PALAEOCLIMATES 135 approach having been pioneered by John Kutzbach and Eric Barron (the standard version of the Hadley centre OAGCM), which uses a horizontal resolution of 1.25 Â 1.25 General Circulation Models GCMs use the laws of physics and an understanding of past geography to simulate climatic responses They are objective in character They require powerful computers to handle vast numbers of calculations Nevertheless, it is now possible to compare the results of different GCMs for a range of times and over a wide range of parameterizations for the past, present, and future (e.g in terms of predictions of surface air temperature, surface moisture, precipitation, etc.) GCMs are currently producing simulated climate predictions for the Mesozoic and Cenozoic that compare favourably with the distributions of the geological climate proxies discussed above They can be used effectively to predict sites of oceanic upwelling and the distribution of petroleum source rocks and phosphorites Models also produce evaluations of parameters that not leave a geological record (e.g cloud cover, snow cover) and quasi-parametric phenomena such as storminess Parameterization is the main weakness of GCMs (e.g palaeogeography, palaeobathymetry, sea-surface temperature, orography, cloud behaviour), and model output for continental interiors is still colder in winter than indicated by palaeontological data The sedimentary and palaeontological record provides an important way of evaluating GCMs, and this is important because the same GCMs are currently being used to predict possible changes in future climate The outputs discussed below were generated by an AGCM (HadAM3) and an OAGCM (HadCM3L), which is currently state-of-the-art The model was developed at the Hadley Centre for Climate Prediction and Research, which is part of the UK Meteorological Office The GCM consists of a linked atmospheric model, ocean model, and sea-ice model The horizontal resolution of the atmospheric model is 2.5 latitude and 3.75 longitude This provides a grid spacing at the equator of 278 km north–south and 417 km east– west The atmospheric model consists of 19 layers It also includes a radiation scheme that can represent the effects of minor trace gases Its land-surface scheme includes a representation of the freezing and melting of soil moisture The representation of evaporation includes the dependence of stomatal resistance on temperature, vapour pressure, and carbon dioxide concentration There is an adiabatic diffusion scheme, to simulate the horizontal mixing of tracers The ocean model has the same spatial resolution as the atmosphere model and 20 vertical layers, with a time step of 30 This contrasts with HadCM3 Palaeoclimate of the Mesozoic – Model Output and Geological Data The Mesozoic Earth was an alien world, as illustrated here by reference to a Triassic GCM simulation and geological data Throughout the Mesozoic dense forests grew close to both poles, experiencing months of continual daylight in warm summers and months of continual darkness in cold snowy winters Neither Triassic nor Jurassic oceanic sediments provide good evidence, but in the Late Cretaceous, from ODP (Ocean Drilling Program)/DSDP (Deep Sea Drilling Program) data, the ocean depths appear to have been warm (8 C or more at the ocean floor), and reefs grew 10 further north and south than at the present time During the era the whole Earth was warmer than now by at least C, giving more atmospheric humidity and a greatly enhanced hydrological cycle However, from modelling studies, it seems that much of the rainfall could have been predominantly convective in character, often focused over the oceans The model output might help to explain geological data suggesting that major desert expanses extended across the continents in low latitudes From the model, polar ice sheets are unlikely to have been present because of the high summer temperatures The model suggests the possibility of extensive sea ice in the nearly enclosed Arctic seaway through parts of the year, but there is as yet no proxy data against which such predictions may be tested The Triassic world was a predominantly warm world; the model outputs for evaporation and precipitation conform well to the known distributions of evaporites, calcretes, and other climatically sensitive facies Triassic: Comparison of Model and Proxy Data – A Case Study Modelled temperatures (Figure 2) A significant feature of the Triassic Earth is that the landmasses were almost symmetrically distributed in a broad arc about the equator (see Mesozoic: Triassic) A major aspect of the modelled Earth is its overall warmth Despite temperatures plunging to À20 C and below over Siberia in the northern-hemisphere winter and to similarly low values over southernmost Gondwana in the southernhemisphere winter, the annual average temperature is subdued in these high-latitude areas because of the high summer values achieved there (ca 24 C) These high summer values preclude the possibility of yearround ice and snow