Simulation of the mid-Pliocene Warm Period using HadGEM3: experimental design and results from model–model and model–data comparison
Authors: Williams Charles J. R. , Sellar Alistair A. , Ren Xin , Haywood Alan M. , Hopcroft Peter , Hunter Stephen J. , Roberts William H. G. , Smith Robin S. , Stone Emma J. , Tindall Julia C. , Lunt Daniel J. 
Published: October 2021

Abstract. Here we present the experimental design and results from a new mid-Pliocene
simulation 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 using
this 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 been
run this far back in time. The mid-Pliocene Warm Period, ∼3 Ma, is of particular interest because it represents a time period when the
Earth was in equilibrium with CO2 concentrations roughly equivalent to
those of today, providing a possible analogue for current and future climate change. The implementation of the Pliocene boundary conditions is firstly described
in detail, based on the PRISM4 dataset, including CO2, ozone,
orography, ice mask, lakes, vegetation fractions and vegetation functional
types. These were incrementally added into the model, to change from a
pre-industrial setup to a Pliocene setup. The results of the simulation are then presented, which are firstly compared
with the model's pre-industrial simulation, secondly with previous versions
of the same model and with available proxy data, and thirdly with all other
models included in PlioMIP2. Firstly, the comparison with the pre-industrial simulation
suggests that the Pliocene simulation is consistent with current
understanding and existing work, showing warmer and wetter conditions, and
with the greatest warming occurring over high-latitude and polar regions.
The global mean surface air temperature anomaly at the end of the Pliocene
simulation is 5.1 ∘C, which is the second highest of all models
included in PlioMIP2 and is consistent with the fact that HadGEM3-GC31-LL
has 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 most
recent simulation presented here, however, appears to show poorer agreement
with the proxy data compared with HadGEM2 and may be overly sensitive to
the Pliocene boundary conditions, resulting in a climate that is too warm.
Thirdly, the comparison with other models from PlioMIP2 further supports
this conclusion, with HadGEM3-GC31-LL being one of the warmest and wettest
models in all of PlioMIP2, and if all the models are ordered according to
agreement with proxy data, HadGEM3-GC31-LL ranks approximately halfway among
them. A caveat to these results is the relatively short run length of the
simulation, meaning the model is not in full equilibrium. Given the
computational cost of the model it was not possible to run it for a longer period; a
Gregory plot analysis indicates that had it been allowed to come to full
equilibrium, the final global mean surface temperature could have been
approximately 1.5 ∘C higher.