Tuning Climate Models

Lucia has a post on a topic that doesn't get enough discussion -- how climate models are tuned.  Those who are so heavily invested in models (any claim of CAGW requires dependence on the models) stamp their feet and insist there isn't any tuning.  But any look at the influence of aerosols will note that every model uses precisely the amount of aerosol forcing needed to bring their model into coherence with past climate.  This is despite the fact that so many models vary dramatically in their claims of sensitivity to CO2.