Pliocene climates: the nature of the problem

The Pliocene may have been the last time period when global temperatures were greater than present. The Pliocene data base is sufficiently complete to provide a valuable test for climate model predictions for warm time periods. This paper reviews some key issues with respect to understanding and verifying theories for the origin of Pliocene warmth. There are two main factors cited to explain Pliocene warmth—higher CO₂ levels or higher levels of ocean heat transport. The two explanations may not be exclusive; for example CO₂ increases may drive ocean heat transport changes. However, initial proxy-CO₂ reconstructions suggest that the CO₂ perturbation is very small (~100 ppm) to effect such large changes in climate. Considerably more data are needed to evaluate the magnitude of Pliocene CO₂ changes. Although more modeling work is also required, it is necessary to continue evaluation of input data used to force model results (particularly sea surface temperatures). Continued simulations and evaluation of pCO₂ and SST results should enable us to better understand how the earth responds during a global warming and determine whether our models are properly simulating such changes.