Not flying doesn't really reduce emissions though... Those planes are going to fly whether or not you are on it.
The new Dreamliner jet is a good example of this (once they solve the battery problem and can fly the sucker...). It's massively more efficient than its predecessors (20% more efficient than the 767 according to Boeing). That's a huge jump.
On an individual level, sure, but not flying reduces demand, which in turn reduces the number of flights.
Try to get flights from London to Paris or from Paris to Lyon. Not flying stops planes from flying very quickly.
This is an extreme example of tragedy of the commons type thinking. Why bother doing anything because someone else will do it anyways?
A lot of cars are used by a single person. Almost never (generalised) to it's full potential.
I'm uncertain about this, but I think most planes are very well utilised (closer to the full potential rather than to the opposite) - since it's so uneconomical to go empty or close to empty utilisation.
I'm not however disputing that a plane emits a lot more emissions than road bound vehicles.
Average CO2 per person in the US is around 17 tonnes per year[1].
A single flight from LA to NYC (round trip) is equivalent to about 2 tonnes after taking into account some high-altitude effect[2].
Driving 12,000 miles per year at 25mpg releases about 4 tonnes[3].
So, if you cut your driving in half and then take one long domestic flight, you are just breaking even. If you take multiple long flights per year, you really have no hope of staying below the average carbon footprint for an american. Unless maybe you live in a tree or something.
I'm not saying this to be critical. I fly, and I am not ashamed of that. But we should be honest with ourselves, and not think that riding a bus once in a while and buying locally-sourced vegetables makes up for taking three international trips a year.
[1] http://en.wikipedia.org/wiki/List_of_countries_by_carbon_dio... [2] http://en.wikipedia.org/wiki/Environmental_impact_of_aviatio... [3] http://www.stewartmarion.com/carbon-footprint/html/carbon-fo...