Results from new simulations of the Early Jurassic climate show that increased ocean heat transport may have been the primary force generating warmer climates during the past 180 m.y. The simulations, conducted using the general circulation model (GCM) at the Goddard Institute for Space Studies, include realistic representations of paleocontinental distribution, topography, epeiric seas, and vegetation, in order to facilitate comparisons between model results and paleoclimate data. Three major features of the simulated Early Jurassic climate include the following. (1) A global warming, compared to the present, of 5-degrees-C to 10-degrees-C, with temperature increases at high latitudes five times this global average. Average summer temperatures exceed 35-degrees-C in low-latitude regions of western Pangaea where eolian sandstones testify to the presence of vast deserts. (2) Simulated precipitation and evaporation patterns agree closely with the moisture distribution interpreted from evaporites, and coal deposits. High rainfall rates are associated primarily with monsoons that originate over the warm Tethys Ocean. Unlike the "megamonsoons" proposed in previous studies, these systems are found to be associated with localized pressure cells whose positions are controlled by topography and coastal geography. (3) Decreases in planetary albedo, occurring because of reductions in sea ice, snow cover, and low clouds, and increases in atmospheric water vapor are the positive climate feedbacks that amplify the global warming.Similar to other Mesozoic climate simulations, our model finds that large seasonal temperature fluctuations occurred over mid- and high-latitude continental interiors, refuting paleoclimate evidence that suggests more equable conditions. Sensitivity experiments suggest that some combination of ocean heat transport increase, high levels of CO2, and improved modeling of ground hydrological schemes may lead to a better match with the geologic record. We speculate, also, that the record itself is biased toward "equable" climatic conditions, a suggestion that may be tested by comparing GCM results with more detailed phytogeographic analyses.
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