A climate modeling study was carried out which used boundary conditions created by the USGS PRISM group. The model used for this study is the National Center for Atmospheric Research (NCAR) GENESIS climate model. GENESIS is an atmospheric general circulation model (GCM) that is coupled to a land-surface model and contains submodels for snow and soil. Boundary conditions incorporated into the model which were specific to the middle Pliocene ( ∼ 3 Ma) are (1) present day continent configuration, modified by a 35 m rise in sea level, (2) modified present day elevations, (3) reduced continental ice cover on both Greenland and Antarctica, (4) Pliocene vegetation, and (5) Pliocene sea surface temperatures (SSTs) and sea ice distribution. Atmospheric pCO2 and orbital parameters were set at current values. The greatest influence of the specified warm SSTs upon the resulting climate was occurrence of warmer and more humid high latitudes than observed in the current climate, especially in the winter season. The global mean surface temperature was 3.6°C warmer than in a present day control case, and global mean precipitation increased by 5% relative to the control case, with most excess precipitation occurring over the African continent and the oceans. Zonal winds weakened slightly in the Pliocene case but Hadley cell extent and jet stream locations were unchanged relative to the control case. Model results predict specific areas in which middle Pliocene climate should have differed substantially from the present climate, which can be tested with paleoclimate data.
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