Scoping the 5th Assessment Report of IPCC

                                                       The core of a climate model.

The upcoming 5th Assessment Report of IPCC AR5 was scoped in IPCCs 30th Session in Antalya, April 21-23, 2009, as a Proposal for an IPCC Expert Meeting on Assessing and Combining Multi Model Climate Projections:

• Climate model results provide the basis for IPCC projections of future climate change. Previous  assessment reports included model evaluations but avoided weighting or ranking models.  
• Defining a set of minimum criteria for a model to be 'credible' or  agreeing on a metric of performance is therefore difficult and the criteria are likely to depend on the  variable and timescale of interest. Combined with an estimated data volume exceeding 1000  Terabytes, the AR5 faces immense obstacles in trying to make sense of the deluge of model runs and  data that it will produce. 
• Recent studies have started to address these issues by proposing ways to weight or rank models, based  on process evaluation, agreement with present day observations, past climate or observed trends.  While there is agreement that 'the end of model democracy' may be near, there is no consensus on how  such a model selection or weighting process could be agreed upon. 

Reading these reservations, we get a warning that we should not expect any significant improvement of climate model reliability from AR4 to AR5. World leaders preparing far-reaching reductions in CO2 emissions thus have to look to the stars for guidance, or simply rely on the predictions of IPCC based on climate models of unknown but most likely low reliability.