PAST CLIMATES

TESTING MODELS AGAINST THE GEOLOGICAL RECORD

Paul Markwick, Bruce Sellwood (PRIS)

A wide range of different indicators can be used to estimate climate in the past. For Pre-Pleistocene palaeoclimates (climate prior to about 1.8 million years ago), most climate indicators suggest that it was substantially warmer than present. In particular, for the Mesozoic era (240 to 65 million years ago) and for the Early Eocene (55 million years ago) continental interior temperatures are estimated to be above freezing for the whole year. Mean annual temperatures (MAT) are 10 deg C or more warmer than present. There were little or no permanent ice sheets at either pole. Carbon dioxide concentrations were higher than the present, perhaps by as much as 8 times.

Such a radically different climate to today provides a dramatic test of climate models. Results from the UGAMP GCM suggests that there are some substantial disagreements between the models and the data. Current work is investigating whether a better representation of vegetation and/or ocean circulation can explain these discrepancies.

ABOVE: Model simulation of Early Eocene mean annual temperatures (MAT). Thick red line is the 16ƒC isotherm, which represents the limit of modern crocodilian tolerances. White circles are fossil crocodilian localities. Note that most lie within the 16ƒC limits predicted by the model, which suggests that the model is predicting MAT relatively well. However, confidence in such a conclusion requires confirmation from additional geological climate proxies. Contour interval is 2ƒC.

For futher information on the geological record of palaeoclimate see Paul Markwick's homepage.