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

摘要

We present results from modelling of the mid-Pliocene warm period (3.3–3million years ago) using the Earth system model of intermediate complexityCLIMBER-2 analysing the effect of changes in boundary conditions as well asof 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 mid-Plioceneorography and vegetation and a present-day orbital configuration. Simulatedglobal Pliocene warming is about 2.5 °C, fully consistent withresults of atmosphere–ocean general circulation model simulations performedfor the same modelling setup. A factor separation analysis attributes1.5 °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 withtime-dependent orbital forcing as well as interactive ice sheets andvegetation give a global warming varying within the range1.9–2.8 °C. Ice sheet and vegetation feedbacks in synergy act asamplifiers of the orbital forcing, transforming seasonal insolationvariations into an annual mean temperature signal. The effect of orbitalforcing is more significant at high latitudes, especially during borealsummer, when the warming over land varies in the wide range from0 to 10 °C. The modelled ice-sheet extent and vegetation distributionalso show significant temporal variations. Modelled and reconstructed datafor Northern Hemisphere sea-surface temperatures and vegetation distributionshow the best agreement if the reconstructions are assumed to berepresentative for the warmest periods during the orbital cycles. Thissuggests that low-resolution Pliocene palaeoclimate reconstructions canreflect not only the impact of increased CO₂ concentrations andtopography changes but also the effect of orbital forcing. Therefore, theclimate (Earth system) sensitivity estimates from Pliocene reconstructionswhich do not account for the effect of orbital forcing can be biased towardhigh values.