Sensitivity of the Pliocene Climate to CO2, Orbital Forcing and Vegetation.

The climate of the Late Pliocene (3.60-2.58Ma), has been extensively studied using models and data, as it represents the most recent period in Earth history where CO₂ levels were similar to the present day.   Within this interval, Marine Isotope Stage (MIS) KM5c (~3.205Ma) also had similar to present day orbital configuration, and hence was the subject of the second phase of the Pliocene Modelling Intercomparison Project (PlioMIP2). 

Phase 3 of PlioMIP (PlioMIP3) is now underway and includes a number of sensitivity experiments to assess how the Late Pliocene climate would have been expected to respond to different CO₂ levels (280ppmv, 400ppmv, 490ppmv and 560ppmv), extreme orbits and vegetation.   

Here we will present results from these sensitivity experiments, which have been run using the Hadley Centre Climate Model (HadCM3).  We find that the CO₂  changes have a slightly smaller effect on temperatures when using Late Pliocene boundary conditions, than when using preindustrial, however the differences are region dependent.  For example, Southern Ocean warming due to CO₂ is notably lower with Pliocene boundary conditions than with preindustrial.  This is partly because non-CO₂ Pliocene forcing has already warmed this region significantly, however non-linearities will be discussed.  

Results from the Late Pliocene experiment with a warm northern hemisphere summer orbit, and a warm southern hemisphere summer orbit will also be presented.  This will allow us to assess how temperature and precipitation patterns could have varied throughout the Late Pliocene.   The relative importance of paleogeography changes, CO₂ changes, orbital changes and vegetation changes on Pliocene warming will be analysed.