Matt Ridley has an excellent article in the Financial Post, looking at the IPCC's greenhouse gas concentration pathways. He finds that some of them are a trifle odd and that it's rather hard to produce predictions of catastrophe from them:
...even if you pile crazy assumption upon crazy assumption till you have an edifice of vanishingly small probability, you cannot even manage to make climate change cause minor damage in the time of our grandchildren, let alone catastrophe. That’s not me saying this – it’s the IPCC itself.
This video, of Matt speaking in Canada is also well worth a look.