What's Your Probability of 2.5° C Warming?

Phil at Statistical Modeling asks everyone to get a little bit more quantitative about their estimates of global warming. He suggests:

Maybe we should start characterizing people by a single number... What probability do you assign to the following statement: increasing the atmospheric carbon dioxide concentration above 800 ppm will change the global average surface temperature by more than 2.5 degrees C (4.5 F)? This would imply a climate sensitivity somewhat below the extreme low end of what the Intergovernmental Panel on Climate Change says is credible... I've chosen 800ppm, which is a bit less than triple the pre-industrial level. It's likely that we're going to zoom right past 800ppm, too.

His number is 90%. Unfortunately the question is slightly incomplete because he doesn't include a deadline. Here are a few possibilities for the deadline: 1 year after we pass 800 ppm, 10 years after we pass 800 ppm, or 30 years after we pass 800 ppm. This would be a "last chance" deadline. Meaning, if the global average temperature increased by more than 2.5° C before then, that would count too. The global average temperature in 2008 was 14.3° C according to the UK's Met; a Google search of this took way too long to find the answer to such a simple question due to the hackery and charts of "temperature anamolies."

Hmm... so let's take the 10 years past 800 ppm deadline for checking to see if global temps have risen. Here's my probability, then: 20%. This is poorly calibrated, but I don't like the idea of either going higher or lower, so I'm sticking with it. Note also that I still favor higher prices for fossil fuels via a revenue-neutral carbon tax. I don't support subsidies, cap-and-trade, or other measures that epitomize the tryanny of the minority over the majority. In an ideal world, we would vote on values, but bet on these kinds of beliefs.