On the effect of orbital forcing on mid-Pliocene climate, vegetation and ice sheets

We present results from modelling of the midPliocene warm period (3.3–3 million years ago) using the Earth system model of intermediate complexity CLIMBER2 analysing the effect of changes in boundary conditions as well as of orbital forcing on climate. First we performed equilibrium experiments following the PlioMIP (Pliocene Model Intercomparison Project) protocol with a CO2 concentration of 405 ppm, reconstructed midPliocene orography and vegetation and a present-day orbital configuration. Simulated global Pliocene warming is about 2.5 C, fully consistent with results of atmosphere– ocean general circulation model simulations performed for the same modelling setup. A factor separation analysis attributes 1.5 C warming to CO2, 0.3 C to orography, 0.2 C to ice sheets and 0.4 C to vegetation. Transient simulations for the entire mid-Pliocene warm period with time-dependent orbital forcing as well as interactive ice sheets and vegetation give a global warming varying within the range 1.9–2.8 C. Ice sheet and vegetation feedbacks in synergy act as amplifiers of the orbital forcing, transforming seasonal insolation variations into an annual mean temperature signal. The effect of orbital forcing is more significant at high latitudes, especially during boreal summer, when the warming over land varies in the wide range from 0 to 10 C. The modelled ice-sheet extent and vegetation distribution also show significant temporal variations. Modelled and reconstructed data for Northern Hemisphere sea-surface temperatures and vegetation distribution show the best agreement if the reconstructions are assumed to be representative for the warmest periods during the orbital cycles. This suggests that low-resolution Pliocene palaeoclimate reconstructions can reflect not only the impact of increased CO2 concentrations and topography changes but also the effect of orbital forcing. Therefore, the climate (Earth system) sensitivity estimates from Pliocene reconstructions which do not account for the effect of orbital forcing can be biased toward high values.