For many years now, a lot of researchers (ourselves included) have been working under the assumption - or at least hope - that models which better simulate the past are likely be more reliable for the future.
This assumption was dealt a bit of a blow by Michel Crucifix's paper in 2006, when he showed that, among the 4 GCMs for which outputs were available, there was no relationship between their sensitivity to the negative forcings of the LGM state, and their response to increased CO2. However, only had 4 models available to him at that time, and it would be hard to show significant results (the r-squared of a correlation would have to be 0.95 or higher to be statistically significant). Anyway, research continued, since paleoclimate simulations remain the only real opportunity to test the models' ability to simulate the large changes in climate that arise from large changes in forcing.
At the workshop we attended in Hawai'i early this year, there was talk of trying to write a review - or more accurately a "preview" - paper about the potential for using past modelled climates to constrain uncertainty in future climate in the context of the upcoming new multi-model climate ensemble (PMIP3/CMIP5). It occurred to me that people hadn't looked much at the spatial pattern of past and future correlation in the PMIP2/CMIP3 ensemble. We did this for our little MIROC ensemble a few years ago, and had seen quite a strong latitudinal variation, but I had been put off from doing something similar in PMIP2 by the slight inconsistencies between the model versions used for the past and future in PMIP2/CMIP3. I originally planned to wait until we had results from PMIP3/CMIP5 which should be a much larger and more consistent ensemble. However, for a preview paper I thought we could give it a shot with the old models, just to see what it looked like. There was the added incentive that writing the preview based on those models meant that in the future, when PMIP3 is complete, we will be able to see how totally wrong we were.
So, my approach was to bin the outputs of the PMIP2 simulations for the Last Glacial Maximum onto a 10x10 degree grid, correlate these local temperature anomalies with the models' climate sensitivties and hope to see a nice big correlation in the tropics where the dominant forcing for the LGM is due to GHG changes. And that's what we got. Only we also got a weird correlation the other way round in the southern ocean! That we don't understand, but at least it explains why there is still no correlation on the global scale despite the larger ensemble (7 models). In fact, when we looked again more carefully at Michel's paper, we were reminded that he had also looked at the tropics. He had compared past to future tropical changes which isn't quite the same as our analysis (we don't have spatial maps of future temperature for all models), but although his results weakly positive, his ensemble was too small for the result to be significant.
I've not really been so interested in climate sensitivity since we settled the matter to our satisfaction several years ago, but the rest of the world has been slow to catch up. So, it is natural to use this relationship, together with James' new LGM temperature estimate.
One approach is to use the linear regression (and its associated predictive uncertainty) to map the observationally-derived temperature estimate (for tropical temperature) to climate sensitivity. This is basically what Bo did for sea ice. An alternative approach is to weight the GCMs according to how well they match the data. Both methods have their strengths and weaknesses, but in practice it doesn't seem to matter that much. analyses point towards a moderate climate sensitivity, though there are some significant caveats in the result, which we hope the paper makes clear enough.
At some point in the summer, one of the IPCC authors contacted us to ask if we had any new climate sensitivity. Meanwhile, the "preview" paper s slow in progressing, as these things tend to be. So, although our analysis was originally intended just as a modest constribution to that larger piece of work, we , submitted to GRL, and somewhat to our surprise it sailed through review with only minor revision.