Abstract
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.
Highlights
The warm climate production runs were undertaken following the spin-up phase, with the climatology of each simulation being compared to that from the piControl, as well as available proxy data, using either annual means or summer/winter seasonal means. For the latter, depending on the availability of the proxy data, Northern Hemisphere summer is defined as either June–August (JJA) or JAS, and Northern Hemisphere winter is defined as either December–February (DJF) or January–March (JFM) – and vice versa for Southern Hemisphere summer/winter
As an example and for comparative purposes, the same figure but with the data based on the modern calendar is shown in the Supplement (Fig. S5); this suggests that the impact of the calendar adjustments on this field and at this spatial and temporal scale is negligible, with the only obvious impact occurring over the Northern Hemisphere polar regions during JJA in both simulations but more so in the lig127k simulation
This study is novel, being the first time this version of the UK model has been used to conduct any palaeoclimate simulations and being the first time we are in a position to include them as part of the UK’s contribution to CMIP6/PMIP4
Summary
Simulating past climates has been instrumental in improving our understanding of the mechanisms of climate change (e.g. Gates, 1976; Haywood et al, 2016; Jungclaus et al, 2017; Kageyama et al, 2017, 2018; Kohfeld et al, 2013; Lunt et al, 2008; Otto-Bliesner et al, 2017; Ramstein et al, 1997), as well as in identifying and assessing discrepancies in palaeoclimate reconstructions (e.g. Rind and Peteet, 1985). The orbital forcing primarily acts on short-wave radiation, whereas greenhouse gas changes primarily act upon the long-wave radiation flux, and the orbital forcing can lead to uneven horizontal and seasonal changes, whereas greenhouse gas forcing can cause more uniform anomalies (it should be noted that whilst a precise calculation of the radiative forcing due to changes in MH and LIG greenhouse gases is beyond the scope of this study, such a calculation could follow the methodology of Gunnar et al, 1998) Despite these differences in driving mechanism, these past high-latitude (and mainly Northern Hemisphere) warm intervals are a unique opportunity to understand the magnitudes of forcings and feedbacks in the climate system that produce warm interglacial conditions, which can help us understand and constrain future climate projections
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