Jurassic climates

The geological record shows, compared to the present, that the Jurassic Earth was an exotic world. The climatic processes operating are illustrated by reference to Late Jurassic palaeoclimate reconstructions generated on a computer-based General Circulation Model. Throughout the Period, and indeed through much of the Mesozoic, dense forests grew close to both poles and experienced months-long daylight. Models suggest that they experienced warm summers, and months-long darkness in cold, sometimes snowy, winters. Ocean deeps were warmer, perhaps by 8°C, than the present. Reefs grew 10° of latitude further north and south than at the present time but with corals as a subordinate component. The whole Earth is modelled to have been warmer than now by 5° C to 10° C, causing a higher atmospheric humidity and greatly enhanced hydrological cycle. Modelling also suggests that much of the rainfall was convective in character and focused under the Intertropical Convergence Zone over the oceans, leaving major desert expanses on the continents. There is no direct geological evidence for polar ice sheets which, from model output, are unlikely to have been present for most of the time because of the high summer temperatures in these areas. Oxygen isotopic ratios in marine shells are more negative (i.e. show less 18O enrichment) than during later times, reflecting an absence of major terrestrial ice caps which would have stored water enriched with 16O. Localized mountain glaciers cannot be ruled out, particularly in high altitude southern polar terrains, but there is no convincing evidence of short-term and large-scale eustatic changes associated with major glaciation and deglaciation comparable with those of the Neogene. Model results for the Jurassic do, however, suggest the possibility of upland ice sheets during orbitally induced climatic minima. During the Jurassic, the world was predominantly warm with at least four times the present level of atmospheric CO2, and model outputs for evaporation and precipitation generally conform well with the known distributions of evaporates, calcretes and other climatically sensitive facies.