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3 Earth System History and Modeling OVERVIEW Contribution of Geologic Studies to Global Change The geologic record preserves the integrated response of the earth system to a large number of perturbations, including those from human activities, that occurred in the past. Thus the record provides opportunities for compre- hensive case studies that can improve our understanding of future changes in the global environment. Furthermore, the geologic record is the only source of information (Table 3.1) on how the climate system has evolved through time. Observations often challenge our constructs of how the earth operates as a system and provide a valuable time perspective for under- standing the consequences of future environmental change. Specific impor- tant geoscience contributions to global change research include the follow- ~ng: 1. The geologic record provides an independent data set for validating This chapter was prepared by the working group on Earth System History and Modeling established under the Committee on Global Change. Members of the working group were Ellen Mosley-Thompson, Ohio State University, Chair; Eric Barron, Pennsylvania State University; Edward A. Boyle, Massachusetts Institute of Technology; Kevin Burke, National Research Council; Thomas Crowley, ARC Technology; Lisa Graumlich, University of Arizona; George Jacobson, University of Maine; David Rind, Goddard Institute of Space Studies; Glen Shen, University of Washington; and Steve Stanley, Johns Hopkins University. Richard Poore, U.S. Geological Survey, and William Currey, National Science Foundation, participated as liaison representatives from the Committee on Earth Sciences. 67

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68 RESEARCH STRATEGIES FOR THE USGCRP TABLE 3.1 Characteristics of Natural Archival Systems (modified from IGBP, 1989, p. 4) Maximum Temporal Extent Derived Record Precision (years) Parametersa Historical records Daily 103 T P B V M L S Tree rings Seasonal 104 T P Ca B V M L 5 Ice cores Polar lyr 1Os TPCaBVMS Mid-laUtUde 1 yr 104 T P Ca B V M S Corals 1 yr 105 T CwL Pollen and other 100 yr 1Os T P B fossils Sedimentary deposits Aeolian 100 yr 106 T P B V M Fluvial 1 yr 106 P VML Lacustnne 1 yr 106 TB M Marine 100 yr 107 TCw B M Soils 100 yr 1Os TPB V aParameters ate as follows: T= temperature P = precipitation, effective moisture, or humidity C = chemical composition of air (a) or water (w) B = vegetation biomass or composition V= volcanic eruptions M= magnetic field L = sea level S = solar activity the response of climate models to altered boundary conditions. Such infor- mation is critical for assessing the capabilities of models to predict accu- rately the future consequences of human activities. 2. The geologic record provides valuable information about how differ- ent components of the environment are coupled. This information contrib- utes to one of the key goals of the USGCRP. 3. The geologic record is the only available source of information on how the biosphere responds to large changes in the environment. Such information will be particularly valuable for assessing the consequences of future environmental perturbations on the biosphere. Previous studies demonstrated how the geosciences have contributed sig- nificantly to our understanding of how the earth works as a system. For

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EARTH SYSTEM HISI ORY AND MODELING 69 example, the CLIMAP group (CLIMAP Project Members, 1976, 1981) pro- vided the first comprehensive maps of the surface of the earth during the last glacial maximum (18,000 before present (B.P.~. These boundary con- ditions have been used in a number of modeling experiments, which have provided significant insight on high-latitude climate patterns. Subsequent observational studies by the COHMAP group (COHMAP Members, 1988) extended paleoclimatic mapping efforts to the last deglaciation and the present Holocene interglacial and demonstrated some excellent agreement between models and data for the evolution of the African-Asian monsoon during the early Holocene. Results from the SPECMAP group verified a strong rela- tionship between changes in the earth's orbit and fluctuations of Pleistocene climate (Hays et al., 1976; Imbrie et al., 1984, 1989~. Additional studies have shown that natural variations in carbon dioxide and abrupt transitions are an integral part of glacial-interglacial climatic changes (Barnola et al., 1987; Broecker and Denton, 1989; Fairbanks, 1989~. Although significant strides have been made in understanding past envi- ronments, there are a number of important problems that require enhanced study. These problems are the targets of the research initiatives outlined in this chapter. In keeping with the philosophy of the USGCRP, both observa- tional and modeling needs are identified for each goal. Before the initia- tives are discussed, it should be noted that the emphasis in this report is strongly oriented toward climate, the closely linked environmental changes (e.g., those having to do with oceanic or atmospheric chemistry), and their interactions with the biosphere. Considerations of the role of solid earth processes in global change form the focus of a different element of the program. Specific Research Initiatives The research proposed in this chapter addresses three primary topics, each of which falls naturally into a different time scale. An important element of the research program will be to develop global-scale data bases to understand the processes operating within a certain time scale and pos- sible interactions among processes operating on different time scales. The specific initiatives that the committee recommends are as follows: to establish an integrated set of globally extensive, high-resolution records of the Holocene (last 10,000 years) as a frame of reference for comparison with any future warming due to greenhouse gases. to understand glacial-interglacial fluctuations of the Quaternary. This research will focus on determining how the climate system responded to known forcing (Milankovitch cycles). Such studies will provide valuable information about interactions among components of the climate system, especially biogeochemical cycles and climate. These studies should also

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70 RESEARCH STRATEGIES FOR THE USGC8P enhance our understanding of instabilities in the climate system, as these processes appear to have played a key role in glacial-interglacial climatic changes. to examine the system response to large changes in forcing due to carbon dioxide and land-sea distribution changes. This research will pro- vide insight into the nature of warm climates, offer a strong test of climate models, and produce the only available information on the effects of large environmental perturbations on the biosphere. Priorities Within these three main areas, the committee recommends that the fol- lowing topics be given highest priority: . Holocene high-resolution records for the last 1,000 to 2,000 years; Glacial-interglacial cycles, with special emphasis on (a) abrupt system changes and the deglaciation sequence, (b) the carbon cycle, (c) tropical environments at the last glacial maximum, and (d) coupling of different components of the climate system. System response to large forcing, with special emphasis on (a) the environment of extreme warm periods such as the Pliocene warm interval (3 to 5 million years ago (Ma)) and (b) evaluation of the climate-biosphere connection during periods of major climatic change such as the Eocene- Oligocene (30 to 40 Ma) transition. Themes of the Proposed Research Cutting across all of the proposed topics are some unifying themes: (1) abrupt transitions; (2) climate of warm periods; (3) system response to known forcing; (4) biotic response to climatic change; and (5) coupling between different components of the geosphere and biosphere, with particu- lar emphasis on the carbon cycle. All of these themes represent unique contributions of geologic studies to global change. Implementation of the Research Plan Previous research experience demonstrates that progress in understand- ing past environments has resulted from both individual research projects and more organized efforts such as CLIMAP and COHMAP. Although indi- vidual research projects will continue to be an important component of future investigations, it is also apparent that fully successful implementa- tion of some elements of the research plan will require some sustained

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EARTH SYSTEM HISTORY AND MODELING 71 levels of coordination in which production of large-scale data sets will be necessary. HOLOCENE HIGH-RESOLUTION ENVIRONMENTAL RECONSTRUCTIONS During the next few decades a major warming is anticipated in response to the enhanced greenhouse effect. However, there is considerable uncer- tainty regarding the potential magnitude and regional response to the pertur- bation. One significant concern is the fact that climate projections do not match the global temperature record of the last century. This disagreement stems in part from the fact that other processes operating in the climate system (e.g., solar forcing, volcanism, and internal variations in the ocean- atmosphere system) may significantly modify temperatures and perhaps mask any greenhouse signal during the early stages of a perturbation. To clarify the course of future climatic change, it is essential to under- stand the origin of the natural variability within the environmental system on a time scale ranging from years to centuries. The Holocene (last 10,000 years) record of climatic change offers the temporal and spatial detail nec- essary to characterize that variability. To date, much of our understanding of Holocene climate is based on a spatially limited data set drawn largely from Western Europe and North America. A broader spatial distribution of historical and proxy records is needed to provide the critical perspective, or backdrop, against which the impact of recent anthropogenic perturbations to the global system can be assessed. Considerably more work is required to develop an adequate understanding of the processes operating on this time scale. The major focus of this initiative involves determining and understand- ing decadal- to millennial-scale climate variability by developing a high- resolution global data set. A two-pronged approach is proposed to address this problem: (1) development of a high-resolution global network of cli- mate fluctuations for the last 1,000 to 2,000 years, with special emphasis on the Little Ice Age (LIA) and on process studies for some key regions and (2) development of longer, multiproxy histories devoted to understanding other centennial- and millennial-scale fluctuations in the Holocene. The Last 1,000 to 2,000 Years The latest part of the Holocene provides the best opportunity to study such decadal- and centennial-scale processes in more detail because the observational data base is the most extensive and there have been numerous oscillations during this interval (Figure 3.1~. However, the processes re

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72 RESEARCH STRATEGIES FOR THE USGCRP 1000 1200 05 o.o c, 0 5 o - 1 .0 -1.5 -2.0 .8 .4 x A) 1 0 C 0.6 0.2 Years A.D. 1400 1600 1800 2000 _ , , _ Phenological Temperature West China Tree Rings ;~ an If, Winter Temperature, , ~ -4t Yangize Valley ~l ~Counties affected .. 400 - by drought c_, 600 - -V'v' ~ o x ~2 3 4 -28 o -29 so -30 1.5 CD 0.0 so -1.5 lot on o o.o a' ~ - 0.5 - 1.0 Dust Rain Frequency ~: Quelccaya Ice Cap, Peru _ South Pole Station, Antarctica 4~ ~_ 1000 1200 1400 1600 1800 2000 Years A.D. FIGURE 3.1 Evidence for decadal- and centennial-scale oscillations in records spanning the last 1,000 years. Note that oscillations are recorded in several differ- ent indices for China (temperature, tree rings, counties affected by drought, and dust rain frequency) and that there is a broad similarity, at least in terms of the time scale of response, with fluctuations in Greenland, Antarctica, and the Peruvian Andes. (Source: Modified from Mosley-Thompson et al., 1990, and Zhang and Crowley, 1989.)

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EARTH SYSTEM HISTORY AND MODELING 73 sponsible for these changes are not well understood. This interval is also of particular interest because it encompasses the time of most significant hu- man disturbance of the environment. Global Network of Environmental Change Although we have some idea of the frequency and magnitude of climate fluctuations in different regions (Figure 3.1), much less emphasis has been directed toward systematic, detailed comparison of different records to test for synchroneity of change. There are also data gaps in some regions (e.g., parts of the southern hemisphere). The primary goal of this research initia- tive is to develop a network of 1,000- to 2,000-year records that is suffi- ciently dense to test for synchroneity of global warming and cooling. Once compiled, these records may be compared with different proposed forcing functions to determine the amount of variance explained by each mechanism. Observational Needs. Observations are needed to determine the timing and spatial variability of past environmental changes on decadal time scales. A global data base of paleoclimate obser- vations is needed. Fortunately, a great diversity of paleoenvironmental sensors are available, many with annual resolution (e.g., direct observa- tions, historical documents, anthropological records, tree rings, ice cores, lake and ocean sediments, and corals). These records must be correlated with an error of less than a decade. An important and nontrivial task will be the development of explicit strategies for combining paleoclimate prox- ies of varying precision that monitor different elements of the system (e.g., seasonal and geographic sensitivities). Such efforts will require a substan- tial level of coordination and collaboration. to quantify the observed environmental changes in terms of tempera- ture, precipitation, and so on. Preliminary work indicates that cooling dur- ing the LIA was on the order of l.Oo to 1.5C in many places, but it is un- clear whether these estimates are mean-annual or seasonal in nature. These efforts should include focused studies, which are essential for understanding the causes and consequences of environmental changes, particularly on regional scales. The latter may be of great significance, as many human activities that are particularly sensitive to environmental perturbations (e.g., food pro- duction and transportation) are organized on similar spatial and temporal scales. to determine the timing and magnitude of potential changes in forcing. At present the Holocene data base is too sparse to map specific responses at the level of detail necessary to elucidate cause-and-effect relationships. Three likely mechanisms for climatic change on decadal time scales include solar

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74 RESEARClI STRATEGIES FOR THE USGCRP variability, volcanism, and internal nonlinear interactions in the ocean-at- mosphere system. Although the timing of solar variability events, based on carbon-14 and beryllium-10 records, is relatively well known (Beer et al., 1988; Stuiver and Braziunas, 1988), the equivalent change in solar forcing is unconstrained. Volcanic fluctuations have also been linked to climatic change (LaMarche and Hirschboeck, 1984~. However, one potential record of volcanism (sulfate fluctuations in ice cores) may be complicated by dimethylsulfide (DMS) release in response to changes in oceanic productiv- ity. DMS can be converted into sulfate. It is desirable to acquire methanesulfonic acid (MSA) measurements from ice cores as an independent indication of the ocean productivity component of the ice core sulfate record. Nonlinear interactions in the ocean-atmosphere system may also cause decadal-scale temperature changes (Gaffin et al., 1986; Hansen and Lebedeff, 1978~. Testing this idea requires better correlations between changes in the deep ocean (cf. Keigwin and Jones, 1989) and on land. Finally, a quantitative assessment must be made of the amount of variance explained in the climate record by each of these mechanisms. Modeling Needs. Efforts are required to model the time-dependent varia- tions in temperature as a function of solar variability, volcanism, and ocean- atmosphere coupling. Once observational results allow quantification of the relative magnitude of different forcing agents, various models must be tested to determine if they have the correct sensitivity. Little Ice Age A period of special interest is the Little Ice Age (approximately 1450 to 1880 A.D.~. In many areas, maximum cooling occurred in the seventeenth century, although not all regions appear to have cooled synchronously. For example, maximum cooling in China may have occurred in the mid-1600s, while in Europe it occurred in the 1690s. During the LIA, there is also evidence for enhanced interannual variability and a stronger meridional cir- culation. The latter feature may explain some of the regional differences in climate patterns. There are also some indications that transitions into and out of the LIA were relatively abrupt (Thompson and Mosley-Thompson, 1987~. Overall, the spatial extent, synchroneity, and magnitude of LIA variations need to be better known. Observational Needs. Observations are needed to develop detailed information about the timing, regional extent, and magnitude of LIA variations, with particular emphasis on the seventeenth century. Although it may not be possible to produce a uniformly dense map

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EARTH SYSTEM HISI ORY AND MODELING 75 of regional climatic change, enough potential information is available from different regions to enhance the synoptic picture of this period. Consider- ably denser coverage is needed than for the global time series developed for reconstruction of the general patterns of fluctuations over the last 1,000 years discussed above. Information is especially sparse from the tropics and marine areas. In some cases, these voids can be filled by sampling of corals, tropical trees, ice cores, and near-shore or high-resolution marine sediments. to better specify the relationship of variability in precipitation and temperature during the LIA. Their trends do not exhibit a simple relation- ship. In fact, evidence from tree rings (LaMarche, 1974), ice cores (Thompson et al., 1986), and dust records (Zhang and Crowley, 1989) indicates that there were two phases of LIA precipitation, with cool, moist conditions prevailing in the first half and cool, dry conditions dominating the latter half (1700 to 1880 A.D.~. . to investigate apparent abrupt transitions into and out of the LIA. These studies may lead to identification of potentially important, but less obvious, causal mechanisms operating on shorter time scales within the Holocene. These may arise from changes in transient geochemical reser- voirs (e.g., ice and labile carbon stores), strong feedbacks (e.g., albedo and carbon dioxide), or volcanism (Berger and Labeyrie, 1987~. The Holocene record is rich in evidence for rapid climate changes in many regions, and a systematic search for widely correlated events reflecting large-scale, short- term climate shifts is recommended. Modeling Needs. Models are needed to construct and test three-dimensional circulation models of the atmosphere and oceans relating specific forcing mechanisms and known system responses (i.e., observations). Model re- sults can be compared to the inferred response in regions where records are available. These experiments will prove valuable for determining the sensi- tivity of models (and the real world) to known forcing. The abundant high- resolution data for the LIA are particularly appropriate for investigating potential forcings and the climatic and biospheric responses. Regional Process Studies Knowledge of the processes responsible for local changes inferred from proxy records is essential for accurate interpretation. Often this informa- tion provides additional insight into regional processes that strongly affect both local- and global-scale circulation systems. Therefore regional climatic chronologies should be developed for areas where episodic regional-scale processes strongly affect both the local climate and global-scale circulation systems. Three candidate areas that should be considered for further study

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76 RESEARCH STRATEGIES FOR THE USGCRP are (1) the subpolar North Atlantic basin, (2) the equatorial Pacific basin, and (3) the Asian monsoon. Observational Needs. Observations are required . to develop a greater understanding of processes occurring in the sub- polar North Atlantic basin. Geologic studies suggest that this region may be a key area for understanding possible changes in the oceanic-atmospheric circulation (see the section "The Last 40,000 Years" below). This area encompasses one of the densest arrays of historical data, and thus it is desirable to determine the pattern of climate fluctuations in this region on decadal scales and to ascertain whether they were accompanied by any changes in the oceanic circulation. A coordinated effort linking the climate of eastern North America, Greenland, Western Europe, and the subpolar North Atlantic is recommended. This effort will require the acquisition of very high sedimentation rate deep-sea records (see section "Sample Acqui- sition" below) from shallow marine areas or sediment drifts for evaluation of possible changes in the surface and deep circulation. . to develop long time series of E1 Nino-Southern Oscillation (ENSO) fluctuations. In the past decade, researchers have demonstrated the large- scale nature of ENSO events and their very important influence on tropical rainfall patterns. Ice cores and corals contain information about interannual climate variability and offer the opportunity to extend these records back several centuries or more. Such results could provide an enhanced under- standing of ENSO. This research will require additional information on tropical rainfall from tree rings, ice cores, and upwelling variations as recorded in coral reefs. . to develop long time series of monsoon fluctuations. The Asian mon- soon is one of the most important features of the planetary circulation, and fluctuations in its intensity affect the lives of nearly 2 billion people. Long time series are available from India extending back about 100 years, and historical time series from China extend back at least 500 years (Zhang and Crowley, 1989~. However, more information is needed to understand the temporal variations. Modeling Needs. Models need to be developed . to simulate many of the features of observed oceanic-atmospheric anomalies such as ENS O events (Cane et al., 1986~. Using existing models, the sensi- tivity of such regional processes to observed or suspected changes in other components of the system can be examined. Conversely, if observations clearly indicate a change in frequency or character of oceanic-atmospheric anomalies, models may suggest potential causes. These efforts will contrib

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EARTH SYSTEM HISTORY AND MODELING 77 ute to validating the utility of such models for predicting possible future changes in ENSO-type events due to global warming. to link climate fluctuations on decadal- and centennial-time scales with the present generation of atmospheric and oceanic models used to study the above regional processes. At present the models can theoretically generate variance on decadal and longer time scales, but key parameterizations in the models are not well constrained by observations. Approaching the problem from both an observational and a modeling viewpoint for the last 500 years may provide additional insight to processes occurring on shorter time scales. Earlier Holocene Millennial-Scale Fluctuations Geologic records indicate that LIA-type fluctuations occur on a charac- teristic time scale of 2,000 to 3,000 years over much of the last 20,000 years (Figure 3.2~. Therefore, any explanation for climatic variability in the last several thousand years should be applicable to these earlier fluctua- tions. Observational Needs. Observations are needed to develop time series of system response from selected regions for the last 10,000 years. Informa- tion is available from such areas as mountain glaciers, the central Asian highlands, and African lakes (Rothlisberger, 1986; Street-Perrott and Harrison, 1984; Thompson et al., 1989~. Some additional high-resolution marine records are critically needed. High-resolution records in the North Atlantic might provide information about fluctuations of the ocean on this time scale. Although records of solar variability extend back to 9,600 B.P. (Stuiver and Braziunas, 1988), the record of volcanism in both hemispheres is not as well documented. High-resolution terrestrial records, especially ice cores, should contribute substantially to reconstruction of the earth's volcanic history during the Holocene. Modeling Needs. Efforts are needed to test models of climate variability developed for the last 1,000 years against longer records. Any explanation for decadal- to millennial-scale fluctuations of the last 1,000 years should also be applicable to earlier time intervals. Specific models should test this hypothesis. GLACIAL-INTERGLACIAL CYCLES The U.S. Global Change Research Program seeks to improve our under- standing and predictive capabilities of the climate system's response to

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EARTH SYSTEM HISTORY AND MODELING 97 30 Ma. The largest biotic turnover in the Cenozoic occurred around the same time as the Eocene-Oligocene transition. There were significant changes in marine organisms, terrestrial flora, and terrestrial vertebrates. The tim- ing of some of these changes is still uncertain, along with their degree of abruptness and the cause of the overall pattern. The end of the Cretaceous marks one of the most spectacular events in the earth's history the probable impact of a 10-km halide that is associ- ated at least in part with widespread extinctions used to define the end of a geologic era. The discovery of the famous iridium layers in K-T sections has provoked some of the most stimulating geoscience research of the last decade. Although much has been learned about the K-T, a number of outstanding problems remain. Observational Needs. Observations are needed to develop a comprehensive reconstruction of the physical changes in the environment across the abrupt events. Detailed time series are needed for key variables and in key regions in order to delineate the timing of the system response and the relationships between different components. The comprehensive reconstructions must focus on the physical, chemical, and biological state of the system at "snapshots" that span the abrupt event. The essential physical climate requirements are the distribution of surface temperatures, hydrologic state, seasonality of temperature and precipitation, cryosphere state, distribution and intensity of winds, and water mass distri- bution. Carbon dioxide levels are a key element of the chemical state of the system and the record of the carbon system, including productivity and carbon burial, are essential requirements. to develop a complete description of the distribution and character of the biosphere, including correlation of terrestrial floras, vertebrates, the faunas of the marginal marine environment and shelf, and planktonic and benthic organisms from the open ocean, in order to describe the ecological dynam ~cs. Modeling Needs. Modeling work is required to determine the origin of the abrupt transitions in the Cenozoic. The climate transition appears to reflect some type of instability in the climate system. However, it is not known what types of instability may have been involved and how they may have been triggered by long-term changes in continental position, orographic forcing, carbon dioxide, or ocean circula- tion. Modeling studies should focus on quantitative estimates of changes in these boundary conditions. A different type of modeling study should examine the effect of the long-term changes on more idealized models that can be

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98 RESEARCH STRATEGIES FOR THE USGCRP used to explore properties of unstable systems. Additional modeling studies for the K-T should include examining the climate and chemical perturba- tions associated with asteroid impacts and large volcanic eruptions. . to understand the relationship between abrupt environmental forcing and observed biotic responses. Although we have an approximate idea of the coincidence of the environmental and biotic transitions, the physical explanations for the biospheric response are still lacking. CRITICAL PROGRAM ELEMENTS Substantial progress in earth system history research has resulted from a proper balance between individual research projects and larger, coordinated efforts. In some instances the magnitude of the effort and the diversity of the expertise required to accomplish the objectives of the USGCRP will require research programs that are interdisciplinary, multiinstitutional, and international. However, the importance of maintaining smaller, single-in- vestigator programs is also recognized. To maximize further progress, some techniques and facilities need to be expanded and refined, and some new methods need to be developed. Ex- amples include development and maintenance of facilities (e.g., drills) to acquire samples, calibration of the environmental records, and development of techniques and strategies to correlate diverse paleoenvironmental histo- ries. Listed below are some of the major issues that must be addressed to implement the earth system history and modeling initiative in the USGCRP. Sample Acquisition The drilling of ice, lake, and ocean sediment cores will be the backbone of the proposed initiatives. Therefore drilling capability must be developed and maintained to meet anticipated needs. Producing high-temporal-resolu- tion records is contingent on obtaining a sufficient amount of material to measure small sample volumes for multiple parameters. It is recommended that drills for collecting both marine sediments and ice cores be designed to take larger-diameter and higher-quality cores. The committee has identified the following needs: . ice cores (see the sections 4`The Last 1,000 to 2,000 Years,', Pearlier Holocene Millennial-Scale Fluctuations,'3 '`Abrupt Change," and ``The Last Glacial Cycled. Ice cores are a treasure trove of information about past climates. The status of U.S. ice core drilling was reviewed recently, and recommendations were made (NBC, 1986) to develop and maintain a suite of drills for diverse programs in diverse areas. These may range from high (>18,000 feet), remote ice caps in the tropics and mid-latitudes to the polar ice caps, which are up to 3,000 m thick. Drilling these deep cores is

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EARTH SYSTEM HISTORY AND MODELING 99 expensive; however, it may be necessary to retrieve other long ice core records after the GISP IVGRIP drilling in Greenland (see the section "Abrupt Changes"~. The committee recommends that this possibility be explored along with the potential of international cooperation (see the section "Inter- national Cooperation" below). long terrestrial records. Although we are making substantial progress in unraveling the history of the ocean basins, the terrestrial record is less well developed. Filling this gap requires expanding our capability to take long cores on land from sediments such as the thick loess sequences in China. Again, international cooperation on this project should be explored (see the section "International Cooperation"~. enhanced drilling of marine records (see the sections "Holocene High- Resolution Environmental Reconstructions," "Glacial-Interglacial Cycles," and "System Responses to Large Changes in Forcing"~. The need for more marine cores will require additional drilling equipment and more efficient use of existing facilities. Currently, sediment sequences from the sea floor are recovered principally by the Ocean Drilling Program (ODP) or indi- vidual efforts by investigators on ships from oceanographic institutions. Meeting the needs of the USGCRP for long, continuous, high-resolution sequences will require some augmentation of drilling efforts. An enhanced ocean drilling effort dedicated to paleoceanography should allow for larger- diameter cores, multiple cores at each site, and perhaps the development of new capabilities for drilling very high sedimentation rate records in conti- nental margins or sediment drifts. Cooperation with the ODP on this issue should be explored. Environmental Calibration The value of proxy records stems from our ability to reconstruct some aspect of the climate system. Implicit in this statement is that we under- stand the climatic signal embedded in the proxy. This requires careful study of modern conditions, as all proxies must be calibrated in terms of current conditions. Unfortunately, available data often are insufficient to perform this critical task. It is essential to allocate resources to the study of modern processes as an integral part of paleoclimatic and paleoenvironmental reconstructions and for refining and developing sets of modern observations to enhance proxy records. The following is needed: process studies of modern environments. Quantitative specification of the physical and chemical processes creating the preserved proxy record must precede the development of empirical transfer functions used to ex- tract paleoenvironmental information. Especially critical is explicit docu- mentation of lags, thresholds, nonlinearities, and interactions between vari

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100 RESEARCH STRATEGIES FOR THE USGCRP ables governing the responses preserved in paleoclimatic records (Graumlich and Brubaker, 1986~. Examples of critical areas that would benefit from such process studies include fractionation of isotopes in precipitation, plankton, and tree rings; entrapment of gases within ice; and incorporation of trace metals into corals. Fairly long-term observations may be required in order to provide a statistically meaningful data set for calibration purposes. development of new proxy techniques to estimate environmental change. One major goal of the earth system history and modeling initiative is to provide quantitative estimates of environmental variabilities at key intervals of relevance to global change research. Achieving this requires expanding our capability to estimate such variables as temperature, salinity, and phos- phorous. New geochemical methods may prove invaluable. For example, a new technique for estimating sea surface temperature or bottom-water tem- perature would be invaluable for separating the ice volume, salinity, and temperature signals from the oxygen isotope record in marine carbonates. Correlation of Records Before any definitive statements can be made about the climate at a certain time, samples must be temporally correlated with a high degree of accuracy. Current efforts to integrate different chronologies must be en hanced. These efforts include enhanced capability for radiometric dating. A substantial number of accelerator carbon-14 dates will be required to accomplish the USGCRP goals. In addition, new applications of the cosmogonic isotopes (chlorine- 36, beryllium-10) should provide valuable insights. Currently, AMS facili- ties exist at six institutions, and an additional facility is scheduled for completion in 1991 at Woods Hole Oceanographic Institution. Based on existing and planned facilities, Elmore et al. (1988) have identified a minimum annual shortage of approximately 4,000 non-carbon-14 AMS analyses to meet cur- rent program needs. The USGCRP will increase current demand for both carbon-14 and cosmogonic isotope measurements. The adequacy of exist- ing U.S. facilities must be reassessed. extension and improvement of the current radiocarbon chronology. Separate efforts must be launched to update older, possibly erroneous mea- surements, as well as to extend the known radiocarbon chronology beyond that available from tree rings (9,600 B.P.~. improvements in chronostratigraphic techniques. Beyond the range of carbon-14, additional techniques may be used to correlate paleoclimatic records. These include isotope stratigraphy, biostratigraphy, tephrochronology, and paleomagnetic stratigraphy. For example, land and sea records can be linked using pollen, dust, paleomagnetics, and tephrochronology. Although

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EARTH SYSTEM HISTORY AND MODELING 101 incremental advances in these areas are expected, a more focused research effort to improve these correlations may be required in some cases. Data Management Successful completion of many of the tasks outlined requires establish- ment of data bases ranging from those limited in scope to the needs of an individual project to global data sets needed for large international programs. The committee recommends that large-scale data bases be developed (1) when it becomes apparent that lack of organization is a deterrent to contin- ued progress and (2) for projects requiring considerable coordination (e.g., the global network for the last 1,000 years (see the section "Global Network of Environmental Change"), the Little Ice Age (see the section "Little Ice Age"), and scenarios for greater warmth (see the section "Environments of Extreme Warm Periods". The design of the data banks must be carefully considered by the project participants. INTERNATIONAL COOPERATION Informal international cooperation has led to abundant and fruitful scien- tific advances in studies of earth system history. Several of the proposed initiatives may require a more formal level of cooperation: acquisition of long ice cores. A number of groups in countries includ- ing the United Kingdom, Denmark, Switzerland, France, the USSR, and Australia might be interested in collaborating in efforts to drill and analyze these cores. . acquisition of long land records with enhanced coring capability. Special priority should be given to collaborative research with the USSR and China, whose land masses account for such a large fraction of those in the northern hemisphere. acquisition of more long paleoceanographic records and development of new methods to take high-sedimentation-rate deep-sea cores. Progress in this area may require either close coordination with the ODP or establish- ment of separate arrangements. If coordination with the international ODP develops, it is essential to recognize that pre-Pleistocene studies currently are not part of the plans for the International Geosphere-Biosphere Program and that a U.S. connection with an international ODP is insufficient to ensure that pre-Pleistocene studies will be included. time slice reconstructions of past climates. These constitute major efforts that will require international participation and support. Discussions between U.S. and Soviet scientists are already under way for collaborative studies of the early Pliocene warming. The committee recommends ex- panded activities in this area.

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102 RESEARCH STRATEGIES FOR THE USGCRP In addition, ache following are critical components of the USGCRP that could begin immediately at Me international level: (1) coordination of existing data bases and sample collections, (2) planning for the coordination of new data bases, and (3) preliminary discussions of the scientific potential and logistical support necessary to mount large regional programs. REFERENCES Alvarez, L.W., W. Alvarez, F. Asara, and H.V. Michel. 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208:1095-1108. Arthur, M.A., W.E. Dean, and L.M. Pratt. 1988. Geochemical and climatic effects of increased marine organic carbon burial at the Cenomanian~uronian bound- ary. Nature 235:714-717. Barnola, J.M., D. Raynaud, Y.S. Korotkevich, and C. Lorius. 1987. Vostok ice core provides 160,000-year record of atmospheric CO2. Nature 329:408~14. Barron, E.J. 1985. Explanations of the tertiary global cooling trend. Palaeogeogr. Palaeoclimatol. Palaeoecol. 50:45-61. 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. Palaeoceanography 5:319-337. Barron, E.J., S.L. Thompson, and S.H. Schneider. 1981. An ice free Cretaceous? Results from climate model simulations. Science 212~4494~:501-508. Beer, J., et al. 1988. Information on past solar activity and geomagnetism from lobe in the Camp Century ice core. Nature 331:675-679. Berger, A.L., and L.D. Labeyrie. 1987. Abrupt climatic change an introduction. Pp. 3-22 in W.H. Berger and L.D. Labeyrie (eds.), Abrupt Climatic Change. D. Reidel, Dordrecht, The Netherlands. Berger, A.L., J. hnbrie, J.D. Hays, G. Kulda, and B. Salt~nan (eds.~. 1984. Milankovitch and Climate. D. Reidel, Dordrecht, The Netherlands. 895 pp. Boyle, E.A. 1988. Vertical oceanic nutrient fractionation and glacial/interglacial CO2 cycles. Nature 331:55-56. Boyle, E.A., and L. Keigwin. 1987. North Atlantic thermohaline circulation during the past 20,000 years linked to high-latitude surface temperature. Nature 330~6143~:35-40. Brass, G.W., E. Saltzman, J.L. Sloan II, J.R. Southam, W.W. Hay, W.T. Holser, and W.H. Peterson. 1982. Ocean circulation, plate tectonics, and climate. Pp. 83-89 in Climate in Earth History (Studies in Geophysics). National Acad- emy Press, Washington, D.C. Broecker, W.S. 1982. Ocean chemistry during glacial time. Geochim. Cosmochim. Acta 46:1689-1705. Broecker, W.S., and G.H. Denton. 1989. The role of ocean-atmosphere reorganiza- tions in glacial cycles. Geochim. Cosmochim. Acta 53:2465-2501. Broecker, W.S., D.M. Peteet, and D. Rind. 1985. Does the ocean-atmosphere sys- tem have more than one stable mode of operation? Nature 315:21-26.

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