Came across a 2007 study concerning climate sensitivity. It was published by the Journal Nature, but only as a letter. Can't say exactly why they do that, but it probably hasn't been scrutinized as closely as it would be otherwise. Anyhow, in this study, atmospheric CO2 concentrations over the past 420 million years were compared to computer models. The best fit was obtained using a climate sensitivity of 2.8C/CO2 doubling. There are of course other factors that influence CO2 levels: Global Co2 degassing Organic and Carbonate burial rates Land plant populations Solar Radiation Palaeogeography Palaeolithology Palaeohydrology When these other factors were adjusted within physically reasonable ranges, it was concluded that climate sensitivity is unlikely to be less than 1.5C/CO2 doubling. The only way the computer model would reasonably fit historical data with a climate sensitivity of less than 1.5C/CO2 doubling was by assuming that CO2 optimally fertilizes all land plants as if there were never any limits to nutrients, water or light. However, we know this is unlikely as there are typically large desert areas on earth and plants grow better with more light. Here is a link to the paper: http://www.gfdl.gov/~ih/jerusalem_papers/royer.pdf [Broken] Climate sensitivity constrained by CO2 concentrations over the past 420 million years While this is an interesting study, it is IMO not particularly news worthy. It is however, consistent with the broad range of values found by most other studies.