http://climate.calcommons.org/article/why-so-many-climate-models
“The above figure, which comes from the report High Resolution Climate-Hydrology Scenarios for San Francisco’s Bay Area, amply illustrates the conundrum facing all those who would make use of climate projection data. In this figure, 18 different climate models are represented, with temperature change values ranging from less than 1 degree to 6 degrees increase, and precipitation change values ranging from a 20% decrease to a 40% increase. With such a wide range, what value does one use for planning? Why is there such a plethora of models?”
“Despite the existence of a wide variety of climate models, there are conceptual problems in treating these as independent entities, amenable to statistical treatments such as averaging or taking standard deviations. To begin with, climate models have a shared history, or in other words a genealogy (Masson and Knutti 2011, Winsberg 2012). There is common code between many of these models. Technical knowledge moves from modeling group to modeling group as scientists relocate. At present we lack a detailed characterization of the shared history of climate models, and it is not at all clear what we do with such a treatment in a statistical analysis sense. It is certainly inappropriate to treat different climate models as randomly sampled independent draws from a hypothetical model space, which is what would be required by rigorous statistical analysis. (Winsberg 2012).”