Pliocene climate and the high latitudes: a data/model perspective

It is well established that during intervals of the Pliocene epoch climatic conditions were both warmer and wetter than the pre-industrial era. Since the first global compilations of geological proxy data and climate modelling studies, it has been known that the pattern of surface temperature change during the Pliocene was not spatially uniform, with both geological data and models showing an amplification of surface temperature change at higher latitudes. This is a trait which the Pliocene shares with other warm(er) climate states in Earth history. Over the last two decades our appreciation of the character of climate and environmental change in the high latitudes has evolved significantly with (a) the availability of new and multi-proxy reconstructions, and (b) through the application of different climate, vegetation and ice sheet models, methodologies and intercomparison projects (PlioMIP1 and 2 and PLISMIP). We have become increasingly aware of the complex interaction of different sources of uncertainty (in proxies, models, model boundary conditions and forcings) when assessing the degree to which climate models are able to reproduce the magnitude of climate change indicated by geological data. It is clear that broad and simple assumptions cannot be made regarding the efficacy of either proxy reconstructions or climate model simulations for the Pliocene high latitudes, and that the picture of Pliocene climate at the higher latitudes, and how well models simulate it, is a nuanced one.

Whilst modelling studies have tended to agree in demonstrating the primacy of greenhouse gas forcing on Pliocene warming as a global average, it is increasingly apparent how important other factors such as palaeogeography and ice-sheet reconstructions can be in determining the local and regional pattern of climate change in the high latitudes. Yet, at present many of these aspects remain poorly constrained.

Energy balance analyses have demonstrated the importance of clear sky albedo, cloud albedo and heat transports in determining the degree of warming at the high latitudes in climate models. This has helped to inspire new climate modelling studies using perturbed physics in order to explore model uncertainty. However, our focus has largely been on the assessment of the ability of climate models to simulate mean annual temperature change, rather than the seasonal expression of temperature change. Recent work has demonstrated that a large ensemble of climate models is generally able to simulate warm season temperatures in the high latitudes of the Northern Hemisphere, with the apparent discrepancy in mean annual surface temperatures being driven largely by climate models underestimating the magnitude of warming during the cold season. If this is true, it would place a useful additional constraint on how Pliocene climate simulations need to evolve in order to match proxy reconstructions more closely.