Jul 2006

Plane Vs. Coach

In the comments to my last post, my old friend Philippe challenges the notion that carbon emissions / pollution from air travel is substantially worse than that generated by road travel (in this case, coach). Philippe used a website called Carbon Debt Calculator and came up with the following numbers…

Dublin – London 300miles
Driving emissions: 160kg CO2
Flying: 130kg
Train: 90kg

This flabbergasted me. It flew in the face of everything I’d been reading about the carbon emissions of air travel. Could it be true that flying 300 miles emits less CO2 than driving the same distance? This didn’t just fly in the face of everything I’d been reading, it flew in the face of common sense! How could it be, that the fuel consumed by picking up a 737 and hurling it into the air at 600kph to a height of 30,000 feet would be less than that consumed by rolling a far lighter vehicle the same distance along flat surfaces?

So I decided to do a little research. I couldn’t verify that Phil’s numbers are indeed the ones produced by the Carbon Debt Calculator (I can’t seem to get it to work… if I type 300 miles into the air travel box and hit ‘calculate’, it responds with “0 tons of CO2”). But based upon barely two hours of internet research and some excel spreadsheetery, I can confidently state that the Carbon Debt Calculator is a total bunch of arse should Philippe’s numbers be representative.

It’s just wrong.

First up: The Coach

The Dublin-London trip is complicated a little by a 70 mile stretch of water between the two. But in the interests of making this a more general (and therefore useful) comparison, let’s assume that both vehicles travel the same distance. In reality I suspect that the 70 mile “piggyback” that the coach receives from the predominantly freight-carrying ferry would reduce the total emissions generated by the journey.

OK… let’s work out the CO2 emitted by the coach on the 300 mile trip between Dublin and London. The vehicle was run by Bus Éireann who use Scania Irizar PB buses. According to the manufacturer, they get 8.15km per litre (let’s call it 7km to take account of potentially inflated claims by the manufacturer). Using a 1:0.62 conversion rate, that’s 4.3 miles / litre.

So the coach will consume approximately 70 litres of fuel during the trip. From here (PDF) we discover that the specific gravity of diesel is 0.88. That equates to a weight of 0.88 kg/litre. So the trip burns 61.6 kg of diesel oil. Remember…

When fuel oil is burned, it is converted to carbon dioxide and water vapour. Combustion of one kilogram of fuel oil yields 3.15 kilograms of carbon dioxide gas. Carbon dioxide emissions are therefore 3.15 times the mass of fuel burned.

Calculating the Environmental Impact of Aviation Emissions | An Oxford University Study (download PDF)

So the total CO2 generated by my 300 mile coach journey is roughly 194kg. Based on 85 passengers per coach, that’s 2.3 kg per traveller.

And now: The Plane

For this I’ll mostly be sourcing my data from the above-cited “Calculating the Environmental Impact of Aviation Emissions”. This is a fairly short report, but I recommend you download and read it. It’s interesting stuff.

With respect to our 300 mile journey, Table 1 of the report allows us to calculate the fuel consumed by a 737 between Dublin and London. Ryanair (they of the 99cent flights) have a fleet of Boeing 737s, so we’ll use them as our plane of choice. And according to that table, the 737 will consume approximately 2,200kg (2.2 tonnes) of fuel covering that distance.

Using the same factor of 1kg fuel = 3.15kg CO2, it appears that the 737 will emit 6,930kg of CO2 (almost 7 tonnes). Based on a capacity of 189 passengers, that results in 36 kg per traveller.

This is not a trivial difference… 2.3kg Vs. 36kg. It demonstrates that emissions generated by flying are a whole order of magnitude greater than covering the same distance by coach. What it doesn’t factor in, however, is the difference between emissions made at altitude and those made at ground level. The Oxford University report spends most of it’s time grappling with this issue and proposing a variety of metrics (multipliers) to take into account the altitude. For instance, it suggests that “the full climate impact of aviation is deemed to be between 2 and 4 times greater than CO2 alone”.

So best case scenario, you’re actually looking at 2.3kg Vs. 72kg per passenger.

Carbon offsetting: A bunch of arse

In the comments, it was also suggested that I could take the convenient and comfortable flight and then offset the carbon emitted either through a payment to a carbon-neutralising fund, or through “good works” of some kind.

Merrick‘s article Carbon offsets are a fraud is a good place to start on this subject. The simple reality is this: Carbon offsetting is a fraud. See? Just like the title to Merrick’s article. The planet has two carbon cycles. One takes place over geological periods of time and includes the carbon locked in fossil fuels. The other takes place over far shorter periods – the life-cycles of plants and animals.

You cannot compensate for the burning of fossil fuels by planting trees. It’s as simple as that.

But even more fundamentally. And this is the kicker. I have a very serious moral problem with the attitude that carbon-offsetting engenders. The suggestion that flying to London is OK so long as I pay someone to plant the trees to capture 72kg of carbon per flight. It’s the whole notion of paying to pollute. Of placing a cash value on environmental damage. Quite aside from it being profoundly undemocratic, it’s just plain wrong.

We don’t own the environment, and we have an obligation – to those that follow us – to minimise the damage we do. One option is to do just that… take the obligation seriously… actively minimise the damage you do (2.3kg Vs. 72kg). The other option is to ignore how much damage you’re doing and hope that by giving someone some cash (based on absurd estimates generated by rose-tinted websites) that they’ll be able to fix the problem at a later date sometime maybe.

Sorry, but that option’s just not good enough.

Posted in: Opinion