[Response: First off, he is confusing models that include the carbon cycle with those that have been used in hindcasts of the 20th Century and are the basis of the detection and attribution of current climate change. (realclimate.org)
Uncertainty is propagated forward in time by assuming that the fractional error found in model hindcasts of global mean temperature change will remain constant in projections of future changes. (ipcc.ch)
The retrospective investigation consists of a model hindcast for more than a half century using «reality - based» atmospheric forcing to drive the model. (psc.apl.uw.edu)