J. Scott Armstrong, noted professor of marketing at Wharton and author one of one of the best-written textbooks I've ever read--Long Range Forecasting, out of print but now available free--concludes that forecasts of global warming are not scientific (the paper is co-authored with Kesten C. Green). A couple of the highlights:
We have concluded that the forecasting process reported on by the Intergovernmental Panel on Climate Change (IPCC) lacks a scientific basis.
1. No scientific forecasts of the changes in the Earth’s climate. Currently, the only forecasts are those based on the opinions of some scientists. Computer modeling was used to create scenarios (i.e., stories) to represent the scientists’ opinions about what might happen. The models were not intended as forecasting models (Trenberth 2007) and they have not been validated for that purpose. Since the publication of our paper, no one has provided evidence to refute our claim that there are no scientific forecasts to support global warming.
We conducted an audit of the procedures described in the IPCC report and found that they clearly violated 72 scientific principles of forecasting (Green and Armstrong 2008). (No justification was provided for any of these violations.) For important forecasts, we can see no reason why any principle should be violated. We draw analogies to flying an aircraft or building a bridge or performing heart surgery—given the potential cost of errors, it is not permissible to violate principles.
2. Improper peer review process. To our knowledge, papers claiming to forecast global warming have not been subject to peer review by experts in scientific forecasting. . . .
5. Forecasts are needed of the costs and benefits of alternative actions that might be taken to combat climate change. Assuming that climate change could be accurately forecast, it would be necessary to forecast the costs and benefits of actions taken to reduce harmful effects, and to compare the net benefit with other feasible policies including taking no action. Here again we have been unable to find any scientific forecasts despite our appeals for such studies.
(The whole site the paper is posted on, The Global Warming Challenge, is worth looking at.)