Much of the climate debate turns on a single logical fallacy. This fallacy is clearly on display in some comments by UK Prime Minister David Cameron:
It’s worth looking at what this report this week says – that [there is a] 95 per cent certainty that human activity is altering the climate. I think I said this almost 10 years ago: if someone came to you and said there is a 95 per cent chance that your house might burn down, even if you are in the 5 per cent that doesn’t agree with it, you still take out the insurance, just in case.”
"Human activity altering climate" is not the same thing as an environmental catastrophe (or one's house burning down). The statement that he is 95% certain that human activity is altering climate is one that most skeptics (including myself) are 100% sure is true. There is evidence that human activity has been altering the climate since the dawn of agriculture. Man's changing land uses have been demonstrated to alter climate, and certainly man's incremental CO2 is raising temperatures somewhat.
The key question is -- by how much? This is a totally different question, and, as I have written before, is largely dependent on climate theories unrelated to greenhouse gas theory, specifically that the Earth's climate system is dominated by large positive feedbacks. (Roy Spenser has a good summary of the issue here.)
The catastrophe is so uncertain that for the first time, the IPCC left estimates of climate sensitivity to CO2 out of its recently released summary for policy makers, mainly because it was not ready to (or did not want to) deal with a number of recent studies yielding sensitivity numbers well below catastrophic levels. Further, the IPCC nearly entirely punted on the key question of how it can reconcile its past high sensitivity/ high feedback based temperature forecasts with past relative modest measured warming rates, including a 15+ year pause in warming which none of its models predicted.
The overall tone of the new IPCC report is one of declining certainty -- they are less confident of their sensitivity numbers and less confident of their models which have all been a total failure over the last 15 years. They have also backed off of other statements, for example saying they are far less confident that warming is leading to severe weather.
Most skeptics are sure mankind is affecting climate somewhat, but believe that this effect will not be catastrophic. On both fronts, the IPCC is slowly catching up to us.