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. Palaeoclimate model simulations are an important tool to improve ourunderstanding of the mechanisms of climate change. These simulations alsoprovide tests of the ability of models to simulate climates very differentto today. Here we present the results from two brand-new simulations usingthe latest version of the UK's physical climate model, HadGEM3-GC3.1; they are themid-Holocene ( ∼6  ka) and Last Interglacial ( ∼127  ka) simulations, both conducted under the auspices of CMIP6/PMIP4. Thisis the first time this version of the UK model has been used to conductpalaeoclimate simulations. These periods are of particular interest to PMIP4because they represent the two most recent warm periods in Earth history,where atmospheric concentration of greenhouse gases and continentalconfiguration are similar to the pre-industrial period but where there weresignificant changes to the Earth's orbital configuration, resulting in avery different seasonal cycle of radiative forcing. Results for these simulations are assessed firstly against the same model'spre-industrial control simulation (a simulation comparison, to describe andunderstand the differences between the pre-industrial – PI – and the two palaeo simulations)and secondly against previous versions of the same model relative tonewly available proxy data (a model–data comparison, to compare allavailable simulations from the same model with proxy data to assess anyimprovements due to model advances). The introduction of this newlyavailable proxy data adds further novelty to this study. Globally, formetrics such as 1.5 m temperature and surface rainfall, whilst both therecent palaeoclimate simulations are mostly capturing the expected sign and,in some places, magnitude of change relative to the pre-industrial, this isgeographically and seasonally dependent. Compared to newly available proxydata (including sea surface temperature – SST – and rainfall) and also incorporating data from previousversions of the model shows that the relative accuracy of the simulationsappears to vary according to metric, proxy reconstruction used forcomparison and geographical location. In some instances, such as meanrainfall in the mid-Holocene, there is a clear and linear improvement,relative to proxy data, from the oldest to the newest generation of themodel. When zooming into northern Africa, a region known to be problematicfor models in terms of rainfall enhancement, the behaviour of the WestAfrican monsoon in both recent palaeoclimate simulations is consistent withcurrent understanding, suggesting a wetter monsoon during the mid-Holoceneand (more so) the Last Interglacial, relative to the pre-industrial era.However, regarding the well-documented “Saharan greening” during themid-Holocene, results here suggest that the most recent version of the UK'sphysical model is still unable to reproduce the increases suggested by proxydata, consistent with all other previous models to date.

climate of the past

CMIP6/PMIP4 simulations of the mid-Holocene and Last Interglacial using HadGEM3: comparison to the pre-industrial era, previous model versions and proxy data