Modelling the mid-Pliocene warm period using HadGEM2

Here, for the first time, we present simulations of mid-Pliocene climate using a UK IPCC AR5-class model (HadGEM2). The global annual mean surface air temperature increases by 3.4∘C compared to the preindustrial control, with warming amplified towards the poles. The overall sensitivity of surface air temperature and polar amplification in response to the specification of the Pliocene boundary conditions is greater in HadGEM2 than in a previously utilised UK model (HadCM3). The simulated temperature anomaly is also at the upper range of that produced by the first phase of the Pliocene Model Intercomparison Project ensemble. Energy balance analysis indicates that the polar amplification of the mid-Pliocene warming in HadGEM2 is due to greenhouse gas emissivity changes and surface albedo changes. Approximately 5 × 106km2 of Arctic sea-ice is lost in the HadGEM2 Pliocene simulation and the global precipitation increases by 0.18 mm/day, these anomalies are approximately twice as large as seen in HadCM3. HadGEM2 can retain a much larger amount of soil moisture than HadCM3, such that the amount of evaporation (and precipitation) over the land surface in the mid-Pliocene simulation is not as strongly constrained by water availability. These results highlight the importance of using more recently developed climate and Earth System Models to simulate the past. They further underline that our appreciation of Pliocene climate is model dependant and ultimately limited by our physical understanding of the climate and the way this is represented in models.