There was a flurry of PMIP-related activity over the Christmas and New Year “holidays” because of the need to get papers describing experimental results that are going to be cited in the next Working Group 1 assessment of the Intergovernmental Panel on Climate Change (IPCC). I’ve said it before (and I’ll say it again) that setting arbitrary deadlines to match demands for assessment reporting is not a great way of getting good science done. But I was reminded about Mary Beard’s blog comment working on days that “might in other circumstances be to devoted to jollity is, I am afraid, getting to be a habit”, so perhaps if the IPCC did not impose silly deadlines, the academics themselves would. Anyway, it’s good to see that the first of the PMIP4 pre-deadline papers is now out in discussion in Climate of the Past ( The paper describes the major features of the PMIP mid-Holocene simulations and briefly compares them with climate reconstructions. The good and the bad news? The PMIP4 simulations are really no different from the PMIP3 simulations: they get the same things right, and the same things wrong. It’s clear what’s bad about this: all that time and effort and we are still not closer to being able to predict large climate changes accurately at a regional scale even if the global-scale features (like land-sea contrast and polar amplification) are fine. But what’s so good about the situation? Well it does mean that we can consider the PMIP3 and PMIP4 simulations as a single ensemble, and think of all that lovely statistical power that gives us.

For me, this is the killer figure. A comparison of the two ensembles with respect to different sets of variables, where the purple areas are places where there might be statistically significant differences between the two ensembles. Not a lot of places really and a quick calculation shows that this much difference is to be expected by chance!


Brierley, C. M., Zhao, A., Harrison, S. P., Braconnot, P., Williams, C. J. R., Thornalley, D. J. R., Shi, X., Peterschmitt, J.-Y., Ohgaito, R., Kaufman, D. S., Kageyama, M., Hargreaves, J. C., Erb, M. P., Emile-Geay, J., D’Agostino, R., Chandan, D., Carré, M., Bartlein, P., Zheng, W., Zhang, Z., Zhang, Q., Yang, H., Volodin, E. M., Tomas, R. A., Routson, C., Peltier, W. R., Otto-Bliesner, B., Morozova, P. A., McKay, N. P., Lohmann, G., Legrande, A. N., Guo, C., Cao, J., Brady, E., Annan, J. D., and Abe-Ouchi, A.: Large-scale features and evaluation of the PMIP4-CMIP6 midHolocene simulations, Clim. Past Discuss.,, in review, 2020.