Constraining uncertainties in climate models using climate change detection techniques

Predictions of 21(st) century climate by different atmosphere-ocean general circulation models depend on the sensitivities of the models to external r adiative forcing and on their rates of heat uptake by the deep ocean. This study constrains these properties by comparing radiosonde-based observation s of temperature trends in the free troposphere and lower stratosphere with corresponding simulations of a fast, flexible climate model, using objecti ve techniques based on optimal fingerprinting. Parameter choices correspond ing either to low sensitivity, or to high sensitivity combined with slow oc eanic heat uptake are rejected provided the variability: estimates used fro m the HadCM2 control run are correct. Nevertheless, the range of acceptable values is significantly wider than that usually quoted. The IPCC's range o f possible sensitivities, 1.5 to 4.5 K, corresponds at best to only an 80% confidence interval. Therefore, climate change projections based on current general circulation models do not span the range of possibilities consiste nt with the recent climate record.