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. Here we present the experimental design and results from a new mid-Pliocenesimulation using the latest version of the UK's physical climate model,HadGEM3-GC31-LL, conducted under the auspices of CMIP6/PMIP4/PlioMIP2.Although two other palaeoclimate simulations have been recently run usingthis model, they both focused on more recent periods within the Quaternary,and therefore this is the first time this version of the UK model has beenrun this far back in time. The mid-Pliocene Warm Period, ∼3 Ma, is of particular interest because it represents a time period when theEarth was in equilibrium with CO2 concentrations roughly equivalent tothose of today, providing a possible analogue for current and future climate change. The implementation of the Pliocene boundary conditions is firstly describedin detail, based on the PRISM4 dataset, including CO2, ozone,orography, ice mask, lakes, vegetation fractions and vegetation functionaltypes. These were incrementally added into the model, to change from apre-industrial setup to a Pliocene setup. The results of the simulation are then presented, which are firstly comparedwith the model's pre-industrial simulation, secondly with previous versionsof the same model and with available proxy data, and thirdly with all othermodels included in PlioMIP2. Firstly, the comparison with the pre-industrial simulationsuggests that the Pliocene simulation is consistent with currentunderstanding and existing work, showing warmer and wetter conditions, andwith the greatest warming occurring over high-latitude and polar regions.The global mean surface air temperature anomaly at the end of the Pliocenesimulation is 5.1 ∘C, which is the second highest of all modelsincluded in PlioMIP2 and is consistent with the fact that HadGEM3-GC31-LLhas one of the highest Effective Climate Sensitivities of all CMIP6 models.Secondly, the comparison with previous generation models and with proxy data suggests a clear increase in global sea surface temperatures as the model has undergone development. Up to a certain level of warming, this results in a better agreement with available proxy data, and the “sweet spot” appears to be the previous CMIP5 generation of the model, HadGEM2-AO. The mostrecent simulation presented here, however, appears to show poorer agreementwith the proxy data compared with HadGEM2 and may be overly sensitive tothe Pliocene boundary conditions, resulting in a climate that is too warm.Thirdly, the comparison with other models from PlioMIP2 further supportsthis conclusion, with HadGEM3-GC31-LL being one of the warmest and wettestmodels in all of PlioMIP2, and if all the models are ordered according toagreement with proxy data, HadGEM3-GC31-LL ranks approximately halfway amongthem. A caveat to these results is the relatively short run length of thesimulation, meaning the model is not in full equilibrium. Given thecomputational cost of the model it was not possible to run it for a longer period; aGregory plot analysis indicates that had it been allowed to come to fullequilibrium, the final global mean surface temperature could have beenapproximately 1.5 ∘C higher.

Simulation of the mid-Pliocene Warm Period using HadGEM3: experimental design and results from model–model and model–data comparison