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TROPICAL CLIMATE STABILITY AND IMPLICATIONS FOR THE DISTRIBUTION OF LIFE 112 tropical temperatures too high for many organisms. Crowley (1991) calls attention to the issue of tropical temperatures during warm climates in general. As stated earlier, a wide variety of atmospheric GCMs (Washington and Meehl, 1984; Manabe and Bryan, 1985; Hansen et al., 1988; Schlesinger, 1989) predict increased tropical sea-surface temperatures for a doubling of carbon dioxide. Schlesinger and Mitchell (1987) illustrate results from models of the National Center for Atmospheric Research (NCAR), the NOAA Geophysical Fluid Dynamics Laboratory, and the Goddard Institute for Space Studies. In each, the doubling of carbon dioxide resulted in a 2 to 4Â°C increase in tropical sea-surface temperatures. In the geologic record, tropical warming may be the product of changes in carbon dioxide (e.g., as proposed by Berner et al., 1983; Berner, 1990), and by changes in geography. Barron and Washington (1985) specifically examine the warmth of the mid-Cretaceous utilizing a version of the NCAR Community Climate Model. The specification of Cretaceous geography without polar ice resulted in a 2 to 3Â°C increase in tropical sea-surface temperatures. However, the global warming was insufficient to explain most of the geologic observations at higher latitudes. The addition of four times the present-day atmospheric carbon dioxide concentration produced a climate with temperatures high enough to satisfy most geologic observations. In this case, tropical sea surface temperatures are more than 5Â°C higher than present-day values (Figure 6.2). Similar experiments with a full seasonal cycle using the GENESIS GCM also produced tropical temperature increases of 3 to 4Â°C for 4Â´ present day CO2 (Barron et al., 1993b). These model predictions for the mid-Cretaceous are within the interpretations proposed based on the oxygen isotopic data. Although still limited in scope, the results from comprehensive climate models supported by the oxygen isotopic data provide the best case for a working hypothesis on tropical temperature variation during Earth history. Figure 6.2 Cretaceous zonally averaged surface temperature (K) limits in comparison with Cretaceous modelderived surface temperatures for the geography and geography plus CO2 quadrupling experiments EVIDENCE FOR TROPICAL SALINITY DIFFERENCES Much of the discussion of tropical climates has centered on temperature analyses. However, salinity is also a major control on the distribution of organisms. Unfortunately, little or no information on salinity has been derived from either geochemical or biological paleoclimatic indices. Only recently (Barron and Peterson, 1989; 1990) have ocean GCM been utilized to derive ocean salinity maps for different periods in Earth history that provide a basis for examining the potential importance of salinity variations. Figure 6.3 illustrates salinity predictions for the mid-Cretaceous, Paleocene, Eocene, Miocene, and Present day continental geometries utilizing the ocean GCM. Substantial ranges in salinity are projected, largely as a result of changes in the area of the oceans within the subtropical arid zone, the restriction of the tropical and subtropical basins and the degree of warmth. In the Eocene and the mid-Cretaceous, salinity predictions for substantial areas of the subtropics exceed 38 parts per thousand (%o) and a range of several parts per thousand is evident within the tropics throughout the Cenozoic. The results from the ocean GCM studies are highly preliminary, but suggest that large salinity variations are also plausible in response to climate and geographic changes. The salinity variations projected are sufficient to influence the distribution of organisms. Interestingly, the high salinities for some time periods (e.g., the Eocene) would also serve to increase the isotopic temperature for the tropics by approximately 2Â°C (J. Zachos and L. Sloan, personal communication). SUMMARY OF TROPICAL CLIMATE EXTREMES In summary, a combination of model sensitivity studies and isotopic temperature analyses supports the conclusion that the tropics have been subjected to substantial climatic variation during Earth history. A temperature range of 3 to 5Â°C and a salinity range of several parts per thousand are reasonable hypotheses for variation within the tropics during the Mesozoic and Cenozoic. The case for warmer, and potentially more saline, tropical and subtropical oceans presents interesting prospects for biogeography and the response of tropical organisms to global warming. This case, perhaps exemplified by the mid-Cretaceous, is par