In the peer reviewed literature, I have emphasized that the IPCC multi-decadal global climate runs, while they refer them as “projections” and also “scenarios” are actually model sensitivity studies since all of the important climate forcings and feedbacks are not included; e.g. see
Pielke, R.A., 1998: Climate prediction as an initial value problem. Bull. Amer. Meteor. Soc., 79, 2743-2746.
Pielke Sr., R.A., 2002: Overlooked issues in the U.S. National Climate and IPCC assessments. Climatic Change, 52, 1-11.
A summary of the types of climate models is given in the weblog
Now, from an unlikely source (Real Climate) have come the statements
“A scenario only illustrates the climatic effect of the specified forcing – this is why it is called a scenario, not a forecast. To be sure, the first IPCC report did talk about “prediction” – in many respects the first report was not nearly as sophisticated as the more recent ones, including in its terminology. “
“One should not mix up a scenario with a forecast – I cannot easily compare a scenario for the effects of greenhouse gases alone with observed data, because I cannot easily isolate the effect of the greenhouse gases in these data, given that other forcings are also at play in the real world.”
Real Climate states that the scenarios can
“….. become obsolete, and….. cannot be verified or falsified by observed data, because the observed data have become dominated by other effects not included in the scenario.”
This is the definition of a sensitivity experiment! In other words, policymakers are being given global and regional multi-decadal model results by the IPCC which are not predictions but sensitivity model runs since a variety of important first order climate forcings and feedbacks are not included in the models! [e.g. as reported in Radiative forcing of climate change: Expanding the concept and addressing uncertainties. ]. Real Climate now has finally reported to us this serious limitiation to the interpretation of the results from climate models.