Different Pathways to an Early Eocene Climate.

The early Eocene (approximately 50 million years ago) was characterised by much higher temperatures globally as well as a low equator-to-pole surface temperature gradient. Comprehensive climate models are quite successful in simulating many features of that climate, at least in the annual average. However, good simulations of the seasonal variations, and in particular the much reduced Arctic land temperature seasonality, have proven more difficult. Further, aside from an increased level of greenhouse gases, it remains unclear what the key processes are that give rise to an Eocene climate, and indeed whether there is a unique combination of factors that leads to agreement with available proxy data. In this paper, we use a very flexible General Circulation Model to explore this question, and in particular the sensitivity of the modelled climate to differences in CO2 concentration, land surface properties, ocean heat transport, and cloud extent and thickness. Even in the absence of ice or changes in cloudiness, increasing the CO2 concentration leads to a polar-amplified surface temperature change due to increased water vapour and the lack of convection at high latitudes. Decreasing the surface albedo has a similar impact to increasing CO2, as it also leads to increased water vapour. Prescribing additional low clouds over Arctic land helps to achieve an Eocene climate, as it decreases summer temperature for all simulations and increases winter temperatures at low CO2. Prescribing additional high clouds over the Arctic ocean only has a small impact on Arctic land temperatures. Changes in prescribed ocean heat transport have a very small effect on Arctic temperatures and seasonality. An increase in the land surface heat capacity, which is plausible given large changes in vegetation and landscape, successfully decreases the Arctic land seasonality to values consistent with proxy data. Different combinations of factors -- high CO2 levels, changes in low-level clouds, and an increase in land surface heat capacity -- can therefore lead to a simulation consistent with proxy data.