SEDIMENTOLOGICAL EVALUATION OF GENERAL-CIRCULATION MODEL SIMULATIONS FOR THE GREENHOUSE EARTH - CRETACEOUS AND JURASSIC

Conceptual climate models, based on the workings of the present-day cl imate system, provided a first-order approach to ancient climate syste ms. They are potentially very subjective in character. Their main draw back was that they involved the relocation of continents beneath a sta ble atmospheric circulation modelled upon that of the present. General circulation models (GCMs) use the laws of physics and an understandin g of past geography to simulate climatic responses. They are objective in character. However, they require super computers to handle vast nu mbers of calculations. Nonetheless it is now possible to compare resul ts from different GCMs for a range of times and over a wide range of p arameterisations. GCMs are currently producing simulated climate predi ctions which compare favourably with the distributions of climatically sensitive facies (e.g. coals, evaporites and palaeosols). They have b een used effectively in the prediction of oceanic upwelling sites and the distribution of petroleum source-rocks and phosphorites. Parameter isation is the main weakness in GCMs (e.g. sea-surface temperature, or ography, cloud behaviour). Sensitivity experiments can be run on GCMs which simulate the effects of Milankovitch forcing and thus provide in sights into possible patterns of climate change both globally and loca lly (i.e. provide predictions that can be evaluated against the rock r ecord). Future use of GCMs could be in the forward modelling of sequen ce stratigraphic evolution and in the prediction of the diagenetic cha racteristics of reservoir units in frontier exploration areas. The sed imentary record provides the only way that GCMs may themselves be eval uated and this is important because these same GCMs are being used cur rently to predict possible changes in future climate.