Posted on behalf of Jeff Tollefson:
Paleoclimate researchers are mounting a new modelling exercise to assess their skills at reconstructing not the actual climate during the last millennium, but a pseudo climate generated by current global models.
The goal of the “Paleoclimate Reconstruction Challenge”
is to get around an inherent problem: Climate reconstructions are difficult to validate because by definition nobody knows exactly what the actual climate looked like. Instrumental data only goes back about 150 years, and proxy data used to calculate temperatures is sparse beyond about 400 years. 
In this case, the teams will be able to compare their reconstructions to an actual climate simulation, which will remain secret until the end. They will then be able to assess in detail where things went wrong.
Caspar Ammann, a paleoclimatologist at the National Center for Atmospheric Research in Boulder, Colorado, has secured about $450,000 over three years from the National Oceanic and Atmospheric Administration for the project. He says the exercise will be open to the entire paleoclimate community, including sceptics who have long questioned previous reconstructions.
Participants will be given realistic data from ice cores, tree rings and the like, but these proxies will be generated from climate models – as opposed to the actual data used in past reconstructions. The final step will be an analysis of lessons learned that can be applied to existing climate models.
The exercise is in part a response to scientific and political controversy regarding the “hockey stick graph,” a generalized reconstruction of average global temperatures during the last millennium that depicts a sharp spike after the industrial revolution. The graph has largely held up under scrutiny, but organizers say the shortened name, “PR Challenge,” has a second role: public relations.
The exercise should build confidence in the models while identifying areas that still need work, Ammann says. “The most important goal is to find out what we might still be missing.”
Image: the hockey stick graph, as featured in the 2001 IPCC report