@Charles: Dr. Dyson says the climate models are full of fudge factors:

http://motls.blogspot.ca/2013/04/dyson-climatologists-are-no-einsteins.html

Dr. Motl:

"At any rate, he is saying some things that should be important for everyone, especially every layman, who wants to understand the climate debate. The climatologists don't really understand the climate; they just blindly follow computer models that are full of ad hoc fudge factors to account for clouds and other aspects."

"Freeman Dyson also mentioned that increased CO2 is probably making the environment better and he estimated that about 15% of the crop yields are due to the extra CO2 added by the human activity. I agree with this estimate wholeheartedly. The CO2 is elevated by a factor of K = 396/280 = 1.41 and my approximate rule is that the crop yields scale like the square root of K which is currently between 1.15 and 1.20."