The claim by the IPCC that an imposed climate forcing (such as added atmospheric concentrations of CO2) can work through the parameterizations involved in the atmospheric, land, ocean and continental ice sheet components of the climate model to create skillful global and regional forecasts decades from now is a remarkable statement. That the IPCC states that this is a 'much more easily solved problem than forecasting weather patterns just weeks from now' is clearly a ridiculous scientific claim.

"A Short Summary of Why Skillful Climate Prediction Is Much More Difficult than Skillful Weather Prediction," Climate Science: Roger Pielke Sr. Research Group Weblog (2007-05-23)