I agree with Hainey for once on this subject!
The nature of the problem is that you can't 'prove' either way which is why the IPCC goes with probabilities to describe the possible outcomes. All we have to go with is our current knowledge of the basic science behind and the available evidence. It's not something which we want to get wrong though and hindsight would be useful on this one 😉
I also agree with previous posters that other reason that people get so fussed about the subject is that it will require a change in behaviour. Some people will respond to scare stories, others to arguments based on equity and fairness. For most I think that the only way to get people to make the required changes is not to try and frighten into changing but to present them with a positive image of a future where we're not back into the caves but live using more resouce efficiently, less polluting and consumption based, and in my opinion, more pleasant lives. Yes, we can still keep our bikes!