Simulation of the mid-Pliocene Warm Period using HadGEM3-GC31-LL: Pliocene climate relative to the pre-industrial era, previous model versions, other climate models and proxy data

<p>To better understand the processes contributing to future climate change, palaeoclimate model simulations are an important tool because they allow testing of the models&#8217; ability to simulate very different climates than that of today.&#160; As part of CMIP6/PMIP4, the latest version of the UK&#8217;s physical climate model, HadGEM3-GC31-LL (hereafter, for brevity, HadGEM3), was recently used to simulate the mid-Holocene (~6 ka) and Last Interglacial (~127 ka) simulations and the results were compared to the preindustrial era, previous versions of the same model and proxy data (see Williams et al. 2020, Climate of the Past).&#160; Here, we use the same model to go further back in time, presenting the results from the mid-Pliocene Warm Period (~3.3 to 3 ma, hereafter the &#8220;Pliocene&#8221; for brevity).&#160; This period is of particular interest when it comes to projections of future climate change under various scenarios of CO₂ emissions, because it is the most recent time in Earth&#8217;s history when CO₂ levels were roughly equivalent to today.&#160; In response, albeit due to slower mechanisms than today&#8217;s anthropogenic fossil fuel driven-change, during the Pliocene global mean temperatures were 2-3&#176;C higher than today, more so at the poles.</p><p>&#160;</p><p>Here, we present results from the HadGEM3 Pliocene simulation.&#160; The model is responding to the Pliocene boundary conditions in a manner consistent with current understanding and existing literature.&#160; When compared to the preindustrial era, global mean temperatures are currently ~5&#176;C higher, with the majority of warming coming from high latitudes due to polar amplification from a lack of sea ice.&#160; Relative to other models within the Pliocene Modelling Intercomparison Project (PlioMIP), this is the 2^nd warmest model, with the majority of others only showing up to a 4.5&#176;C increase and many a lot less.&#160; This is consistent with the relatively high sensitivity of HadGEM3, relative to other CMIP6-class models.&#160; When compared to a previous generation of the same UK model, HadCM3, similar patterns of both surface temperature and precipitation changes are shown (relative to preindustrial).&#160; Moreover, when the simulations are compared to proxy data, the results suggest that the HadGEM3 Pliocene simulation is closer to the reconstructions than its predecessor.</p>