Update 7/25/12: Just a note: By its actions, the IPCC Admits Its Past Reports Were Unreliable. There seems to be some indication that the IPCC reports was a little more advocacy and a little less science.
An issue with climate models is whether or not they have any capability to actually predict anything. The amazingly complicated climate system of this planet, for example.
The hysterics place great value in the models, those that produce the “We’re all gonna die!!!” results.
Willis Eschenbach on WUWT posts some thoughts on scientific models in general:
There’s a lovely 2005 paper I hadn’t seen, put out by the Los Alamos National Laboratory entitled “Our Calibrated Model has No Predictive Value” (PDF).
The paper’s abstract says it much better than I could:
Abstract: It is often assumed that once a model has been calibrated to measurements then it will have some level of predictive capability, although this may be limited. If the model does not have predictive capability then the assumption is that the model needs to be improved in some way.
Using an example from the petroleum industry, we show that cases can exist where calibrated models have no predictive capability. This occurs even when there is no modelling error present. It is also shown that the introduction of a small modelling error can make it impossible to obtain any models with useful predictive capability.
We have been unable to find ways of identifying which calibrated models will have some predictive capacity and those which will not.
There are three results in there, one expected and two unexpected.
Interesting stuff. Read it all (and 300 comments) here: A Modest Proposal—Forget About Tomorrow
Are Computer Models Reliable – can they be used to predict the future climate?
No: says the IPCC (Chapter 14, 18.104.22.168, Working Group 1, The Scientific Basis)
Third Assessment Report: “In sum, a strategy must recognise what is possible. In climate research and modeling, we should recognize that we are dealing with a coupled nonlinear chaotic system, and therefore that long-term prediction of future climate states is not possible.”
All the computer models do is produce scenarios based on the assumptions and limited understanding of climate programmed into them. They do not do clouds, some of the assumptions are estimates that vary by orders of magitude, we have different scenarios varying from 1.0C to 6.0c, some scarier scenarios of 10 – 12 C for the next century. These assume ‘climate sensitivity’ – ‘feedbacks’ due to CO2 which amplify the simple physics of per doubling of CO2 in the atmosphere of ~1.0C , to increasing temperatures due to positive feedbacks producing the various accelerated global warming scenarios… Yet the order of magnitude of the sensitivity is unknown.
In fact the sign is unknown, it is assumed positive, is could equally be a negative feedback, it is currently uncertain.
I do hope that some scientists somewhere are doing some actual experimentation with observable data to establish this. The fact that the Earth has not experienced this behaviour in the past at point when CO2 were much higher naturally, must demonstrate these assumptions of strong positive feedbacks are wrong.
The original post, well worth reading: Are Computer Models Reliable – Can they Predict Climate?
But then… a commenter on the WUWT post adds:
“Sir, you say the IPCC does not make predictions. The IPCC says the world is going to warm. I call that a prediction.”