SIMULATING THE MID-PLIOCENE WARM PERIOD WITH CCSM4

We use the Community Climate System Model (CCSM), version 4, to simulate the mid-Pliocene Warm Period (mPWP) as part of the Pliocene Model Intercomparison Project (PlioMIP), using fully-coupled ocean, land, atmosphere and sea ice models at a 1^o ocean and atmosphere resolution. The mid-Pliocene Warm Period (ca. 3.3 to 3.0 Ma) is the last period of sustained warmth before the onset of Pleistocene glaciation. Temperature estimates suggest a 2 to 3 ^oC increase in mean global surface temperature over present day with high latitude temperatures as much as 15 to 20 ^oC warmer than modern. The mPWP is of particular interest because it is also the last prolonged period in Earth history when CO₂ concentrations were similar to present day, and unlike other warm periods in Earth history, we have relatively abundant proxy estimates of both land and ocean temperatures. The enigma of the mPWP is that although continental configurations and ocean bathymetry were close to modern and estimated CO₂ concentrations were only incrementally higher than present day values, proxy evidence suggests a much lower pole-to-equator temperature gradient and a more equable seasonal climate overall. Results from the CCSM4 mPWP simulation show a 1.9^o C increase in global mean annual temperature, with a polar amplification of ~3 times the global warming. Global precipitation increases slightly and monsoonal rainfall is enhanced, particularly in the Northern Hemisphere. Areal sea ice extent decreases in both hemispheres but persists through the summers. CCSM4 underestimates Northern Hemisphere warming relative to ocean temperature proxy reconstructions, particularly in the North Atlantic.