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6
Tropical Climate Stability and Implications for the Distribution of Life

ERIC J. BARRON

The Pennsylvania State University

ABSTRACT

The tropics are generally viewed as an environment in which the physicochemical factors are not undergoing major changes, and therefore the biotic composition and character are defined largely by biological competition. However, many equatorial species are also characterized by narrow environmental tolerances, which suggests that relatively small climate changes may result in a substantial biologic response. The climatic stability of the tropics is therefore a central issue in global change research. Evidence from the geologic record and from climate models suggests that a temperature variation of 3 to 5°C and a salinity variation of several parts per thousand from present values are plausible. The key question becomes the significance of changes of this magnitude for the distribution and character of tropical life. Data on the tolerances of tropical organisms and a case study for the mid-Cretaceous indicate that climate change may substantially influence tropical life. The changes in tropical organisms and their distribution through Earth history should be viewed as a rich, underutilized record that can provide new insights into the climate sensitivity of the tropics. This record provides the only major source of data on the biologic response to global change.

INTRODUCTION

The tropical biological environment is strongly associated with the notion of physical and chemical stability. However, there is also abundant evidence indicating that climate is a significant limiting factor in the distribution of life (e.g., Valentine, 1973; Stanley, 1984a,b). Tropical organisms may be sensitive to climate, in particular, because of their narrow environmental tolerances. Even small climate changes in the tropics can have a substantial impact on life. The question of tropical climate stability with respect to future global change therefore becomes a central issue of research (Crowley, 1991).

Oxygen isotopic paleotemperatures (Douglas and Savin, 1975; Savin, 1977; Shackleton, 1984) can be interpreted as evidence for large variations in tropical temperatures. However, these interpretations have been questioned by Matthews and Poore (1980) and Horrell (1990). Some



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Effects of Past Global Change on Life 6 Tropical Climate Stability and Implications for the Distribution of Life ERIC J. BARRON The Pennsylvania State University ABSTRACT The tropics are generally viewed as an environment in which the physicochemical factors are not undergoing major changes, and therefore the biotic composition and character are defined largely by biological competition. However, many equatorial species are also characterized by narrow environmental tolerances, which suggests that relatively small climate changes may result in a substantial biologic response. The climatic stability of the tropics is therefore a central issue in global change research. Evidence from the geologic record and from climate models suggests that a temperature variation of 3 to 5°C and a salinity variation of several parts per thousand from present values are plausible. The key question becomes the significance of changes of this magnitude for the distribution and character of tropical life. Data on the tolerances of tropical organisms and a case study for the mid-Cretaceous indicate that climate change may substantially influence tropical life. The changes in tropical organisms and their distribution through Earth history should be viewed as a rich, underutilized record that can provide new insights into the climate sensitivity of the tropics. This record provides the only major source of data on the biologic response to global change. INTRODUCTION The tropical biological environment is strongly associated with the notion of physical and chemical stability. However, there is also abundant evidence indicating that climate is a significant limiting factor in the distribution of life (e.g., Valentine, 1973; Stanley, 1984a,b). Tropical organisms may be sensitive to climate, in particular, because of their narrow environmental tolerances. Even small climate changes in the tropics can have a substantial impact on life. The question of tropical climate stability with respect to future global change therefore becomes a central issue of research (Crowley, 1991). Oxygen isotopic paleotemperatures (Douglas and Savin, 1975; Savin, 1977; Shackleton, 1984) can be interpreted as evidence for large variations in tropical temperatures. However, these interpretations have been questioned by Matthews and Poore (1980) and Horrell (1990). Some

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Effects of Past Global Change on Life simple physical arguments involving changes in evaporative cooling with warming (Newell et al., 1978; Newell and Dopplick, 1979) and questions on the mechanisms of tropical temperature change (e.g., Horrell, 1990) have also been utilized to support the notion of tropical temperature stability. However, a number of more comprehensive model experiments suggest that variations within a limited, but significant, range cannot be ruled out (Washington and Meehl, 1984; Manabe and Bryan, 1985; Hansen et al., 1988; Schlesinger, 1989). In combination, the isotopic data and the model studies support the hypothesis that past climate changes should have had a substantial impact on the character and distribution of life within the tropics. The climate stability of the tropics and its implications for the distribution and character of life are addressed here by (1) consideration of the oxygen isotopic records of low latitude temperature variations; (2) discussion of the physical arguments for temperature stability within the tropics; (3) examination of climate model-derived tropical temperatures; (4) examination of model evidence for tropical salinity differences between different time periods in Earth history; (5) consideration of the climate tolerances of tropical organisms; and (6) consideration of a mid-Cretaceous case study in which simulated climate changes in the tropics can be compared with the biological record. The primary conclusions are that (1) throughout Earth history there has been significant variation in tropical temperature (3 to 5°C differences from the present day) and salinity (several parts per thousand); (2) these variations are large enough to have substantial impact on life; and (3) greater study of the geologic record within the tropics will yield important insights into climate sensitivity and into the biologic response to global change. OXYGEN ISOTOPIC RECORDS OF LOW LATITUDE TEMPERATURES The oxygen isotope method of determining paleotemperatures has been widely utilized to study the Cenozoic and the Cretaceous (Savin, 1977). These paleotemperature determinations for the tropics suggest substantial variation. Isotopic measurements on apparently unaltered planktonic foraminifera from the Shatsky Rise, near the equator during the mid-Cretaceous, yield temperature values of 25 to 27°C (Douglas and Savin, 1975), if an ice-free Earth is assumed. These values can be taken at face value and used to indicate little change in tropical temperatures or slightly lower temperatures than are present (e.g., Horrell, 1990). However, several factors (regional variations, habitat, and selective preservation) must be considered in interpreting isotopic measurements on planktonic foraminifera. First, the isotopically lightest measurement (27°C) is likely to represent the shallowest dwelling foraminifera. Even present day shallow-dwelling foraminifera give isotopic temperatures that are 3 to 5°C cooler than the surface. Further, selective dissolution of the more fragile, shallow-dwelling forms tends to bias estimates in the cold direction (Savin et al., 1975). Consequently, a reasonable interpretation of the isotopic data within the Cretaceous tropics is surface temperatures of 27 to 32°C. The range of possible interpretation is from similar to the present day (28°C) to several degrees higher than at present (Figure 6.1). Pre-Pleistocene Cenozoic isotopic temperatures are also substantially different from the present day. Shackleton (1984) presents data yielding isotopic paleotemperatures as low as 18°C for the low latitude Pacific from the Maastrichtian to the Late Miocene. Values similar to the present day occurred only in the late Neogene. Early Eocene and Early Miocene values represent tropical ocean sea-surface temperature minima in the Shackleton (1984) analysis. The Early Eocene low-temperature values have received particular attention (Shackleton and Boersma, 1981). Recent synthesis and interpretation of these and other isotopic data (Sloan, 1990) suggest that at a maximum, Early Eocene tropical surface temperatures were about 24°C, about 3 to 5°C lower than present values. Analysis of tropical sea-surface temperatures during the last glacial maximum also contributes to the notion of tropical temperature variation. Early estimates of tropical sea-surface temperatures from oxygen isotopes for ice age FIGURE 6.1 Cretaceous mean annual temperature limits in comparision with modern values (Barron, 1983). Some of the major constraints based on oxygen isotopes(benthic and plankontic foraminifera and bellemnites), reef distribution, and the absence of permanent ice. Solid line is "warmest" Cretaceous, dotted line is "coolest" Cretaceous, and dot-dashed line is present day.

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Effects of Past Global Change on Life 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,

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Effects of Past Global Change on Life if strong enough to alter sea-ice distributions, may also be self-sustaining in terms of radiative balance. Global warming of near 6°C could be achieved by about 50% increases in poleward ocean heat transport. A greater role for the oceanic thermohaline circulation may be a plausible mechanism for an increased role of the oceans, because of their heat capacity. Today, deep water forms in geographically restricted regions, for which a density contrast from the main ocean occurs through interaction with the atmosphere. The source of deep water depends on the buoyancy flux (Brass et al., 1982), which is a function of the density contrast and the volume flux. Brass et al. (1982) demonstrate the plausibility of a thermohaline circulation very different from today, including the potential for subtropical deep water formation. Interestingly, ocean GCM simulations completed for several periods during the Cenozoic predict warm saline deep water formed within the subtropics during the Eocene (Barron and Peterson, 1990). Modeling work by Ogelsby and Saltzman (1990) gives added support to the concept of warm salty bottom-water formation. Given the large volume fluxes of the deep circulation in the ocean and the evidence for the possibility of warm saline deep water formation, the thermohaline circulation is a plausible candidate for an increased role by the oceans in poleward heat transport. Much additional research is required to examine the potential role of changes in the thermohaline circulation. In summary, the calculation of Horrell (1990) may render unlikely scenarios in which tropical ocean sea-surface temperatures are as low as 15°C. This conclusion is also supported by the recent analysis of Crowley (1991). However, decreases in tropical sea-surface temperatures of 3 to 5°C are plausible. Third, Matthews and Poore (1980) argue, in part based on the conclusions of Newell and others described above, that the low latitude surface ocean had a stable temperature very close to present-day values. The difference between the isotopic temperature and the actual temperature is explained by changes in the oxygen isotopic composition of the oceans due to the storage of isotopically light waters as snow and ice in ice caps. The argument of Matthews and Poore (1980) essentially proposes extensive ice on Antarctica during much of the Cenozoic. The occurrence of ice on Antarctica is a matter of substantial debate, however, and Shackleton (1984) takes exception to the interpretation that the difference in tropical values is due to ice volume. Shackleton points out that such an interpretation would require more ice on Antarctica during the warm middle Miocene interval than today and would also require very large fluctuations in ice volume in the Early Miocene. Such a result appears unlikely and is unsubstantiated. The arguments for Cenozoic tropical sea-surface temperatures near 15°C have also been challenged by Adams et al. (1990), based on biotic evidence. Adams and others note that the existence of Eocene mangroves, corals with zooxanthellae, and larger foraminifera preclude such low temperatures. These forms have minimum temperature limits closer to 18 to 20°C. If the temperature tolerances estimated for these organisms are accurate, then very cool tropical temperatures can be rejected. However, temperature variations within 5°C of modern values cannot be eliminated by these data. In conclusion, none of the discussions presented above provides convincing arguments for tropical temperature stability, only limits to temperature variation. Tropical temperatures near 15°C can probably be rejected based on heat transport arguments and the biotic composition within the tropics during the Cenozoic. However, tropical temperature variations within 3 to 5°C of present-day values are not eliminated by any of the physical or biological arguments proposed to date. MODEL-DERIVED TROPICAL TEMPERATURES Interestingly, much of the emphasis on the interpretation of tropical temperatures and the evaluation of estimates using geologic data described above has focused on cases in which tropical temperatures may have been lower than at present. Unfortunately, there is a notable failure of atmospheric GCMs to simulate tropical climates with lower temperatures than the present day (Barron, 1987). If the conclusions from the experiments of Covey and Thompson (1989) are correct, and an increased role by the ocean in poleward heat transport is required to achieve cooler tropical temperatures, then this problem is explained by the lack of an explicit ocean formulation in current atmospheric climate models. At present, the debate over tropical cooler temperatures cannot be addressed explicitly by current climate models. The results from ocean GCM experiments (Barron and Peterson, 1991) for an ocean driven by an Eocene atmospheric simulation in an uncoupled mode, which produced deep water within the subtropics, are suggestive of a different role for the oceans. However, to date, climate models have not simulated reduced tropical sea-surface temperatures based solely on physical processes incorporated within the model. The conditions for reduced tropical temperatures in models remain problematic. The prospect of higher tropical ocean surface temperatures during warm climates, perhaps the most interesting case for future global change projections, has received much less attention than the "cool" tropics cases. Barron and Washington (1985) noted that higher carbon dioxide climates proposed for the mid-Cretaceous might result in

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Effects of Past Global Change on Life 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 4°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 3°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 5°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 4°C for 4´ 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 2°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 5°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-

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Effects of Past Global Change on Life FIGURE 6.3 Simulated surface salinity (%0) with contour interval of 1%0: (a) mid-Cretaceous, minimum 34.5%0; (b) Paleocene, minimum 32.5%0; (c) Eocene, minimum 33.5%0; (d) Miocene, minimum ,<32.5%0; (e) present day, minimum <32.5%0. ticularly interesting for global change studies and for considering the tropical response to increased levels of carbon dioxide in the near future. The key question becomes the significance of increases of 3 to 5°C in surface temperature and of several parts per thousand in salinity on the distribution and character of tropical organisms. CLIMATE TOLERANCES OF TROPICAL ORGANISMS Abundant evidence exists demonstrating that climate is a significant limiting factor in the distributions of organisms (e.g., Valentine, 1973; Stanley, 1984a,b). The tropics are generally viewed as an environment in which the physicochemical constraints are not undergoing major changes, resulting in an environment that is limited largely by biological competition. However, many equatorial species are also characterized by narrow environmental tolerances, suggesting that relatively small temperature and salinity changes could result in a substantial biologic response. Stanley (1988) suggests that tropical cooling could result in extinction if the temperature decrease removed the warm climate conditions required by many tropical organisms. In the case of hermatypic coral diver-

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Effects of Past Global Change on Life sity, Stehli and Wells (1971) describe the importance of tropical temperature by noting the decrease in diversity with temperature from 50 genera at a mean temperature of 27°C to half that number at only 24°C. Tropical warming, on the other hand, might expand the latitudinal range of some organisms or displace organisms outside the tropics if some upper limit in temperature or salinity were exceeded. Quantitative data on tolerances of organisms from the geologic record are obviously very limited. Experiments with modern organisms provide some guides. Vaughn and Wells (1943) performed a number of tank experiments on hermatypic corals for different temperatures and salinities. Tolerance limits were defined by evidence for notable damage or death due to exposure of only 12 to 24 hr. The optimum temperature range for corals was found to be 25 to 30°C, with minimum and maximum tolerances of 18 to 20°C and 35 to 37°C, respectively. The optimum salinity for growth was found to be 36%o, with minimum and maximum tolerances of 17 to 28%o and 40 to 48%o, respectively. Since, the minimum and maximum tolerances were defined by damage over a very short time (24 hr), logically longer exposures could well result in narrower temperature limits or competitive disadvantage. Knowledge of organism tolerances provides crucial data for examining the question of tropical temperature variation. A MID-CRETACEOUS CASE STUDY Four lines of evidence—isotopic paleotemperatures, climate model results, the distribution of climate-sensitive organisms, and quantitative estimates of tropical tolerances—provide the basis for case studies of tropical climate sensitivity and the response of organisms during Earth history. From the viewpoint of both the availability of all lines of evidence and the interest in warm geologic climates for future global change considerations, the mid-Cretaceous is an interesting and valuable case study. Mid-Cretaceous atmospheric GCM experiments have been completed for Cretaceous geography with the present level of atmospheric CO2 and Cretaceous geography with four times the present-day levels of atmospheric CO2 (Barron and Washington, 1984). These atmospheric simulations were utilized to drive an ocean GCM (Barron and Peterson, 1990) that simulates surface ocean temperatures and salinities. Figure 6.4 illustrates the tropical areas that exceed the temperature optimum (30°C) and the salinity optimum (37%o) for each of the two simulations. A substantial portion of the Tethyan Ocean exceeds the optimum conditions for corals as described by Vaughn and Wells (1943). There are substantial differences between the two simulations. In the high CO2 simulation, the optimum is exceeded over much of the tropical latitudinal band. In the simulation with present-day CO2 levels, the optimum is exceeded largely within Tethys. Figure 6.4 Ocean GCM predictions above the optimum conditions for coral growth for temperature (=30°C) and salinity (=37%o): (a) mid-Cretaceous simulation; (b) mid-Cretaceous simulation with atmospheric CO2  concentrations at four times the present value. The region of warm temperatures and higher salinities corresponds closely to the Supertethys zone of Kauffman and Johnson (1988), which was dominated by rudistid bivalves. The history of reef-building rudists has been documented by numerous authors (Kauffman and Sohl, 1974; Scott, 1988; Kauffman and Johnson, 1988; Scott et al., 1990). During the Early Cretaceous, rudists became important in shallow water communities, tending to be more important in restricted environments such as lagoons and intrashelf basins. From the Hauterivian to the Albian, rudists became increasingly successful as reef dwellers and became the dominant tropical reef organism by the late Albian. Regionally, corals and rudists coexisted (e.g., Texas, Arizona margin of Tethys in the mid-Cretaceous), but rudists displaced corals in large measure. Corals decreased substantially within the tropics, and stromatoporoids almost disappeared. Many hypotheses have been offered, based on both competition and environment, to explain the displacement of corals by rudists. The model results presented here fit well with the speculation by Barron (1983)

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Effects of Past Global Change on Life 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 5°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. For this reason, changes in tropical organisms and their distribution should be viewed as a rich, underutilized record that can provide many new insights into the climate sensitivity of the tropics. This record provides the only major source of data on the biologic response to global change. REFERENCES Adams, C. G., D. E. Lee, and B. R. Rosen (1990). Conflicting isotopic and biotic evidence for tropical sea-surface temperatures during the Tertiary, Palaeogeography, Palaeoclimatology, Palaeoecology 77, 289-313. Barron, E. J. (1983). A warm, equable Cretaceous: The nature of the problem, Earth-Science Reviews 19, 305-338. Barron, E. J. (1987). Eocene equator-to-pole surface ocean temperatures: A significant climate problem? Paleoceanography 2, 729-739. Barron, E. J., and W. H. Peterson (1989). Model simulation of the Cretaceous ocean circulation, Science 244, 684-686. Barron, E. J., and W. H. Peterson (1990). Mid-Cretaceous ocean circulation: Results from model sensitivity studies, Paleoceanography 5(3), 319-337.

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