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TROPICAL CLIMATE STABILITY AND IMPLICATIONS FOR THE DISTRIBUTION OF LIFE 110 climates were nearly 5°C lower than present-day values (Emiliani, 1970). However, these estimates were followed by a period of active debate over the relative role of temperature variation and ice volume changes in governing oxygen isotopic composition of foraminifera (e.g., Emiliani and Shackleton, 1974), with the general consensus that the signal was dominated by ice volume changes. CLIMAP (1981) reconstructions for 18,000 yr ago give tropical temperature values approximately 1 to 2°C lower than present-day values in most areas. The argument may still be unsettled, however; Rind and Peteet (1985) suggest that much cooler tropical temperatures are required to explain the snowline in low latitudes during the last glacial maximum. Rind and Peteet (1985) suggest that a 3 to 4°C lower tropical temperature in comparison with the present day would represent a better fit of the terrestrial observations. Without critique, oxygen isotopic data suggest that tropical sea-surface temperatures have varied at least within ±5°C of present-day values. There is, however, substantial room for reinterpretation and debate. ARGUMENTS FOR TROPICAL TEMPERATURE STABILITY Three physically-based arguments, and additional evidence from biota, have been presented for essentially stable tropical temperatures and then utilized to question the validity of the oxygen isotopic analysis. First, the conclusions of Newell et al. (1978) and Newell and Dopplick (1979) using a simple balance between radiative energy input and evaporation have been cited as an argument for tropical temperature stability. This argument is based on the fact that saturation vapor pressure, a measure of the amount of moisture contained within a parcel of air, is a strong nonlinear function of temperature. With increasing temperatures, evaporation should increase substantially, thus limiting any increase in surface temperature. Newell et al. (1978) and Newell and Dopplick (1979) argue that present-day sea-surface temperatures are at a maximum for present-day energy input from the Sun. The problem with the argument presented by these authors is that the role of the atmosphere is basically ignored in their simple surface energy budget model. Higher temperatures and increased moisture should also lead to a greater greenhouse effect, thus promoting surface warming. In every major climate model study in which the atmosphere is included in detail and for which a forcing factor is incorporated that promotes warming (e.g., increased carbon dioxide), the tropical surface temperatures increase (e.g., Washington and Meehl, 1984; Manabe and Bryan, 1985; Hansen et al., 1988; Schlesinger, 1989). These results reflect the increased radiative forcing from carbon dioxide and the importance of water vapor feedback at warmer temperatures. The arguments by Newell and others are not supported by these more comprehensive experiments and should not be a basis for assuming tropical temperature stability. A second major argument centers on whether cool tropical temperatures (e.g., for the Eocene) could be explained plausibly by increased poleward heat transport, as suggested by Shackleton and Boersma (1981) and Barron (1987). Rind and Chandler(1991) and Barron et al. (1993) demonstrate that reasonable changes in ocean poleward heat transport can explain the equator-to-pole surface temperature distribution for time periods of past warm climates such as the Cretaceous or the Eocene. However, using a simple energy balance climate model, Horrell (1990) suggests that the heat transport required to maintain very low tropical temperatures (e.g., 15°C) would be a factor of two or three times the present-day total poleward heat flux. This magnitude of increase would be excessive and unlikely, especially since the Eocene equator-to-pole temperature gradient was small. The question of plausibility in this case depends entirely on the magnitude of the tropical cooling that is proposed. A 3 to 5°C decrease in tropical temperatures in comparison with the present day is quite plausible. Sloan (1990) calculated the total poleward heat transport in an atmospheric general circulation model (GCM), with sea-surface temperatures specified at 3 to 5° C lower than present in the tropics but substantially warmer polar regions in accordance with observations. The atmospheric heat transport in the model decreased from present-day values, as expected because of the decreased temperature gradient from equator to pole and because of the cooler tropical temperatures. The total poleward heat transport increased, which implies a greater role for the ocean in order to achieve warmer poles and cooler tropics. However, the change was not substantial. Covey and Thompson (1989) examined explicitly the role of increased ocean heat flux on the total poleward heat transport and on the latitudinal distribution of surface temperatures. In a case for doubled oceanic poleward heat transport, the total poleward heat transport increased slightly (about 12% at the maximum in midlatitudes), while the role of the atmosphere declined substantially. The tropical sea-surface temperatures decreased by 5°C. Barron et al. (1993a) found similar results (2-3°C decrease) for the Cretaceous for increases of 15 to 30% of observed ocean heat transport. Therefore, relatively small changes in total poleward heat transport can substantially influence tropical sea-surface temperatures. Rind and Chandler (1991) provide a different perspective by calculating the ocean heat transport required to achieve a specific sea-surface temperature distribution. They conclude that perturbations in ocean heat transport,
