The hydrologic cycle: A major variable during earth history

Palaeogeography, Palaeoclimatology, Palaevecology (Global and Planetary Change Section), 75 (1989): 157-174

157

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

THE HYDROLOGIC CYCLE" A MAJOR VARIABLE DURING EARTH HISTORY ERIC J. BARRON 1, WILLIAM W. HAY 2 and STARLEY THOMPSON 3 1 Earth System Science Center, The Pennsylvania State University, 512 Deike Building, University Park, PA 16802 (U.S.A.) 2 University of Colorado, Department of Geological Sciences, Boulder, CO 80309 (U.S.A.) a National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307 (U.S.A.) (Received February 8, 1989; revised and accepted May 15, 1989)

Abstract Barron, E.J., Hay, W.W. and Thompson, S., 1989. The hydrologic cycle: a major variable during Earth history. Palaeogeogr., Palaeoclimatol., Palaeoecol. (Global Planet. Change Sect.), 75: 157-174. Water plays a central role in nearly all Earth processes and in the evolution of the planet. However, despite the significance of water, our knowledge of it as part of the global system is meager. In fact, for paleoclimatology the primary focus on planetary evolution is centered on temperature variations and little attention is directed towards the role of the hydrologic cycle. Model analyses presented here based on a series of simulations utilizing the Community Climate Model (CCM) at the National Center for Atmospheric Research demonstrate that the hydrologic cycle is highly sensitive to climate change and to climatic forcing factors such as changes in atmospheric carbon dioxide, plate tectonics, paleogeography, and orbital variations. The implications of the large sensitivity of the hydrologic cycle are of considerable importance. The role of water in explaining much of the Earth's record has probably been underestimated. The importance of water in global change in Earth history may also suggest that the hydrologic cycle should be of primary interest in studies of future global change.

Introduction

Water plays a central role in nearly all Earth processes and in the evolution of the planet. Water is a necessary and limiting ingredient for life. For example, precipitation rate is the major limiting factor for terrestrial productivity (Fogg, 1975). The phases of water have substantial impacts on the Earth's radiation budget (e.g. clouds, snow cover and ice caps), and in the energy transfer between components of the Earth system. The regional balances of evaporation, precipitation, run-off and sea ice also have strong ties to the global heat transport by the oceans (Goody, 1980), to deep ocean convection processes (Killworth, 1982) and to longer-term ocean circulation changes (Rooth, 1982). Water 0921-8181/89/$03.50

© 1989 Elsevier Science Publishers B.V.

is the primary agent of erosion and sediment transport. The solvent properties of water render the water cycle an essential part of chemical reactions and geochemical cycles. Gases dissolved in rain water interact with soils and rocks, liberating mineral nutrients and other elements to solution. Rates of chemical weathering are closely linked to rainfall amount and associated vegetation cover. In fact, changes in precipitation with global warming or cooling are an essential element of geochemical cycle models which propose continental weathering as a major control on atmospheric CO2 levels through time (Berner et al., 1983). Despite the significance of water, our knowledge of it as a part of the global system is meager. This is particularly true in our efforts to

158 reconstruct and to understand the changes recorded in Earth history. The primary quantitative tools in paleoclimatology measure temperature. Vegetation characteristics, continental weathering products, desert sediment features and evaporite minerals (see for example the discussion by Frakes, 1979) are the primary sources of data used to reconstruct precipitation and evaporation patterns for Earth history. However, the interpretations of these data are usually restricted to qualitative determinations of either wetter or drier conditions. Model studies of Earth history have been equally one-sided. With some exceptions (e.g. Cogley and Henderson-Sellers, 1984), climate model studies of the Archean (e.g. HendersonSellers and Meadows, 1977; Owen et al., 1979), the warmth of the Cretaceous (Barton and Washington, 1984; Crowley et al., 1986), or the Last Glacial Maximum of the Pleistocene (e.g. Manabe and Broccoli, 1985) have been directed toward explaining a record of temperature. These model studies provide an interesting perspective. In several of the cases, the climate models appear to lack the sensitivity required to explain the paleoclimatic temperature data given knowledge of the known forcing factors. However, in pioneering efforts to look at the sensitivity of precipitation to climatic forcing (Kutzbach and Guetter, 1984; Kutzbach and Street-Perrott, 1985), and in the model analyses presented here, the hydrologic cycle appears highly sensitive to climate changes. Many of the forcing factors which fail to yield much temperature response in climate models result in substantial precipitation changes. The implications of the large sensitivity of the hydrologic cycle to climate change and to climatic forcing factors are of considerable importance. First, given the central role of water in so many Earth processes, the variability of the hydrologic cycle implies considerable biologic, geochemical and sedimentologic variability. Perhaps, the role of temperature in explaining much of the Earth's record has been overestimated. Second, demonstration of the importance of water in global change in Earth history suggests that the hydrologic cycle should be of primary

interest in studies and predictions of future global change.

Model description The simulations described in this study are based on two versions of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM). The CCM has evolved from the spectral climate model of Bourke et al. (1977) and McAvaney et al. (1978). The model uses a sigma vertical coordinate system with nine levels. The truncation for this spectral model is at wave number 15. The associated Gaussian grid has 40 latitudes ( - 4 . 4 ° resolution) and there are 48 longitudinal grid points 7.5 ° longitudinal resolution). The model includes atmospheric dynamics based on the equations of fluid motion and includes radiative processes. Also included are convective processes, evaporation and condensation as described by Ramanathan et al. (1983) and P i t c h e r et al. (1983). T h e radiation-cloudiness formulation introduced by Ramanathan et al. (1983) is one of the principal advances in this CCM in comparison with earlier versions. The radiation-cloudiness formulation includes an absorptance formulation for CO.~, which is in excellent agreement with observations, and an interactive cloudiness scheme. Convective clouds occur whenever the vertical gradient of equivalent potential temperature is less than zero and the local relative humidity is greater than 80%. If the vertical gradient is greater than or equal to zero the atmosphere is assumed to be stable and nonconvective clouds occur whenever the relative humidity reaches 80%. In both cases precipitation occurs at the same relative humidity threshold that governs cloudiness. Additional details can be found in Ramanathan et al. (1983). A second improvement from earlier model versions is an improved treatment of surface hydrology. The surface hydrologic scheme uses model-derived precipitation and evaporation rates to simulate the change of soil moisture and snow cover as described by Washington and

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Williamson (1977). The field capacity, or saturation point, for soil moisture is assumed to be 15 cm. An excess above the field capacity is considered to be run-off. The two model versions utilized in this study are based on two different formulations for the ocean. Model simulations with annually averaged solar insolation use an energy balance ocean model which lacks ocean heat transport or heat storage. A sea ice layer up to two meters thick forms when the surface ocean temperature falls below 2.0°C with associated changes in albedo and surface energy balance characteristics. This model version is useful for first-order experiments, particularly with the advantage that it comes to equilibrium rather quickly so multiple experiments can be performed. The mean annual version of the CCM does a very good job of simulating many features of the present-day atmosphere, including the latitude-height temperature distribution and the zonal winds (Washington and Meehl, 1983). The second ocean formulation is an energy balance model which includes heat storage. The specification of a 50-m thick mixed layer allows a realistic simulation of the full annual cycle. In this case, sea ice forms and grows when surface temperatures are below -1.2°C. Washington and Meehl (1984) favorably compare the results of seasonal simulations with present-day observations. For the purposes of this paper, the model capability to simulate precipitation is most important. Pitcher et al. (1983) compare CCM simulations for January and July experiments with specified sea surface temperatures. The CCM is successful in simulating many of the general features, including the intense equatorial rainfall belt and mid-latitude maxima. Precipitation associated with storm tracks and with monsoonal circulations are well simulated. The primary difficulty is a precipitation rate in the tropics which is larger than observed. This problem is accentuated in simulations with energy balance oceans (Washington and Meehi, 1984). In other cases, the locations of high rainfall areas are only approximate. Both the model resolution and the averaging time for precipita-

tion, which is characterized by high spatial and temporal variability, may explain many of these deficiences. In addition, the simple scheme for surface hydrology may be limiting. In general, the large scale features and areas of geographically-forced precipitation are well-simulated. The experiments described here should be viewed as sensitivity experiments. The CCM has continued to be the subject of improvements, particularly with regards to the factors which influence the model response to increased levels of carbon dioxide (Washington and Meehl, 1986). The mean annual version of the CCM appears to be less sensitive to changes in carbon dioxide level than the annual cycle version. The experiments are most valuable for a first-order view of the variability and importance of the hydrologic cycle, rather than as a predictive tool. However, comparison with observations from Earth history suggests that the model has a remarkable ability to predict the areas of high rainfall.

Global temperature and the rate of precipitation Paleotemperature data suggest that the Earth has experienced a considerable range of global temperature, even if only the last 100 million years are considered. CLIMAP (1981) data yield a globally averaged surface temperature 3-5°C lower than at present for the Last Glacial Maximum (LGM) and estimates of global warmth for the Cretaceous (65-140 m.y. ago) are in the range of 6-12°C higher than present-day values (Barron, 1983). To a first approximation, global warming or global cooling should result in an increase or a decrease, respectively, in global rates of precipitation. This general statement stems from simple balance requirements and thermodynamic arguments. First, on a very short time scale a balance exists between globally averaged evaporation and globally averaged precipitation. Second, the saturation vapor pressure, the pressure exerted by the water vapor of a saturated air parcel over a planar surface of water, depends only on temperature. The change in saturation vapor pressure above a liquid with

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respect to temperature is given by the ClausiusClapeyron relationship. Basically, the evaporative capacity of warm air is much higher than for cool air and this relationship is strongly non-linear. The changes in evaporation rate as a function of latitude follow from this argument. Consequently, a warmer planet should, all other factors being similar, have an intensified hydrologic cycle.

The expected response from this simple reasoning is evident from four experiments using the mean annual version of the CCM, with two different geographies. These experiments yield nearly a 10 °C range in globally averaged surface temperatures. The four cases illustrated in Fig. 1 are: present geography, present geography with 4 x present CO 2 concentration in the atmosphere, mid-Cretaceous geography (Fig. 2) and mid-Cretaceous geography with 4 x present CO.~ concentration in the atmosphere. A simple, apparently linear, relationship is found between increasing globally-averaged surface temperature and increasing global precipitation. Thus, the relationship between temperature and precipitation is established as a first-order control on variations in the hydrologic cycle during Earth history on the basis of simple qualitative arguments and from model experiments. However, the relationship between global warmth and global precipitation presented in Fig. 1 may have only limited application. First, the differences in geography between the present day and the Cretaceous are insufficient to evaluate whether this relationship is independent of continental position and land-sea distribution. Continents may serve to focus or modify rainfall distribution or may influence global rates. Second, the majority of the biological, sedimentologic and chemical factors or processes of significance for the geosciences are dependent

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on the spatial distribution of rainfall and evaporation. The comparison of global averages given in Fig. 1 masks considerable variability over the globe, and different spatial responses to climate change.

The role of geography in the h y d r o l o g i c cycle The role of geography in the hydrologic cycle is two-fold; land-sea distribution may influence the availability or source of moisture for the atmosphere, and land-sea distribution and topography may influence the distribution of rainfall. Again, simple arguments suffice to define the potential of geographic factors to influence the hydrologic cycle. First, large differences in evaporation rate occur with respect to latitude based on the equator-to-pole surface temperature gradient. Consequently, the global water balance is dominated by the high evaporation rates at low latitudes. This suggests that the latitudinal distribution of land has the potential to modify the global water budget. Second, the annual, zonally averaged precipitation is largely a function of the meridional circulation and vertical motions. Simply, rising air cools along a dry adiabat, reaches saturation and hence precipitation occurs. The annual, zonally averaged precipitation (Fig. 3) reflects the rising part of the tropical Hadley circulation as a maximum, the sinking subtropical circulation as minima, and the secondary meridional circulation and position of cyclonic storm tracks at higher latitudes as mid-latitude maxima. Given

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land area, and (b) two polar continental "caps" with present-day total land area. t h a t the meridional circulation is affected by land-sea distribution and topography, a change in continental distribution will result in a change in the spatial distribution of precipitation (i.e. continental rainfall). The zonal precipitation shown in Fig. 3 may be interrupted in two ways. Within a prevailing wind system a significant topographic expression produces high windward precipitation and a rainshadow in its lee. In addition, land-sea distribution influences atmospheric vertical motions due to differential heating between land and sea. The monsoons and winter storm tracks are such examples of geographic controls on precipitation patterns. Both topography and land-sea configuration are substantial variables in Earth history.

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continental distribution using a mean annual version of the CCM and a suite of idealized continental geometries. These experiments provide an interesting test of the relationship presented in Fig. 1. Two experiments provide extreme limits. In these two experiments the present-day land area was confined to a tropical continent (17 °N to 17 °S) and to two polar land "caps" (90 ° to 45°N, s) as pictured in Fig. 4. The polar continent case has a 4.6°C lower globally averaged surface temperature compared to the equatorial continent case, yet a substantially higher globally averaged precipitation. The results (Fig. 5) are clearly at odds with the simple relationship between global temperature and global precipitation rate given earlier. Figure 6 illustrates the distribution of rainfall and Fig. 7 illustrates the zonally averaged precipitation rate for the two idealized continent cases. The tropical land case removes a major source of atmospheric moisture (i.e. ocean surfaces between 17°N and 17°S), and this loss of a moisture source within the warm tropics severely reduces tropical rainfall, as well as rainfall poleward of the tropical land mass. In contrast, the polar continent case has open ocean in the re-

The changes in global precipitation rates illustrated in Figs. 1 and 5 do not necessarily reflect changes in continental precipitation rates, and this is perhaps the more essential variable of interest in geologic studies. Interestingly, if continental precipitation rates from the previously described experiments are plotted as a function of global temperature, the results are somewhat different (Fig. 8). First, a trend with respect to global temperature is more apparent. Second, additional evidence for geographic controls on precipitation is illustrated. The precipitation results for experiments with higher carbon dioxide levels are not significantly different from the companion experiments without high CO2 for both present day geography and Cretaceous geography. One therefore might argue that the geography defines the continental precipitation rates more clearly than the global temperature. However, as will be described later, the distribution patterns and rate of continental precipitation change considerably in the high CO2 simulations. Evidently, these changes tend to average out on a continent-wide basis, at least in the two experiments described here. The geography of the mid-Cretaceous is also a special case in t h a t the rate of continental precipitation appears high with respect to the model-simulated globally averaged surface temperature. A "source" argument may also apply in this case, in that the Cretaceous is characterized by an extensive zonal ocean (Tethys) within the subtropics. A reasonable argument would be to suppose that this extensive seaway provides a major source of moisture for con-

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tinental precipitation on land masses b o t h n o r t h and south of Tethys. Figure 9 illustrates the change in land areas with respect to latitude from the Cretaceous to the present day. T h e most distinctive change in geography is in the area of the T e t h y s Ocean. T h e relationship between the area of the zonal T e t h y s Ocean within the subtropics and the rate of continental precipitation can be determined

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from a third series of mean annual CCM experiments based on a sequence of paleogeographies at t w e n t y million year increments. Figure 10 illustrates continental precipitation rates on all land surfaces poleward of T e t h y s as a function of the area (% ocean in the subtropics) of the T e t h y s Ocean. T h e relationship between higher subtropical ocean areas and higher continental precipitation rates supports an argument t h a t l a n d - s e a distribution influences continental precipitation rates by virtue of modifying the source area of moisture. A global cooling trend also characterized the last 60 million years, b u t this is not a significant factor in the interpretation of Fig. 10, because the CCM fails to generate this cooling trend in experiments where paleogeography is the only forcing factor. T h e r e appears to be no d o u b t t h a t geography can modify the intensity of the global hydrologic cycle a n d / o r the rates of continental rainfall simply by providing a source of moisture. T e m p e r a t u r e and geography are evidently responsible for substantial variations in average precipitation rates on either a global or continental basis.

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All of the model simulations are characterized by regions of focussed high precipitation. Note t h a t the polar continent case illustrated in Fig. 6 has a strong precipitation maxima which corresponds closely to the continental margin. Apparently, the location of the continental margins at 45°N and S controls and accentuates the location of mid-latitude precipitation. The spatial heterogeneity of precipitation with respect to geography has important implications for geochemical cycle models (Kump and Barron, 1988), biogeography and sedimentary processes. The importance of these distinctions in the spatial distribution of rainfall is also evident from comparison of global and continental precipitation rates and run-off rates. Note in Fig. 5, t h a t the polar continent case has a greater global precipitation rate than the equatorial continent case. However, the equatorial continent has a higher average continental precipitation rate because large areas of the polar continents (except for the margins) are relatively dry (Figs. 6 and 8). Yet, if run-off rates are examined (indicating areas of highly positive P - E balance), then the polar continents have a substantially higher run-off rate than the equatorial continent (Fig. 11). These differences in large-scale averages reflect the spatial variability of the rainfall. To some extent the total run-off rate reflects the degree to which precipitation is highly focussed on specific continental areas and therefore persistently exceeds evaporation. The results presented in Fig. 11 are also noteworthy because the differences in global temperature and geography result in large contrasts (about 5 × ) in model simulated continental run-off. The nature of geographically-forced regions of continental rainfall is perhaps one of the greatest differences between time periods in Earth history. Certainly, in an examination of climate model precipitation fields these concentrations of high rainfall are the most obvious aspects of any General Circulation Model experiment for a past geography. Early grid-point CCM simulations for Creta-

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southeast Asia is monsoonal in nature. T h e position of centers of high rainfall are controlled by the land areas within the tropics and subtropics. As further explanation of the results shown in Fig. 10 (relating the size of T e t h y s to adjacent continental precipitation) and the high precipitation on the margins of Tethys, a Cenozoic series of mean annual simulations demonstrates t h a t as T e t h y s becomes less zonal and smaller in size, the mid-latitude precipitation maxima become less well defined and less sharply focussed on the continental margin (Fig. 14). Recently, the importance of l a n d - s e a distribution and precipitation p a t t e r n has been further highlighted by CCM simulations of the

megacontinent Pangea for the period 250-200 m.y. ago (Kutzbach and Gallimore, 1989). T h e large bipolar land masses of Pangea exhibit extreme continentality with hot summers and cold winters, and large-scale summer and winter monsoons occur on the margins of Tethys. Each of the above examples provide imp o r t a n t insights into the potential variation of precipitation in E a r t h h i s t o r y . First, global w a r m t h is an i m p o r t a n t factor in the intensity of the hydrologic cycle. Second, continental positions can alter the hydrologic cycle substantially if these positions affect the supply of moisture to the atmosphere, A cold climate can be a wet climate, or a hot climate can be a wet

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climate depending on continental configurations. Third, coastlines or land-sea configurations can influence the intensity and the location of precipitation maxima. In particular, zonal coastlines within the tropics and mid-latitudes tend to result in focussed rainfall on the continental margins.

Carbon dioxide and precipitation Man-induced increases in atmospheric carbon dioxide have focussed considerable attention on the nature of the greenhouse warming (e.g. Schlesinger and Mitchell, 1987). One of the most

significant aspects of greenhouse warming is the geographic distribution of the climatic response. Experiments with increased CO 2 in the CCM (Washington and Meehl, 1983, 1984) indicate enhanced tropical rainfall, and enhanced midlatitude rainfall, in particular poleward of the mid-latitude maxima in the zonal averages. These increases are consistent with the increased globally-averaged precipitation rates associated with global warming in mean annual models (3.0% for 2 × CO 2 and 6.0% for 4 × CO2). However, the largest increases in precipitation are associated with the Intertropical Convergence Zone (ITCZ) and are located over

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169

the ocean. Many of the central continental areas become drier, particularly in summer. These resuits demonstrate t h a t increased global precipitation does not necessarily result in increased continental precipitation. T he high C02 simulations also offer an opportunity to compare model results using different GCM's (Schlesinger and Mitchell, 1987). In particular, the versions of the CCM cited above have some of the smallest globally averaged temperature responses to a doubling of CO2 (and typically smaller changes in globally averaged precipitation) of all GCM results summarized by Schlesinger and Mitchell (1987). A number of factors, including differences in snow-sea ice albedo parameterizations, differences in high-latitude lapse-rate feedback and differences in the globally averaged surface temperature in the basic states of models, may explain the differences in sensitivities to changes in carbon dioxide (Washington and Meehl, 1986; Schlesinger and Mitchell, 1987). Which models are more correct or better is impossible to determine and not at issue here. However, the already remarkable range of precipitation variability

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A wide variety of evidence from the Pleistocene (Hays et al., 1976) and from the prePleistocene (Fisher, 1980; Arthur et al., 1984) indicates t h a t substantial climatic variance corresponds to the orbital periods of precession, obliquity and eccentricity. For the Pleistocene, this variance is concentrated into three discrete spectral periods of 23,000, 42,000 and 100,000 years respectively. The pre-Pleistocene record approximates these periods. Variations in the Earth's orbital parameters result in seasonal and latitudinal changes in solar insolation. For example, at 9000 yr B.P. the time of perihelion was July 30 rather than January 3 as it is today; the obliquity or tilt of the Earth's axis was 24.23 ° compared to 23.45 ° today. These orbital characteristics increased the amplitude of the annual cycle of solar insolation by 14% for the northern hemisphere continents ( - 7% more in July and - 7% less in January). T he primary climatic response to a change in the amplitude of the seasonal cycle involves the

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171

different thermal properties of land and ocean. The heat capacity (thermal inertia) of the ocean is substantially greater than for the continent. Kutzbach (1981) estimated that at 9000 yr B.P. the low latitude oceans would have been 0.5°C warmer whereas the continental surface would have been 5.0 °C warmer. The increased heating over land would be associated with a surface pressure decrease. The increased pressure differential between land and ocean then results in increased flow of air from ocean to land leading to a strengthened monsoon. Hence, for regions associated with monsoons, orbital periodicities should lead to substantial changes in precipitation. Kutzbach (1981), !~utzbach and Otto-Bliesner (1982) and Kutzbach and Guetter (1984; 1986) have demonstrated the sensitivity of the Asian monsoon and of the cross-flow over Africa to orbital variations over the last 18,000 years. The greatest strength of the monsoon occurs at 9000 yr B.P. with a 30-35% increase in precipitation over the monsoonal area of Asia (2.1 mm day-1 more than the 5.6 mm day-1 in the control run). The Indian monsoon increased from 1 2 m m day -1 to 16 mm day -1. Precipitation and precipitation minus evaporation increased over North Africa, the Middle East, and Southern Asia. The average precipitation over all northern hemisphere land increased by approximately 20% in July. The land area centered at 10°N latitude had a precipitation increase from approximately 6 mm d a y - 1 to 10 mm day-1 (Fig. 17). Similar experiments based on changes in the amplitude of the seasonal cycle of insolation have been completed for the mid-Cretaceous (Glancy et al., 1986, 1989, in press), using the seasonal simulations illustrated in Fig. 13, with a range of orbital conditions typical of the last 18,000 years. Glancy et al. (1989, in press) found 8-12 mm day -~ more precipitation over regions of the northern margin of the Tethys ocean during winter in the case of a larger amplitude of the annual cycle of insolation. The summer monsoon was intensified over Southeast Asia, northern Africa and South America as well. Both the experiments for the last 18,000 years

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and for the mid-Cretaceous demonstrate that the regions of large-scale, geographically-forced precipitation are sensitive to variations in the Earth's orbit. This adds a component of shorter time-scale variability to the changes in the hydrologic cycle associated with global warming or cooling and changes in geography. Discussion The suite of climate model experiments de~ scribed above illustrates the great sensitivity of the hydrologic cycle to climatic change and to a set of specific forcing factors. These experiments establish first-order relationships governing the hydrologic cycle, but imply large and complex variability of the hydrologic cycle during Earth history. Global warmth is a primary control of the intensity of the global hydrologic cycle. However, geography modifies this relationship by influencing the source or availability of moisture to the atmosphere. Through this mechanism, land-sea distribution influences average continental precipitation rates as well as having the potential to modify global rates. As an example, the size and zonality of the Tethys Ocean was a primary control on northern hemisphere continental precipitation over the last 100 million years. Geography also controls the spatial distri-

172

bution of rainfall. Land-sea distribution within the subtropics and mid-latitudes exerts a strong influence on the distribution of regions of highly focussed continental rainfall (e.g. monsoons and winter storm-tracks). The regions of highlyfocussed, geographically-controlled precipitation dominate the model-simulated run-off rates. Changes in precipitation rates and distribution in response to higher carbon dioxide are similar in modern and Cretaceous simulations. The intensity of the global hydrologic cycle reflects the model global warming. However, the precipitation increases are largely over tropical oceans and poleward of the mid-latitude precipitation maxima compared to the control simulations. Many continental areas are considerably more arid. Orbital rhythms also produce substantial model sensitivity in precipitation. Variations in the Earth's orbital parameters modulate the amplitude of the annual cycle of insolation. Because of the different thermal properties of land and sea, orbital rhythms should lead to substantial changes in the strength of the monsoons (or any region of geographically-forced precipitation). The experiments described above are far from comprehensive in addressing the forcing factors which may have influenced the climate of the Earth, nor has the potential response of multiple forcing factors been considered. Even with a limited set of experiments, the model simulations yield a substantial (5 × ) range in continental run-off, factor of two changes in average continental precipitation, nearly a factor of two changes in global precipitation and a large sensitivity to the location and intensity of regions of highly focussed rainfall, Certainly these model experiments demonstrate that the hydrologic cycle is a highly sensitive component of the Earth system. The importance of the model results is strengthened by the correspondence between the results and observations from the geologic record. This correspondence may even provide the opportunity to test various scenarios for past climates. The Cretaceous model experiments suggest that the record from the northern hemi-

sphere mid-latitude continents and the southern margin of Tethys should reflect unusually high precipitation rates. The Cretaceous is characterized by abundant laterites (Khain and Ronov, 1960) that reflect deep chemical weathering and therefore high rainfall. Widespread bauxites with other indications of humid, seasonally wet conditions occur over large areas of North America, Europe and the Middle East (Hallam, 1985), Karst bauxites and kaolinite-rich clays occur throughout southern Europe for example. Cretaceous kaolin deposits in central Georgia and South Carolina are the largest kaolin resource in the United States (Tourtelot, 1983). Many Deep Sea Drilling Project cores in the North Atlantic are characterized by abundant kaolinite and smectites of Cretaceous age, indicative of warm and humid conditions surrounding the North Atlantic (Chamley, 1979). The Cretaceous was also a time of extensive coal formation, and over 50 percent of the Cretaceous coals of North America are of mid-Cretaceous age (Beeson, 1984). The climate model results suggest that the great extent of Cretaceous coals, bauxites and kaolinite reflect an optimal geographic distribution of land and sea and climatic warmth which promoted very high precipitation. The model results for changes in the Earth's orbital parameters are substantiated by the geologic record. Kutzbach and Street-Perrott (1985) demonstrate that the CCM predictions of higher monsoonal rainfall 9000 y B.P. are in excellent agreement with data indicative of paleo-lake levels. Mid-Cretaceous model simulations (Barron and Washington, 1982, 1984) indicate that the regions surrounding Tethys are the most "susceptible" to orbital variations and hence rhythmic variations of precipitation. The record of the distribution of mid-Cretaceous rhythmic carbonate a n d / o r organic carbon sequences, which have been tied to orbital periods, fits with the model results, Pratt (1981, 1984) and Barron et al. (1985) show, using the Bridge Creek Limestone (latest Cenomanian-early Turonian) of the Greenhorn Formation of the Western Interior Seaway of North America, t h a t the rhythmic

173

sediments (carbonate contents from 30 to 90% and organic carbon between 0 and 7%) can be interpreted as precipitation-runoff cycles. These comparisons between model results and observations, although largely qualitative, add confidence in the capability of the CCM to predict large-scale changes in the hydrologic cycle. Further, these results are indicative of the significance of the predicted variations in rainfall for biological, sedimentological and geochemical processes. Certainly, both the degree of variation in continental and global-scale averages and the degree of spatial contrast and sensitivity are significant for models of geochemical cycles (Berner et al., 1983; Kump and Barron, 1988), for ancient vegetation patterns and for sedimentological processes of weathering, erosion, sediment transport and deposition. The implications of the sensitivity of the hydrologic cycle are of considerable importance. Given the central role of water in so many Earth processes, the variability of the hydrologic cycle implies that water places a marked "fingerprint" on the geologic record. The strength of this fingerprint is of particular importance in light of the fact that the primary focus of paleoclimatology has been on temperature changes. The importance of water in global change also suggests that the hydrologic cycle should be of primary interest in predictions of future climate change.

Acknowledgments The authors thank J. Kutzbach, L.A. Frakes and A. Henderson-Sellers for helpful suggestions and critical reviews. E. Barron gratefully acknowledges NSF Grants ATM-8715499 and EAR-8720551. The National Center for Atmospheric Research is sponsored by the National Science Foundation.

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