JOINT INVESTIGATIONS OF THE MIDDLE PLIOCENE CLIMATE .2. GISS GCM NORTHERN-HEMISPHERE RESULTS

Marine microfaunal data and terrestrial pollen records indicate that the middle Pliocene (ca. 3 Ma) climate is the most recent period in geologic history with global temperatures nearly as warm as those predicted for the coming century. We used the GISS GCM to examine the Pliocene climate by specifying sea surface temperatures and vegetation distributions derived from U.S.G.S. data sets. The simulation resulted in 1.4 degrees C warming, annually averaged over the Northern Hemisphere. Warming was greatest at high latitudes; consequently, the equator to pole temperature gradient decreased by 11.5 degrees C. Surface air temperature increases were greatest in winter, as decreased snow and sea ice triggered a positive albedo feedback effect. At low latitudes, temperatures were mostly unchanged except for an anomalous 3 degrees C cooling over eastern Africa. This anomaly is supported by palynological data and, in the simulation, was a response to the weakening of the Hadley circulation, which caused subtropical clouds and evapotranspiration rates to increase. Evaporation and precipitation rates decreased over the oceans and the appearance of negative P-E anomalies might have implications for the Pliocene thermohaline circulation. The hydrological cycle intensified over the continents, where annual evaporation, rainfall, and soil moisture all increased. However, simulated summer drought conditions are not corroborated by terrestrial records, pointing to deficiencies in either the model, the boundary conditions, or the terrestrial data interpretations. The Pliocene SST pattern implicates increased ocean heat flux as a component force behind the middle Pliocene warmth, since levels of CO2, large enough to cause the extreme high latitude temperatures, would generate more tropical warming than is indicated by floral and faunal records. Surface energy fluxes, calculated by the GCM, indicate that an increased meridional ocean heat nux of 32% could reproduce the data-derived SST distribution, despite weakened atmospheric transports. The decreased wind stress values suggest that any increase of ocean heat transports would probably have resulted from a strengthening of the thermohaline circulation.