What was done
The authors state that they analyzed "the spatial expression of seasonal climates of the Mediterranean and northern Africa in pre-industrial and mid-Holocene simulations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5)," which they did by comparing the CMIP5 simulations with "modern observations from the CRU TS3.1 data set," and by evaluating "mid-Holocene simulations using quantitative climate reconstructions derived from pollen records."

What was learned
Perez-Sanz et al. report that "the CMIP5 models fail to reproduce key aspects of both the modern and mid-Holocene climate of the northern Africa and Mediterranean region, including [1] the correct geographical location of zonal precipitation regimes in the pre-industrial simulation and [2] the magnitude of mid-Holocene changes in these regimes." More specifically, they say that most models [3] "overestimate the extent of monsoon influence and [4] underestimate the extent of desert." And they state that they also [5] "fail to reproduce the amount of precipitation in each zone," while noting that [6] "most models underestimate the mid-Holocene changes in annual precipitation."

What it means
When all was said and done, the four researchers were forced to acknowledge that "there are still important discrepancies between the simulated and observed magnitude of changes in precipitation, despite the increasing complexity and resolution of the CMIP5 models compared to earlier generations of models." And they state, in the concluding sentence of their paper, that "the failure to simulate observed mid-Holocene changes in the north African monsoon and the potentially linked failure to simulate the observed shift in rainfall seasonality in the Mediterranean raises concerns about the reliability of model projections of future climates in these regions."