The article I quoted assumes that the risk of the IPCC's worst case scenario is 5% to 10%, from IPCC figures. I'd say that's calculated, perhaps as accurately as they can. It is calculating an estimate on the worst case scenario, not making a worst-case estimate (whatever that means). You can't admit that they give a number and then say that they can't give a number, that makes no sense. They calculated the number, they published it and they used it.The article you linked to starts with assuming that there is a 5% chance of a 12 degree temperature increase, and then does calculations as to what the effect would be. But it even admits that the 5% number is a worst-case estimate. It is that number which can't be calculated, which makes the rest of the calculations pointless. That does not mean that there is no chance it could happen, or that the chance is small, but it does mean that we can not calculate in the manner you described.
If you feel defensive about Gore being an idiot, that is your issue, I just said he was an idiot, I made the point that this was not a reflection on the message. You can choose any popular message, and some of the people presenting it will be idiots.
The point in the last paragraph was that the message of reducing your footprint as being most important thing to do, is a misguided message. A better message would be for people to generate wealth, and direct it towards energy efficient technology. If people did that, the problem would be more likely to be solved. Ironically, generating wealth is exactly what Gore is doing, and if he is offsetting his personal consumption by investing in alternate energy, he is doing what he should be there also. But he is still an idiot.
I don't feel defensive about Gore, in fact I was trying to say that I don't care if he's an idiot or not. I do care that the Gore is a hypocrit argument is another thinly veiled anti-global warming charge. Its a distraction (I think it was point number two on the Exxon memo) that takes time away from the central argument that the science is good. Wasting time talking about Gore is like arguing whether trains exist while you're sitting on the tracks in the way of the express. It doesn't matter other than stopping us from talking about what really matters.
Look up the Jevron principle. The idea is that the more energy efficient we become, the more we use. Got a flourescent bulb, now you can afford to keep it on all day.... Really what we need is a carbon tax, to help make it too expensive to use too much carbon based fuels. Oh, and just to start another argument or two, fewer kids. That's probably the biggest eco footprint decision you can make. Generating wealth to buy better windows won't make half the difference that raising the price of natural gas enough that we'd change the way we heated our houses, maybe heating the rooms we used when we used them instead of the whole house.