Two weeks ago I flew from Budapest to London and took the train back. Approximate figures:
Plane: cost of ticket – £60; time door-to-door – 6 hours; emissions – 300kg
Train: cost of ticket – £275; time door-to-door – 24 hours; emissions – 70kg
So the train cost £215 more and saved 230kg CO2.
On this basis, assuming time is priced at zero, to persuade someone to switch from plane travel to train travel you would need a carbon price of about £930 per tonne CO2, or Euro 1,000. Compared to the market price of Euro 12.50 today.
The alternatives for someone wishing to cut emissions are (i) not to travel, or (ii) offsetting. In the case of the Budapest-London trip you could spend up to £215 on offsetting and it would still be cheaper and more convenient than taking the train. Imagine you spent just £100 on offsetting. That would get you about 10 CERs from CDM projects.
Imagine you don’t rate CERs and think that they deserve a massive 70% environmental discount. So one CER represents only about 300kg of real reductions (rather than 1000kg). Still, you would get 3 tonnes of real reductions to offset 300kg of actual emissions.
This shows that people who criticise offsetting in principle are talking rubbish. It is a question of practice, not principle. For the Budapest-London trip you can fly with more than 10-fold effective offsetting and still save over a hundred quid over the next best alternative of taking the train.
As long as travel and carbon have the current relative costs, compulsory 10-fold offsetting would be a great way to cut emissions, while keeping the airlines in business.