Emission calculation

[Beausoleil]

(This is not a Global Warming Denial thread.)

You may have seen the climatecare.org emissions calculation - you type in your trip and it tells you how many kg of CO2 your trip will generate. It is based on BA operations, essentially dividing fuel usage by passenger miles. (Was any account taken of freight - is that significant?)

http://www.climatecare.org/

I used a trip from MAN-ATL, one way, and was told it generated 0.9 tonnes.

So I tried to work it out for myself.

I used fuel consumption of 16L/km (Wikipedia for a 747 I think)
I assumed 400 passengers
If Jet-A has a density of 0.8 kg/L, and is 75% carbon by mass, the mass of CO2 generated per km is 88 g CO2.
I guessed Manchester to Atlanta was about 4000 km
So I got 0.35 Tonnes CO2 per trip.

Given my guesses, this isn't bad agreement, but I wondered where I was out to lose a factor of 3. I don't exclude bad arithmetic!

Any ideas?

[gpn01]

I don't know about the generally validity of the caluclation but I believe it fails in two key areas:

(1) You calculated the carbon emission for a seat on the aircraft whether you've occupied it or not. The Jumbo is going to be making the journey anyway, so 'your' carbon emission should be calculated as being as a result of the additional fuel burned because you and your luggage were on the aircraft. So, you should work out how much extrat fuel is burned because of your weight. In which case the carbon emission calculations being cited on various websites are a massive over estimate.
(2) If the seat is empty, the airline will tryand make use of the extra capacity that provides by either filling the seat with another passenge or by increasing the cargo carried. Either way, if the airline is efficient, there will b no difference whetehr you fly or not. ergo - the carbon emission as a result of you flying is now zero.

Only problem is that there's some people who've set themselves up as carbon neutral trades whose only real function is to salve people's consciences by allowing them to 'buy' credits to offset against their flying. There's a whole business out there with a vested interest in making carbon emission calculations come up with bigger numbers than reality...

[Beausoleil]

You raise an interesting issue. I'm not sure there is a "correct" answer - what one has to remember when using this number (or indeed any of the statistics used in the political arena) is how the calculation was done. Clearly the net decrease in CO2 emission from my not flying tomorrow would be very small since the plane would go anyway, as you note. We might call this the marginal carbon footprint. However the net decrease in CO2 emission from everybody like me not flying would be more significant since eventually the demand would decrease to the point where some flights were removed from the schedule.

Compare, say, a restaurant. One doesn't expect to pay something calculated as: (cost incurred by restaurant in serving me a meal) - (cost incurred by exactly the same restaurant if I chose not to eat there at the last minute). The restaurant has to pay staff time, ingredients, premises and so on whether I eat there or not. But I don't cover the marginal cost, I cover the unit cost The carbon footprint is analogous to a unit cost, and one would calculate a unit cost by dividing the total cost by the number of units.

Or suppose I told the airline that they are flying the plane anyway so I shouldn't have to pay for the cost fuel incurred in flying any of the mass that isn't part of me or my luggage or a share of the crews salaries or any of the non-flight operating costs. Would they go for it?

My point is that carbon footprint is calculated in a similar way to other costs, so I don't think it is invalid or counterintuitive as such.