Analyzing the climate of the world, and modelling it so that accurate projections of the future can be made is extremely difficult. It is much more difficult than has been acknowledged by James Hansen, Al Gore (who probably isn't enough of a scientist to understand) and the IPCC. Not only are many important parameters not well known, there are some important factors for which the future is unknowable. One of the flaws in the IPCC modelling of the climate is that they started with the premise that increased CO2 levels in the atmosphere caused the temperature increase that occurred from 1975 to 1998, and they calibrated their models to reflect the past with the increase in CO2 levels. This assumption is now obviously flawed because the CO2 levels have continued to increase over the past 10 years but the temperature has not gone up. It is obvious that the IPCC models need to be improved. Here is a paper discussing how other factors can be shown to have caused the temperature increase of the latter part of the twentieth century, and that these factors do a better job of predicting temperatures, even if the contribution of greenhouse gases are ignored.

http://www.worldclimatereport.com/index.php/2008/12/03/rethinking-observed-warming/