During the Pliocene, global temperatures, particularly at high latitudes, are believed to have been significantly warmer than today.
Simulating past warm climates and identifying model/data contrasts for periods such as the Pliocene provide a test of the sensitivity of our primary tool for study future climate change: global climate models. At present, our results do not support the suggestion that Pliocene warming was caused by carbon dioxide increase since such changes are not consistent with the SST distributions derived from deep sea cores. There is evidence that changes in ocean circulation and the amount of heat oceans transport may be one potential cause of the warming.
Still, investigators have found evidence that minor increases in CO2 (up to 380 ppm) did occur in the Pliocene. This causes us to wonder whether it is possible that an, climate feedback, as of yet unknown, associated with small increases in CO2, could lead to the larger changes seen in the ocean circulation? Certainly the evidence for higher levels of CO2 and stronger thermohaline circulation challenges recent results from coupled ocean-atmosphere models, which suggest that thermohaline circulation weakens as global temperature rises. Perhaps the Pliocene warming is uncharacteristic of next century's expected warming, perhaps the causes are different but the effects will be similar, and perhaps the Pliocene is a warning that unkown factors still exist that could exacerbate or mitigate the CO2 increase and global warming.
Successful comparisons, while increasing our confidence in the basic approach, probably occur coincidentally in some cases and such errors would be difficult to identify. Nevertheless, mismatches between data interpretations and model results offer undeniable evidence that either the model, data, or both are innacurrate for a specific region and climate variable. Understanding this allows us to focus resources and efforts on areas that are likely to afford the most gain. Moreover, subsequent iterations, based on new treatments of the data or GCM, test the veracity of previous conclusions.
The GISS Pliocene GCM simulation and the PRISM reconstructions are a first step in the interative process of data collection and analysis, model experimentation and analysis, and data/model comparison; the gridded, boundary condition data sets are continuously being refined, updated, and extended into areas with scarce data. Additional modeling and sensitivity experiments involving new data sets and updated GCM versions will soon begin. Close cooperation between modeling and data groups can achieve an overall better understanding of global climate models, data, data collection and simulation strategies, and the climate changes our society and planet could face relatively soon.