I read Dr. Hansens' letter to the editor of the Wall Street Journal over lunch in which he claims that doubling CO2 will result in atmospheric warming of 2.2 to 4.8°C. His letter was written to criticize an article by Matt Ridley that estimates the atmospheric response to doubling at at the lower rate of 1.5°C (I agree with this value or slightly less).
The problem is that Dr. Hansen's own data does not back up his contention (scroll down to middle of page).
In 1988, Dr. Hansen published a paper where he forecast 2012 temperatures to be in the range of the green and blue lines. I have highlighted those values with a red oval. He also forecast temperatures if the Kyoto treaty had been immediately ratified and CO2 levels started to drop. That is the purple line.
Actual world temperatures -- as calculated by Dr. Hansen himself (the black line) -- are below even the level he was expecting if Kyoto had been ratified.
There is a lot of cognitive dissonance in climate science as actual conditions increasingly diverge from what has been forecast. Dr. Hansen's letter is the latest example.