Nature has Climate sensitivity constrained by CO2 concentrations over the past 420 million years (subs req, of course). Its interesting for two reasons: firstly as yet another way of getting the same range for climate sensitivity (they get 2.8 oC as a best guess). And secondly as an antidote to the septics who assert no correlation of CO2 and T over geological time. Not that the match presented here is perfect, but it does exist.
[Update: FP has a link+pix for those without direct nature access -W]