The results from this new experiment reveal a middle Pliocene global warming of 2.2°C as compared to the simulated current climate. This is a slight increase over the warming found using the 8° X 10° version of the GCM. Much of the warming occurs near the poles, although temperatures increase across most of the globe. Notable exceptions include the tropics, which show little change in temperature, and East Africa, where elevations were higher in the Pliocene, and thus temperatures cooler. The reduction in elevation over Antarctica (resulting from the shrunken ice sheet) and in the western U. S. give rise to large temperature increases in those regions. Those increases can be shown to be primarily the result of temperature adjustments based on the elevation decrease and the lapse rate. However, some warming can be attributed to an albedo decrease caused by the loss of snow and ice and the encroachment of less reflective Pliocene vegetation types in those regions. For example, note that in northern Canada, the one region where snow cover actually increases in the Pliocene, the warming signal is muted.
The hydrological cycle is somewhat intensified in the Pliocene climate, as would be expected in a warmer climate. The intensification is largely driven by the warmer extratropical sea-surface temperatures, which cause greater evaporation from the oceans. The polar regions are especially wet compared with the current climate, and while most continental regions see moisture increases, the United States, Western Europe, and Southeast Asia are simulated as being drier than today. Throughout tropical regions where near-modern SST's were specified, the hydrological cycle does not intensify, weakening slightly. This has the affect, in combination with the reduced pole-to-equator temperature gradient, of damping the thermally- driven Hadley circulation, thus reducing rainfall in the intertropical convergence zone.
As in previous coarse grid simulations, the basic temperature patterns are supported by the terrestrial (mostly palynological) data. The hydrological patterns, however, despite being consistent between soil moisture, surface runoff and precipitation minus evaporation results, reveal large disparities between model and data estimates for North America and Europe. Although some of this discrepancy arises from the GCM's lack of resolution in high relief terrain, it is likely that new models, with improved clouds, convection, and ground hydrology parameterizations will be required to move forward in this area of comparison. New versions of many of the primary GCMs in the U.S. are poised to appear and most focus on improving their hydrological climatologies. The Pliocene may serve as one climate change testing ground for the next generation GCMs. Stay tuned...