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The simulations of the Middle Holocene climate can be compared with previous PMIP results: Although more recent models have a better representation of the pre-industrial climate, they do not yet fully agree with reconstructions from climate archives, especially on monsoons. In particular, most of the models still do not satisfactorily describe the increase of the African monsoon, characteristic of the "humid" Sahara during this period.
This work underlines the interest of evaluating the ability of models to simulate a past climate change, and not only the current or pre-industrial climate.
The climate modeling of the last interglacial is a novelty for PMIP. Depending on the differences in incident solar radiation at the top of the atmosphere, the amplification of the seasonal thermal and monsoon cycle in the Northern Hemisphere may be greater than in the Middle Holocene. The comparison between models and reconstructions is this time globally satisfactory, even if some local characteristics, such as a cooling in the Labrador Sea or along Greenland, are not reproduced by the models.
The climate of this period is characterized by a significant summer melting of the Arctic ice pack, and even, for some models, by an almost total melting in summer. The diversity of the models' responses can probably be explained by the choice of parameters describing the clouds and the albedo of the pack ice.
According to the researchers, these results are close to the projections made with the same models, in a scenario where the atmospheric concentration of CO2 increases by 1% per year from a pre-industrial concentration. In fact, the expanses of summer sea ice obtained by the different models at the time of the doubling of CO2 content (1950-1970) vary (linearly) as those obtained for the last interglacial. This work also shows here the potential of model evaluation, thanks to past climate simulations and reconstructions.
For the pack ice, the current reconstructions are not yet precise enough to identify the most realistic models, but the results obtained show how useful these data would be...
PMIP and CMIP for predicting future climates
Launched almost 30 years ago, following AMIP (Atmospheric Modelling Intercomparison Project), the PMIP project coordinates simulations of key past climates in order to understand their differences from the current climate and to evaluate the models used to predict future climates.
PMIP feeds into CMIP (Coupled Model Intercomparison Project), which brings together simulations of current climate, past climates and future scenarios to understand current climate change, evaluate climate models and address future climate.
The objective of the 4th phase of PMIP (PMIP4) is to coordinate the paleoclimatic simulations of the 6th phase of CMIP (CMIP6).