Geological evaluation of multiple general circulation model simulations ofLate Jurassic palaeoclimate

General circulation models (GCMs) are currently used to predict future glob al change. However, the robustness of GCMs should be evaluated by their abi lity to simulate past climate regimes. Their success in 'retrodiction' can then be assessed by reference to the geological record. Geological evidence provides a database that can be used in the estimation of sea surface temp eratures, orography and other proxy data useful in palaeoclimatic studies, These data can then be used to refine the prescribed boundary conditions fo r running GCMs themselves. Results from a series of modelling experiments, run with Late Jurassic (Kimmeridgian) boundary conditions, and using the UK University Global Atmospheric Modelling Programme (UGAMP) GCM and the UK M eteorological Office GCM are presented. Simulations from these two quite in dependently generated models, although subtly different, confirm a generall y warmer Jurassic Earth with arid zones over the Tethys and SW USA, parts o f Gondwana dominated by 'monsoonal' systems and convective rainfall general ly higher over the oceans than at present. Circum-polar wetlands are also i ndicated. These results generally conform well to the distributions of know n facies in these regions. Modelled cloudiness is also higher in the Jurass ic, and although unconfirmed geologically, such conditions would have contr ibuted to greenhouse conditions at high latitudes and could have influenced both terrestrial biomes and marine ecosystems. Using one of the GCMs (UGAM P) we have also investigated the role of orbital parameters for high latitu de climate. At times of 'minimum seasonal forcing' (comparable with an orbi tal geometry affecting the Earth at 115 ka BP) parts of Antarctica could ha ve sustained a modest ice sheet over areas exceeding 1 km elevation, but su ch modelled sheets would have been ephemeral features and very dynamic in c haracter. (C) 2000 Elsevier Science B.V. All rights reserved.