib, I know: I’ve often said it myself to wind up environmentalists.

The answer reminded me of a brain-teaser that’s been entertaining me for the past couple of months. Since buses often run almost empty, two people sharing a car emit less CO2 per person than do bus passengers. Shouldn’t we then be travelling by car?

The BBC’s in-house environmental activist, Justin Rowlatt, aka “Ethical Man”, recently pondered this question and concluded that, no, he’d still be taking the bus. Why? Because the buses are making the journey anyway.

A pause to run through the statistics. According to my colleagues on the BBC’s More or Less programme, cars emit 127g of CO2 per passenger per kilometre and buses 106g, based on average occupancy. Even London buses average a mere 13 passengers. This is one of the problems of a public transport system: in order to make the system attractive, frequent services need to run off-peak, and in order to make the system work at all, vast chunks of metal need to counter-commute, almost empty, to get back to the start of their rush-hour routes. They are “making the journey anyway”.

There is something strange going on when the environmentalist and the anti-environmentalist use the same excuse – one to justify taking the plane, the other to justify taking the bus. An admittedly unscientific poll of environmentalists at dinner parties suggests to me that they think “the plane is making the journey anyway” excuse is unacceptable but “the bus is making the journey anyway” excuse is spot on – and that they have no coherent justification for the distinction. Their favourite excuse is “you have to set an example” – but surely, before you decide to set an example, you need to be sure that you aren’t setting a bad one.

To cut through the fog we need to rely on some technical language. We must distinguish between average cost, marginal cost, and average marginal cost. The average carbon cost of travelling by car or bus is the total emissions divided by the number of passengers: these are the numbers that are unflattering to buses. The marginal carbon cost is the extra emissions caused by one additional passenger. For planes, trains and buses this is low – unless, that is, the passenger is the straw that breaks the camel’s back, and causes an additional bus, plane or train to be scheduled in future, in which case the marginal carbon cost of that passenger will be gigantic.

The average marginal cost averages out the marginal costs of a large chunk of passengers. (Exactly which chunk to use seems to be a rather black art.) The idea is to share out the cost between the passengers who do not provoke an extra bus or flight, and the passengers who do.

For all you environmentalists out there, then, here is the justification for the double-standard of taking the bus but not the plane: it is that bus schedules might be insensitive to passenger demand, while planes are highly sensitive – and ever more so since the budget airlines arrived on the scene. Your best argument for taking the bus is a perverse one: that, no matter how many people do likewise, it’s the rare public transport tsar that will lay on extra buses.

I’ll be cycling.

Also published at ft.com.

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