CMIP6/PMIP4 simulations of the mid-Holocene and Last Interglacial using HadGEM3: comparison to the pre-industrial era, previous model versions and proxy data
Authors: Williams Charles J. R. , Guarino Maria-Vittoria , Capron Emilie , Malmierca-Vallet Irene , Singarayer Joy S. , Sime Louise C. , Lunt Daniel J. , Valdes Paul J. 
Published: August 2020

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