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TROPICAL CLIMATE STABILITY AND IMPLICATIONS FOR THE DISTRIBUTION OF LIFE 115 and the more comprehensive analysis of Kauffman and Johnson (1988) that the rise of rudist-dominated reefs corresponded to increased tropical warmth and higher salinities. The rise of rudist-dominated reefs may well be one example of the response of tropical organisms to global warmth and increased tropical sea-surface temperatures and changes in surface salinity. DISCUSSION AND CONCLUSIONS Three major conclusions can be derived from the lines of evidence presented in this study. First, substantial variation in tropical temperatures and salinities during Earth history is plausible. Oxygen isotopic data, climate model studies, ocean heat transport experiments, and biologic data support a range of variation within 3 to 5Â°C of the present-day surface temperatures. Limited ocean model studies further suggest that salinities differing by several parts per thousand from present-day values are also reasonable. Although all the sources of data are characterized by uncertainty, and the individual sources of data are probably insufficient to describe these variations quantitatively through time, the data are sufficient to conclude that the tropics are sensitive to global change. Second, the variations in temperature and salinity are large enough to have a substantial impact on tropical organisms. This conclusion is based on relatively limited experiments on the temperature tolerances of living organisms. The case study of the changes in reef communities from coral-dominated to rudist bivalve-dominated during the Cretaceous is perhaps one major example of the tropical response to global warming. Importantly, the emersion experiments on corals are likely to overestimate the optimum environmental range of tropical marine organisms, and such experiments fail to take into account changes in competitive advantage with changes in environmental conditions. More research within this area is required to be able to assess tropical response to global change. Third, greater study of the tropics and tropical biota in Earth history may well yield substantial additional insights into global change research. The sensitivity of the tropics to external forcing factors is a subject of considerable debate, with as yet few data to verify or validate the model simulations. The rise of rudist-dominated reefs is likely to be just one example of the response of tropical organisms to change. The geologic record contains a wealth of other case studies of tropical changes. The mid-Cretaceous case study also suggests that the fabric of the biologic changes contains much more information than evidence of warmth. For example, the fact that corals and rudists were able to compete or coexist at some localities (Scott et al., 1990) has a number of additional implications. First, this may suggest that the temperatures and salinities did not exceed the limit of corals, but rather were likely to be outside the optimum conditions. Second, the model simulations and knowledge of the nature of environments suggest substantial spatial variations in temperature and salinity. Global warming may well cause restricted tropical regions to exceed temperature and salinity tolerances more readily than open ocean regions. Given the large temperature changes with depth, the changes in coral versus rudistid dominance may well exhibit a depth control. Further, the climate model simulations do not suggest a simple belt of above-optimum salinities and temperatures. Rather there is considerable spatial structure, with some areas exceeding only the temperature optimum, others exceeding only the salinity optimum, and still others exceeding both temperature and salinity optima. There may well be substantial structure in the rudist and coral communities that can be tied to the spatial characteristics of the model results. Finally, there are large differences between the Cretaceous and the Cretaceous simulation with high carbon dioxide. The degree of warming, the mechanism of warming, and the history of global warmth throughout the Cretaceous may well be described within the changes in tropical communities during this period. The generally held view that the tropics are an environment in which the physicochemical constraints are not undergoing major changes should not be translated to a view that the tropics are stable to external forcing factors (e.g., increases in atmospheric carbon dioxide). Because of the narrow environmental tolerances of many tropical organisms, tropical biota may be very sensitive to global change. 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