[paul.fr]

I heard theother day that an airline is to offer passengers then option to have a tree planted for them every time they fly, this is designed to "offset their carbon footprint"

Now assuming that the tree is planted somewhere it will not be chopped down in a few years, how long would that tree have to be in the ground to take in enough co2 and output enough oxygen to have "offset" the "carbon footprint" of the flight?

lets assume the passenger took one flight in europe, and another one was transatlantic.

[another_someone]

As far as I can ascertain, it seems that a fully laden flight, while cruising, can do about 70 passenger miles per gallon.  As with all transport, this is an idealised figure, since it assumes all seats are occupied, and will be less efficient on short haul flights than on long haul flights.

On a 3000 mile flight, that would amount 210,000 gallons.  That is the amount of energy that would have to be contained in a tree in order to make up the energy used on a 3,000 mile flight.

Assuming that the energy value of wood was similar to that of aviation fuel (which is clearly nonsense, but is a fair starting point to work from), that would imply that the tree would need to have a volume of 210,000 gallons, which is a little short of 800 cubic metres, or about 28,000 cubic feet.  This would amount approximately to a tree 100 foot high, and 9 foot in diameter (this is ignoring the wood contained in the branches, and just looking at the main trunk - the dimensions might be less than this when one takes into account the wood also contained in the branches).

So, the first question is how long it will take a tree to grow to those dimensions.

The second issue it all the wild approximations I was making.

Another factor is where is the tree planted.  There has been some question about the advisability of tree planting at all in high latitudes, as the tree might reduce the albedo of the planet in areas that are likely to have snowfall in winter, and this could cost more in terms of the greenhouse effect that any change in CO₂ might affect.

[Bored chemist]

To a very rough aproximation the tree wood would need to be twice the volume of the fuel tanks of the plane though Another Someone's approximation is better. Don't forget that the roots of a tree can take up as much space as the bit you see.

[DrDick]

However, the benefit of the tree is that it takes in CO2 from the air, not how much energy you could get out of the tree.  A tree takes in much more CO2 than it uses to build the trunk of the the tree.  There's also the roots, the leaves and flowers (which get produced every year) and the branches.

I found a calculator at the American Forests website that estimates how many trees you need to offset various CO2 emissions.

http://www.americanforests.org/resources/ccc/

Toward the bottom, there's a spot for airline travel.  According to this calculator, it takes 1.9 trees to offset a 3000 mile trip.

Dick

[paul.fr]

However, the benefit of the tree is that it takes in CO2 from the air, not how much energy you could get out of the tree.  A tree takes in much more CO2 than it uses to build the trunk of the the tree.  There's also the roots, the leaves and flowers (which get produced every year) and the branches.

I found a calculator at the American Forests website that estimates how many trees you need to offset various CO2 emissions.

http://www.americanforests.org/resources/ccc/

Toward the bottom, there's a spot for airline travel.  According to this calculator, it takes 1.9 trees to offset a 3000 mile trip.

Dick

would that be fully grown trees?

[another_someone]

However, the benefit of the tree is that it takes in CO2 from the air, not how much energy you could get out of the tree.

The point I was making is that the two are approximately the same thing.

Since the energy obtained from aviation fuel is the convertion of hydrocarbons to water and CO₂; and the energy obtained from burning a tree is the conversion of carbohydrates to water and CO₂; and the carbohydrates themselves are obtained by convertion of water and CO₂.  Thus there is an approximate equivalence between the energy absorbed in photosynthesis; the CO₂ consumed by that process, and the energy released by combustion, and the about of CO₂ that would be released by that combustion.

The main approximation that I made was an assumption that the carbon density of wood was similar to the carbon density of aviation fuel, which it clearly is not.