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Research Strategies for the U.S. Global Change Research Program (1990)

Chapter: 3 Earth System History and Modeling

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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"3 Earth System History and Modeling." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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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

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

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

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

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

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.)

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 1.5°C 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

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

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

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

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

78 RESEARCH STRATEGIES FOR THE USGCRP ~ , .. , . , . . .. . . . , , ~Little Ice Age. | Gloelar Contrecti ~1 _. ~ I , _, l_ I _ I _ - - Glocie' E'Donsion l ~ , _, _ . _ . _ . _ . _ . ~ . _ Glocis, Controction E ;locier Expon~ion ?- ~ I ~ Glocier Controction 1 rSokamI/MocA1pine/Coehrencl _ 1 l l l S104~. ~ - ~ - ~ - t1 c> ._ 0 E ._ <~ ~ Z c _ _ . ~ 0 - Unnomed In1eratode , .., _, I _, _, _ j _j _, _ i ~ - I - ~ Algonquln Stode - I - ~ | , ~, _, _, _, _, _ I _ I _ I _ I _l I Two R1Y.r' oUvonce I i Two Creelle Interetode _ , _ I , . , _ I _, _, _, _, _, _ _~_'Po!t Hur,on'Stod. ~ I 1 1 ~t I Ice Shee' c I Recession u .s _ I _ I _ I - I- I- I-I I I 11 I _ t _ ~ _ I _ I _ I _ I _ I ~ _ I _ I _ I _ I _ I _ I _ I _, _, _, ~ _ I _ I _ I . I _ I _, _ I _ I _ I _ _ I _ I _ I _ I _, . I _ I _ I _ I _ I - I - I Lote Wisconaln I -1 ~ I ~ I ~ Ilaximum of I ~ I ~ Wast Antorctic '_ '_ I ~ _ I-I _ Ice Shee1_ I _ I _ '_ - I - ~ (McMurdo Sound)- I - I I -1-I-I- I- I- ·_ ~ _ ~ _ ~ _1 _ I _' _ I_ ~ _ ~ _,_ I_ I_ ~ _ 1 1 ~ _ I- I- I - I - I - 1- I -I - 1 - -~-I-1-1-1-1 - 1-1-9' I _ I _ I _ I _ ~ - I- I - I 7' I Cory/Por1 Huron Intc =1 c - 1 1 cn ° 0 0 _ I _ ~ _ I _ I _ I _ I _ I _ I _ I _, I - I - I - Cary Stode I - I - I _ ~ , _ ~ . ~ _, . 1 _, _, ~ _ ~ I . ~.=c~ | Erie Interetade 1 2 ° 1 - 1 - I - I - I - I - I - ~ - I _, _ ~ I _ I _ I _ l_ I_ ,_ ~ _ ~ _ ~ _ I _ I . I _ 1 _, _ I ~ I _ ~ I _, _ ~ _ ~ _ I _ Torewell Stode _ I _ ~ ~ _ I _, _ I _ ~ _ ~ _ ~ _ ~ _, _ ~ _ _ ~ _ ~ _ ~ _ ~ _ ~ _, _, _ ~ _, _, ~ _ ~ _ ~ _ ~ _ I - ~ - 1 - I - I - I - - ~ - I - I - I -1-1 - ~ -1-1 -1 l ~ I Connereville Interalade | ~, _, , _, , ._, , _, _, _,, _ `~1 - ' - ~ - , - 1 _ ~ _, _, _, _ ~-`~- Foyctle SIode--l ~ _ ~ _ , _ , _ , _ ~ _ , _ _ O 1000 2000 3000 4000 5000 6000 7000 8000 9000 Q a, 10,000 11 000c o 12 000O ._ o 13 000~ - 14 000 15 000 I6 000 '7 000 t8 000 19,000 20 000 21 000 22 000 FIGURE 3.2 Evidence for repetition of millennial-scale climate oscillations at in- tervals of 2,000 to 3,000 years over the last 20,000 years. Analysis of any specific event, such as the Little Ice Age or Younger Dryas, must take into account the characteristic time scale of these oscillations. (Reprinted, by permission, from P.A. Mayewski et al. (1981~. Copyright @) 1981 by John Wiley and Sons, Inc.)

EARTH SYSTEM HISTORY AND MODELING -0.6 O . O 0.6 -0.64 O . O 0.64 79 A ' ' '1 '.1 ''1 '' B :'': 0.0 1 00. 200. 300. 400. 500. 600. 700. 800. ~ IME (K yes ago) FIGURE 3.3 Filtered oxygen isotope record for the late Pleistocene, illustrating very high coherence between orbital forcing (dotted line) at the (a) obliquity alla (b) precession bands (40,000- and 23,000-year periods, respectively) and global ice volume response (solid line). (Reprinted from J. Imbrie et al. (1984~. Copyright @) 1984) known forcing. Climate records of the late Pleistocene (Figure 3.3) demon- strate significant responses to orbitally induced variations in insolation- the Milankovitch effect (Imbrie et al., 1984~. Carbon dioxide and methane in the atmosphere also vary with glacial-interglacial cycles. The geologic record indicates that the system response is quite complex in space and time and that there are probably instabilities in the climate system. The principal objective of this initiative is to determine the nature of climate system responses to known forcing during the Pleistocene. These studies will provide information about the characteristics of the coupling among different components of the climate system and therefore enable development and testing of models describing these interactions. These studies also will be especially valuable for investigation of climate instabilities and biogeochemical cycles. To meet the objectives of this initiative, the research strategy must be divided into different subtasks. These subtasks fall naturally into different time scales, with processes operating on one time scale sometimes affecting

80 RESEARCH STRATEGIES FOR THE USGCRP processes operating on different time scales. The subtasks involve detailed studies over the range of the carbon-14 time scale (see the section "The Last 40,000 Years" below), climate fluctuations over the last glacial cycle (see the section "The Last Glacial Cycle (Last 130,000 Years)" below), and climate fluctuations over several glacial cycles (see the section "The Last Few Glacial-Interglacial Cycles (Last 500,000 Years)" below). Accom- plishing these goals requires both field programs for data collection and modeling studies. The Last 40,000 Years Numerous studies demonstrate that there have been several glacial cycles of approximately 100,000-year duration during the late Pleistocene (Imbrie et al., 1984~. High-resolution time series of climate variables record a significant response to orbital variations over this interval. However, the exact manner in which the orbital signal is transmitted through the climate system is not understood. The interactions between external forcing and system response involve both gradual processes (COHMAP Members, 1988) and abrupt transitions that may reflect instabilities in the climate system (e.g., Broecker and Denton, 1989~. Much progress has been made in mapping and understanding the evolu- tion of climate over the last 40,000 years (e.g., COHMAP Members, 1988; Crowley and North, 1990~. However, a number of important problems remain that require enhanced study to increase our understanding of the climate system and its response to altered boundary conditions. The committee recommends the following research topics for special emphasis: (1) analyses of the abrupt changes that occurred during deglaciation (14,000 to 10,000 B.P.) and possibly from 40,000 to 30,000 B.P. and (2) resolution of model- data discrepancies such as the characterization of tropical environments at the last glacial maximum and the environment of mid-continental Eurasia at 6,000 B.P. The observational needs and modeling requirements are dis- cussed for each. Abrupt Changes Some of the most spectacular examples of abrupt climatic change in the earth's history are known to have occurred at the end of the last glacial stage (14,000 to 10,000 B.P.~. For at least some regions of the world, there is growing evidence for an abrupt (1,000 years) warming at 14,000 B.P., followed by a cooling at about 11,000 B.P. (termed the Younger Dryas) and another abrupt warming about 10,000 B.P. (Figure 3.4~. New sea level estimates (Fairbanks, 1989) link meltwater pulses to these warming events, revealing that sea level rose 24 m in less than 1,000 years at 12,000 B.P.

EARTH SYSTEM HISTORY AND MODELING LAKE GERZENSEE 50 1 00 15a CH 2213 250 31a2 ~ 350 ~ 400 81 DYE 3 GREENLAND 4c HE (YRS BP) 10 0ao 11,000 13,000 ~ 1 1 ~ -5 - e 018 - 1 5 - 1 0 - 1 763 - 1 778 - 1 780 I 79e - 1 800 -1818 -1820 - I 830 -35 -30 -25 FIGURE 3.4 Evidence from Greenland and Switzerland for rapid environmental oscillations at die end of the last glacial. Note the striking similarity of warming about 14,000 B.P., cooling at 11,000 B.P., and warming again at 10,000 B.P. (From Oeschger (1985~. Copyright @) 1985 by the American Geophysical Union.) There is also evidence for abrupt changes in surface, intermediate, and deep water during the Younger Dryas (Boyle and Keigwin, 1987; Ruddiman and McIntyre, 1981~. Some studies suggest the 10,000 B.P. transition to warm conditions may have occurred in as little as 20 years (Dansgaard et al., 1989~. Abrupt transitions found in the Dye 3 Greenland ice core between 40,000 and 30,000 B.P. may be associated with rapid carbon dioxide fluc- tuations (Oeschger et al., 1985~. However, these earlier events have yet to be reproduced in other ice cores. Analyses of pollen data have shown that the rate of change in terrestrial vegetation has varied considerably through time, with widespread and abrupt changes in eastern North America concentrated especially in the period from 14,000 to 10,000 years ago, probably in response to large-scale changes in atmospheric circulation (Jacobson et al., 1987~. The abruptness and magnitude of the above changes remain unexplained. They represent a dramatic contribution of earth studies to the understanding of global change. Some ideas link the rapid changes to a complete reorga- nization of the ocean-atmosphere system (Broecker and Denton, 1989; Broecker

82 RESEARCH STRATEGIES FOR THE USGCRP et al., 1985~. To better understand this phenomenon, it is necessary to determine the relationship between slowly changing boundary conditions (e.g., Milankovitch forcing; COHMAP Members, 1988) and abrupt system responses. Observational Needs. Observations are required . to determine the magnitude and global extent of abrupt changes dur- ing the last deglaciation (14,000 to 10,000 B.P.~. Previous studies reveal large coherent changes in the North Atlantic basin, with changes at 14,000 to 13,000 B.P. also occurring in the southern hemisphere. However, the chronology of these events must be improved, both on land and sea, and we need better measurements of the extent and magnitude of the system re- sponse. Needed are multiple, independent monitors of changes on land (i.e., temperature, precipitation, and biota), in the surface and deep ocean, and in the atmosphere (i.e., dust and sulfate aerosols) including its chemical composition (carbon dioxide and methane). Also needed is an even more intensive study of climatic and biospheric changes in the North Atlantic basin, as this region seems to be a key to understanding the processes responsible for the changes. A precise and much-improved chronology of events in key parts of the ocean-atmosphere-biosphere system can be devel- oped now by application of atomic mass spectroscopy (AMS) carbon-14 techniques, which have the advantage of requiring much smaller and more reliable samples than earlier techniques. These results should be viewed within the framework of global-scale data bases as mapped by the COHMAP Members (1988~. · to test the existence of rapid carbon dioxide fluctuations from 40,000 to 30,000 B.P. This subtask is closely related to a more comprehensive examination of carbon cycle fluctuations, which is best accomplished when viewed from the perspective of the last 130,000 years (see the section "Glo- bal Carbon Cycle" below). Unlike the deglaciation, in which carbon diox- ide changes apparently were not rapid, studies of the Dye 3 Greenland ice core reveal rapid (centennial- or even decadal-scale) oscillations in carbon dioxide and other climate variables (Dansgaard et al., 1989~. Such discov- eries have prompted hypotheses that the ocean-atmosphere system may have more than one stable state (Broecker et al., 1985~. Those rapid fluctuations have not been detected in the Byrd antarctic ice core (Neftel et al., 1988~. Possible explanations for this contradiction include (1) the Greenland site was exposed to local warming, with seasonal melting affecting carbon dioxide concentrations or (2) the air "closure time" for the Byrd core is too long. To clarify these issues, it is highly desirable to gather ice core records from regions of higher accumulation where air closure times are shorter and time scales can be established with greater confidence. Any evidence linking

EARTH SYSTEM HISTORY AND MODELING 83 rapid changes in both climate and carbon dioxide is important for validating predictions for future climate. The second International Greenland Ice Sheet Program (the U.S. program is GISP II and the European program is GRIP) has just begun drilling in central Greenland to obtain a 150,000-year record with moderate time resolution. Studies of gas bubbles, isotopes, major ele- ment chemistry, and dust have already begun; other measurements should be added to take advantage of this core. In the future it may be necessary to acquire other ice cores (e.g., from Antarctica). to determine the nature of biotic responses to abrupt environmental change. Although major terrestrial extinctions occurred at the end of the Pleistocene, many large environmental changes were not associated with extinctions. The background of gradual biotic responses to orbital forcing has been well illuminated by recent synoptic studies (e.g., COHMAP Mem- bers, 1988; Huntley and Webb, 1989; Huntley and Prentice, 1988; Webb et al., 1987~. There are, nevertheless, events such as the Younger Dryas cooling (ca. 11,000 to 10,000 B.P.) associated with large, rapid changes in some regions of the world. Understanding of the spatial extent and synchroneity of biotic responses to both abrupt and gradual climatic changes must be greatly improved. · to determine the temporal history of millennial-scale fluctuations that may be linked to abrupt transitions. Some data indicate that abrupt transitions such as the Younger Dryas and possible oscillations between 40,000 and 30,000 B.P. may be related to 2,500-year time scale climate oscillations that have been detected in the Holocene (Demon and Karlen, 1973; Figure 3.2), including the LIA, and in some marine records (e.g., Pisias et al., 1973~. At present, studies of such fluctuations are limited by the lack of long time series from high-deposition-rate deep-sea cores. Enhanced coring capabili- ties in deep-sea sediments may be needed (see the section "Sample Acquisition" below). On land, expanded paleoecological and ice core studies with high temporal resolution for the past 40,000 years are needed. Modeling Needs. Models are needed · to develop a better understanding of the causes and processes involved in rapid climate transitions. With the reality of abrupt transitions becoming more and more apparent, a much better understanding of the causes and processes involved in the transitions is needed. Broecker et al. (1985) have proposed that such changes may be caused by major reorganizations of the ocean-atmosphere system. Some modeling studies support elements of this conjecture (e.g., Maier-Reimer and Mikolajewicz, 1989; Man abe and Stouffer, 1988), and detailed modeling comparisons for the Younger Dryas (11,000 to 10,000 B.P.) support the consistency between ocean and land data in the vicinity of the North Atlantic (Rind et al., 1986~. However, future work

84 RESEARCH STRATEGIES FOR THE USGCRP must apply improved oceanic models, assess the global distribution of the changes, and determine how abrupt transitions are related to slowly chang- ing boundary conditions (Milankovitch forcing). Resolving Model-Data Discrepancies over the Last 20,000 Years Detailed global mapping programs of the last decade have provided sub- stantial information about the surface of the Earth over the last 18,000 years (CLIMAP Project Members, 1976, 1981; COHMAP Members, 1988~. These studies have stimulated a number of modeling endeavors, which demonstrate levels of agreement between models and data varying from excellent to poor (cf. summary in Crowley and North, 1990~. In order to establish higher levels of confidence in climate models, it is necessary to reconcile these differences over a time interval that is particularly rich in data. Better understanding of these "snapshot" time intervals may reveal how slowly changing boundary conditions could trigger instabilities in the climate sys- tem (see the section "Abrupt Changes" above). Some special areas of enhanced model-data comparison involve the fol- lowing: (1) resolving tropical sea surface temperature (SST), lowland pre- cipitation, and snowline fluctuations at 18,000 B.P. (Rind and Peteet, 1985; Webster and Streten, 1978~; (2) making more quantitative comparisons of models and observations for the African-Asian monsoon regions at 9,000 and 6,000 B.P. (COHMAP Members, 1988; Mitchell et al., 1988~; (3) clari- fying factors responsible for high-latitude climatic change in the southern hemisphere at 18,000 B.P. (cf. Crowley and North, 1990~; and (4) utilizing oceanic GCMs to understand intermediate- and deep-water circulation changes during the last glacial maximum and deglaciation (Boyle and Keigwin, 1987; Maier-Reimer and Mikolajewicz, 1989~. The committee recommends that resolving model-data discrepancies in the tropics at 18,000 B.P. be given the highest priority. Observational Heeds. Observations are needed · to increase information on regional climate patterns over the last 20,000 years. Although we have good information from some regions, a number of data gaps must be filled. Some of the most important areas of concern are tropical lowlands and SSTs at 18,000 B.P. The tropical lowland informa- tion is especially important for better estimates of temperature and precipi- tation changes at low elevations on the continents. This information would also be extremely valuable for assessing the sensitivity of tropical rain forests to environmental change. These studies should improve our under- standing of high biological diversity in the tropics. · to improve estimates of temperature and rainfall variations in regions where model-data comparisons need to be more quantitative, such as the

EARTH SYSTEM HISTORY AND MODELING 85 lowland tropics and southwestern United States (18,000 B.P.) and the Afri- can-Asian monsoon (9,000 to 6,000 B.P.~. · to develop a more complete assessment of transfer functions used for paleoenvironmental estimates. Paleo-oceanographic data should be care- fully evaluated and compared with other, independent estimates of SST. Temperature depression associated with snowline and paleovegetational descent on tropical mountains should be quantified and dated. Estimates for changes in low-elevation temperatures need substantial improvement. Any new pa- leoecological records from low latitudes for 18,000 B.P. and before will be extremely valuable. Modeling Needs. Models should be used for · increased testing of atmospheric and oceanic models in order to re- solve apparent model-data discrepancies. New GCMs with better horizontal and vertical resolution are needed. Precipitation estimates derived from models (with biospheric feedback) or geological data should be included. Finally, oceanic GCMs should be used to explore additional changes over the last 18,000 years. application of advanced models to address important model-data dis- crepancies over the last 20,000 years. Among the most important modeling problems to address are (1) capability of climate models to generate drier conditions in tropical lowlands at 18,000 B.P.; (2) reconciliation of appar- ently small lowland temperature changes with larger upland temperature changes in the tropics at 18,000 B.P.; (3) improved quantitative agreement between models of enhanced monsoon and southwestern U.S. rainfall fluc- tuations at approximately 9,000 and 18,000 B.P., respectively, and observations; (4) identification of the mechanisms responsible for significant cooling in the high latitudes of the southern hemisphere at 18,000 B.P.; and (5) ability of oceanic models to generate decreased deep-water and enhanced interme- diate-water production rates in the North Atlantic at 18,000 B.P. The Last Glacial Cycle (Last 130,000 Years) Many of the processes that can be studied over the carbon-14 time scale provoke explanations that should be applicable to other time intervals of climatic change during the Quaternary. In particular, a considerable amount of information is available from marine and ice core records revealing the climate of the last full glacial cycle (Figure 3.5~. The growing number of land records available from this time interval will make it possible to map climate evolution over a full glacial cycle as it relates to Milankovitch forcing. Figure 3.5 suggests that carbon dioxide may have played an impor- tant role in climatic change over the last 130,000 years. However, because carbon dioxide lags climatic change in the southern hemisphere at the end

86 2 o 4 -6 -8 13 12 11 a - 10 8 RESEARCH STRATEGIES FOR THE USGCRP 4 r ~ T T ~ . , I . . T | T ~ · ~ r r T I 1 ~ r i-- ~ ~ · ~ . .- r 6 310 -Vostok Temp. - con AD \. , A "cite V i'. W S. Ocean SST ' . ., 1 I_· . 1 · · · ~ . · . I · . · I . . . I . . . I · 0 20 40 60 80 100 120 t 40 160 Time (Ka) 290 270 ~ Q 250 Q o 230 210 190 FIGURE 3.5 Comparison of three high-latitude records from the southern hemi- sphere showing the overall good agreement between carbon dioxide and temperature changes (inferred from AD). However, the carbon dioxide record clearly lags the Vostok (Antarctica) temperature record at the end of the last interglacial. (Re- printed, by permission, from T.J. Crowley and G.R. North (1990~. Copyright @) 1990 by Oxford University Press. Data sources: the Vostok /`D record (Jouzel et al., 1987) and the carbon dioxide record (Barnola et al., 1987) are plotted according to the revised chronology of Petit et al. (1990~.)

EARTH SYSTEM HISTORY AND MODELING 87 of the last interglacial, the climate feedback of this important variable war- rants further evaluation. Thus study of climatic change over the last 130,000 years will make it possible to (1) address more fully the mechanisms re- sponsible for ice age carbon dioxide changes and the magnitude of the carbon-dioxide-climate feedback; (2) investigate both the nature of warmth during the last interglacial and the processes responsible for cooling and ice cap growth at the end of the warm period; and (3) trace the regional varia- tions in climate on both land and sea as the system evolves through an entire glacial cycle. Global Carbon Cycle The ability to study changes in past atmospheric composition from the bubbles trapped in polar ice caps has been one of the major scientific devel- opments of the past decade. Convincing evidence now exists for changes in atmospheric carbon dioxide during the past 150,000 years (Barnola et al., 1987; Neftel et al., 1988~. Ongoing work is documenting variations in methane, nitrous oxide, and the oxygen isotope composition of ancient atmo- spheres; the data show that large changes in atmospheric carbon dioxide have occurred that are approximately in step with the major climatic changes of the last 150,000 years and that significant changes have occurred in other natural greenhouse gases. One particularly intriguing result is the observation that the antarctic climate appears to have cooled substantially before carbon dioxide decreased at the end of the last interglacial period (Figure 3.5~. This observation needs to be confirmed and examined for its climatic implications. The causes of changes in atmospheric carbon dioxide must be sought in models of the oceanic carbon system, which is the only buffer sufficiently massive, yet fast enough to drive the large and rapid changes seen in the ice core record. Several competing ideas remain to be tested concerning atmo- spheric carbon dioxide (e.g., Boyle, 1988; Broecker, 1982; Knox and McElroy, 1984; Sarmiento and Toggweiler, 1984; Siegenthaler and Wenk, 1984), and no consensus exists as to which, if any, of these models provides the correct explanation for the observed atmospheric changes. The continued develop- ment of oceanic carbon system models to explain past changes in atmospheric carbon dioxide should be a high priority in global change research. Observational Needs. Observations are needed to develop a better understanding of the timing of events in the ocean during the last 150,000 years. This information is needed for evaluation of scenarios for climatic change and for measurement of time constants associ- ated with components of the climate system. As mentioned in the preceding section "Abrupt Changes," there have been major changes in both the deep- and the intermediate-water circulations, which have implications for atmo

88 RESEARCH STRATEGIES FOR THE USGCRP spheric carbon dioxide. While the ocean must be the proximate determinant of changes in atmospheric carbon dioxide, it is difficult to evaluate precisely the relative timing of events observed in the ocean cores and ice cores. Furthermore, it is also necessary to understand the temporal relationships among changes in ocean circulation, ocean chemistry, and changes on the continent (e.g., in ice extent, atmospheric dust transport, and vegetation). As another example, we need to know the timing of glacial mass-wasting in various parts of the northern hemisphere ice sheets in relation to changes in oceanic temperature and salinity; can the sequence of events in the ocean be linked to forcing by meltwater and iceberg calving? · to conduct detailed comparisons of ice core, marine, and continental records. Some problems of particular interest that might be examined include the interactions of terrestrial ecosystems, environmental change, and atmo- spheric composition, and the climatic effect of possible variations in cloud cover. The latter might be induced by changes in atmospheric dust measured in both ice cores and deep-sea cores and by changes in DMS emissions, which have been inferred from ice core measurements of the DMS by- products (MSA and excess sulfate). Both of these variables could affect the radiation budget on glacial-interglacial time scales (Charlson et al., 1987; Harvey, 1988~. Modeling Needs. Models are required to understand the origin of ice age carbon dioxide and methane changes. One of the key modeling challenges facing earth scientists is to understand the origin of the ice age carbon dioxide fluctuations. Attempts to elucidate this relationship over the past decade have turned up a surprising number of ways in which the ocean might change the partial pressure of carbon diox- ide (pCO2~. Yet this research has been frustrated by difficulties in accounting for the timing and amplitude of the pCO2 observations. Current understanding of the role of the ocean circulation, chemistry, and productivity does not account for the observed pCO2 record. An effort should be made to im- prove models of oceanic carbon dioxide. Such efforts will be a major contribution of earth system research to the USGCRP. There is a growing need to couple climatic and geochemical models in the future. The Previous Interglacial The previous interglacial was the last time when conditions were as warm as the present (Holocene) interglacial. Although a number of studies have concluded that it was warmer than at present, other investigations suggest that in some cases the warmth was primarily seasonal in nature (e.g., Prell and Kutzbach, 1987~. Except for a few regions, global SSTs may not have

EARTH SYSTEM HISTORY AND MODELING 89 been significantly different from those at present (CLIMAP Project Mem- bers, 1984~. However, sea level was about 5 to 6 m higher than at present (e.g., Mesolella et al., 1969; Dodge et al., 1983~. It has been suggested that the last interglacial may be an "analog" for the early stages of a future greenhouse warming (e.g., Hansen and Lebedeff, 1987~. Acceptance of this suggestion, however, requires clarification of the seasonal (versus year-round) nature of the warmth and whether the warmth was globally synchronous. Present evidence suggests that during the last interglacial globally averaged mean annual temperatures were not signifi- cantly greater than at present (Crowley, 1990) and that the last interglacial should not be cited as a carbon dioxide analog. Nevertheless, it is still desirable to understand the regional patterns of warmth during this period. It is also important to determine which ice sheets contributed to the sea level rise both Greenland and the West Antarctic ice sheets have been suggested (Koerner, 1989; Mercer, 1978~. These two topics-warmth and sea level are critically related because knowledge of the magnitude of warming may help calibrate the sensitivity of the cryosphere to warming trends and thus enhance our ability to predict the course of future changes in sea level. Additional questions related to this time period concern the transition into the last glacial stage. As stated earlier, carbon dioxide lags cooling in the southern hemisphere (Figure 3.5~. Furthermore, recent modeling studies have suggested that the reduced solar insolation of the 115,000 to 105,000 B.P. interval may not have been sufficient to generate or maintain low- elevation ice sheets in the Laurentide area, even with reduced carbon diox- ide (Rind et al., 1989~. Such results call into question either climate model sensitivity or our understanding of the mechanisms whereby orbital varia- tions lead to ice sheet growth, or both. Observational Needs. Observations are required to determine the magnitude and nature of the last interglacial warmth. Two items of paramount importance involve (1) evaluation of whether times of greater warmth were globally synchronous and (2) evaluation of terres- trial paleoclimate proxy-data to determine whether the warmth was seasonal or mean-annual in nature. The latter may require new techniques for esti- mating past temperatures. · to establish the source of global rise in sea level. The higher temperatures presumably triggered a sea level increase of 5 to 6 m. However, we need to know whether the increase resulted from melting in Greenland or in Antarctica, or both. · to clarify the nature of climate transitions into and out of the last interglacial. Research on the last deglaciation suggests that significant cli

9o RESEARCH STRATEGIES FOR THE USGCRP mate oscillations were involved perhaps similar to the Younger Dryas. Are such oscillations characteristic of all deglaciations? Other studies of relevance include the rate at which climate deteriorated at the end of the last interglacial. Some studies suggest that the transition was quite abrupt (Frenzel and Bludau, 1987~. Answering this question involves knowing more about where and when the Laurentide Ice Sheet developed. Modeling Needs. Efforts are needed to develop cryosphere models that incorporate inferred temperature history with fluctuations of ice sheets on Greenland and Antarctica. to utilize ocean models to determine how changes in atmospheric forcing affected oceanic circulation. Of particular interest is the question whether such oceanic changes could account for inferences of increased mean-an . . nua temperatures in some regions. · to continue atmospheric modeling in order to test sensitivity of models to known variations in orbital forcing, in particular with respect to regions of ice sheet growth and decay. It is of special interest to assess the role of carbon dioxide in the onset of glaciation. Regional Variations in Climate over a Glacial Cycle Although we have a reasonable picture of how climate changed in a number of regions over the last glacial cycle, changes in different regions have not been adequately integrated to determine the nature of dynamical linkages. The fairly widespread availability or potential availability of a number of good records over this time interval justifies a period of time to be studied as a "special observing period," for which the various mecha- nisms proposed to explain observed changes may be tested. Observational Needs. Observations are required · to develop long terrestrial records of climatic change. At present knowledge of the land record beyond the range of carbon-14 dating is rather limited. In Europe, studies of several exceptionally long stratigraphic records have revealed the responses of vegetation to climatic changes throughout the entire last glacial-interglacial cycle (e.g., Guiot et al., 1989~. More records should be acquired and correlated with the deep-sea record. Acqui- sition of long terrestrial records may require enhanced coring capabilities (see the section "Sample Acquisition" below). · to refine estimates of variations in dust and clouds. Evidence suggests that these changes may affect the planetary radiation budget and contribute to glacial-interglacial climatic change. The dust record is of large geo- graphic scale (Petit et al., 1990~. Clouds, as inferred from DMS by-prod- ucts in ice cores (Legend et al., 1988), must be better understood.

EARTH SYSTEM HISI ORY AND MODELING 91 The Last Few Glacial-Interglacial Cycles (Last 500,000 Years) Although detailed investigation of the last glacial cycle will provide con- siderable insight into processes responsible for Pleistocene ice sheet fluc- tuations, that information alone will not resolve the problem. These ideas must be tested through several realizations of glacial-interglacial cycles. In addition to the study of the last glacial cycle, the second major proposed task will address selected elements of these longer time series so as to place phenomena from the most recent cycle in perspective. Previous work by the SPECMAP group indicates a strong influence of orbital forcing on the evolution of the earth's climate system (e.g., Berger et al., 1984~. This work represents an excellent opportunity to examine system sensitivity to known forcing. In addition, sampling climate fluctua- tions through several realizations enables gathering of reliable statistics in- dicating how various components of the system are coupled. Such results can make valuable contributions to the USGCRP. For example, studies clearly indicate that there are significant phase offsets among the different compo- nents of the climate system (e.g., Imbrie et al., 1989~. Although significant progress has already been made on this topic, better information about some variables is needed to constrain models of the Pleistocene ice ages. Observational Needs. Observations are required - · to continue measurements of various components of the climate sys- tem over the last few hundred thousand years. More measurements of SSTs, deep and intermediate waters, aeolian fluxes, and various compo- nents of the carbon cycle are needed. It is especially desirable to link these records with the growing number of long land records (e.g., Hovan et al., 1989; Kukla, 1989). ~. Modeling Needs. Efforts are needed · to develop time-dependent models linking various components of the climate system. For example, GCM studies indicate that there is a direct link between seasonal variations in orbital forcing and monsoon variability over the last glacial cycle (Kutzbach and Street-Perrott, 1985; Prell and Kutzbach, 1987~. Observational studies indicate that this monsoon signa- ture is detectable in the deep sea (Pokras and Mix, 1987; Prell, 1984) and may be involved in continental biomass variations (Keigwin and Boyle, 1985) that could affect atmospheric methane (wetlands are an important source of methane). An ocean modeling study supports a direct link be- tween orbitally induced changes in the monsoon and surface circulation in the Indian Ocean (Luther et al., 1990~. More systematic information about these interactions will clarify an important climate problem. Because the relationship between forcing and initial system response is so clear and

92 RESEARCH STRATEGIES FOR THE USGCRP amenable to modeling, the new information will provide an ideal opportu- nity for some advanced modeling studies using, for example, biospheric models and coupled oceanic-atmospheric models. · to develop models of ice age carbon dioxide fluctuations for the last glaciation and glacial cycle that make predictions of the time-dependent history of various components of the climate cycle. Modeling studies need to cast such predictions in the time and frequency domains. SYSTEM RESPONSES TO LARGE CHANGES IN FORCING Prior to the Pleistocene, there were large changes in boundary conditions for the earth's climate system. These changes involved variations in conti- nental position and height, the boundaries of the ocean basins, sea level, and probably the carbon dioxide content of the atmosphere. The evolving boundary conditions were paralleled by large changes in the earth's climate. The major features of the evolution of global climate over the last 100 million years are illustrated in Figure 3.6. Note the long-term cooling trend characterized by relatively abrupt transitions. Although these long-term trends are based on qualitative and semiquantitative information in both marine and continental records, more quantitative information is now be- coming available. The environment of the Cenozoic offers unique opportunities for studies in global change. Topics of special importance involve the response of the earth system during times when climates were substantially warmer than modern and intervals during which the climate system experienced rapid and large changes. An especially important contribution involves the bio- spheric response to these large changes, as the geologic record provides the only information available on the relation between extinction events and environmental change. Pre-Pleistocene climate records also present a formidable challenge to models. Detailed information on past warm intervals will be extremely valuable for testing model ability to simulate warmer climatic conditions that are likely to have resulted for different reasons. As these same models will be used to predict the nature of future greenhouse warming, it is impor- tant to validate the models in an independent manner. The geologic record provides the only independent test of these models. Cenozoic records will be especially useful in testing oceanic models and coupled oceanic-atmo- spheric models. It is especially important to test the highly parameterized coupling between the ocean and atmosphere against independent data sets because the boundary conditions are so radically different. There is a wealth of marine data available from the Ocean Drilling Program (ODP) to validate model simulations. Important questions to consider include whether models are capable of simulating warmer climates under different boundary condi

EARTH SYSTEM HISTORY AND MODELING -2 1 O~r. IIJ IL Z J a: tI: o t1 ~ 2 Z ~ 3 w m O .~. 93 . JEW I1 ? . o 20 40 60 80 100 MILLIONS OF YEAtlS 20 15 0 [LI 10 5 0 AL o In FIGURE 3.6 Deep-water oxygen isotope record for the last 100 million years, illustrating die long-term cooling trend arid the tendency for the system to evolve Trough abrupt transitions (arrows). (Adapted, by permission, from R.G. Douglas and F. Woodruff (1981~. Copyright (3 1981 by Cesare Emiliani.) lions and whether there is a satisfactory explanation for abrupt changes. The committee recommends that research under this initiative fall into two basic types of case studies: (1) environments of extreme warm periods and (2) climate-biosphere connections during abrupt changes. Environments of Extreme Warm Periods The most important intervals to focus on in studying past warm periods in order to test the ability of models to simulate warmer climatic conditions are the Pliocene (3 to 5 Ma), the Early Eocene (50 to 55 Ma), and the mid- Cretaceous (100 Ma). For each of these intervals, tropical biota expanded into higher latitudes than at present and polar ice cover was greatly reduced. There may have been significant changes in oceanic circulation as well. Attempts have been made to model these warm climates (e.g., Barron, 1985; Barron et al., 1981; Crowley et al., 1986~. The major paleogeographic changes of the last 100 million years are not sufficient to model the warm climates. Other factors such as increased atmospheric carbon dioxide may be needed to explain the warm periods.

94 RESEARCH STRATEGIES FOR THE USGCRP Although each of the above intervals is important, the committee gives the Pliocene highest priority because it is the most recent time period for which we have evidence for climates significantly warmer than the present. In general, precision and resolution of climate data will be higher and sam- pling densities will be greater for more recent warm intervals than for older warm intervals. Pliocene flora and fauna are also very similar to modern, Pliocene records are widespread and easily accessible in both continental and marine settings, and most Pliocene records have undergone little alter- aiion. All these features facilitate more quantitative estimates of environmental information and development of regional and even global patterns of climatic data. In addition, Pliocene warm intervals are punctuated by the abrupt development of wide-scale glaciation in the northern hemisphere at about 2.5 Ma (see the section "Climate-Biosphere Connections During Abrupt Changes" below). Observational Needs. Observations are required to develop a global stratigraphy for warm intervals, particularly the Pliocene, that enables correlation of different sections on both land and sea. In order to implement this plan, substantial investments may be required to improve chronologies and acquire long records from terrestrial and marine environments. · to derive quantitative estimates of a variety of climatic parameters and boundary conditions, including estimates of temperature and precipitation as well as seasonality of continental interiors. High-resolution time series and synoptic studies are required. Whenever possible, sampling intervals of time series should be fine enough to detect forcing functions on orbital time scales. At a minimum, synoptic studies should provide regional resolution comparable to model output. Additionally, proxy evidence for carbon diox- ide levels and better assessment of paleorecords of wind, ice extent, and sea level are required. Changing boundary conditions such as oceanic gateways and orography must be better specified. Although individual research projects will be an important component in understanding past climates, fully successful implementation of this research plan and maximum interaction with models will require coordinated, multidisciplinary efforts that integrate work from a wide variety of disciplines and environments. Modeling Needs. Efforts are required to experiment with atmospheric and oceanic models under radically different boundary conditions. These efforts will prove a sturdy test for climate models. For example, an oceanic GCM sensitivity experiment for an open Central American isthmus indicates that North Atlantic deep-water production may have collapsed (Figure 3.7~. Since this same model will

EARTH SYSTEM HISI ORY AND MODELING Atlantic Heat Transport (PWJ 2.0 1.0 0.0 -1 .0 - 95 ~ Present ~ Open C.American Isthmus 90 S 60 30 EQ 30 60 90 N FIGURE 3.7 Ocean general circulation model experiment testing the effect of an open Central American isthmus on North Atlantic d~e~mohaline circulation and poleward ocean heat passport (tile latter is positively correlated with the amount of North Atlantic deep water produced). (Reprinted, by permission' from T.J. Crowley aIld G.R. North (1990~. Copyright @) 1990 by Oxford University Press: after Maier- Reimer et al., 1990.) eventually be used to make greenhouse predictions, it is of interest to deter- mine whether there is any geological support for such a large change. In fact, the record does provide some support for this conclusion (Woodruff and Savin, 1989; Delaney, 1990~. Continued experiments with oceanic models under radically altered boundary conditions (Barron and Peterson, 1990) are needed. Other examples of modeling studies involve atmospheric GCMs and coupled oceanic-atmospheric models (e.g., Washington and Meehl, 1989~. In addition, there is a need for further testing of models of warm, saline bottom-water production (Brass et al., 1982; Peterson, 1979~. The geologic record represents the only realistic test for the parameterizations in these models. Climate-Biosphere Connections During Abrupt Changes A number of significant and relatively abrupt transitions have occurred during the last 100 million years (Figure 3.6~. Some transitions, such as those near the Eocene-Oligocene boundary (34 Ma), appear to be closely associated with ice buildup and reorganization of oceanic circulation due to the tectonic evolution of ocean basins (Corliss and Keigwin, 1986; Kennett et al., 1974~.

96 RESEARCH STRATEGIES FOR THE USGCRP In general, the origin of abrupt transitions is not well understood. There are at least four classes of models exhibiting unstable behavior due to (1) thermohaline instabilities reorganizing the oceanic-atmospheric circulation (Broecker et al., 1985), (2) ice albedo feedback resulting in abrupt changes in ice volume (North and Crowley, 1985), (3) nonlinear feedbacks in the climate system leading to "internal" oscillations in climate (e.g., Saltzman and Sutera, 1984), and (4) carbon dioxide changes due to abrupt changes in ocean productivity (e.g., Arthur et al., 1988~. The importance of abrupt transitions for global change research is clear, as much of the concern about the human impact on the earth system in- volves the unprecedented rate of changes. Study of rapid transitions or abrupt changes between different states offers the potential to monitor effects of rapid change on the environment, including the response of the biosphere. Better understanding of these events will delineate the system response to a large, sudden perturbation. A number of events stand out in their potential to contribute substantially to objectives of global change research. The two most promising intervals for study are the late Pliocene 2.5-Ma onset of mid-latitude northern hemi- sphere glaciation and the Eocene-Oligocene cooling (30 to 40 Ma) marked by expansion of antarctic ice and the largest biotic turnover in the Ceno- zoic. The Cretaceous-Tertiary (K-T) boundary event also merits attention in that it is a major extinction event associated with and likely caused by an asteroid or comet impact (Alvarez et al., 1980~. The development of extensive northern hemisphere ice sheets at about 2.5 Ma is a rapid transition from relatively ice free conditions in polar regions of the northern hemisphere. The 2.5-Ma event follows closing of the Isthmus of Panama, opening of the Bering Strait, and continued moun- tain building in Tibet and western North America (Ruddiman and Raymo, 19883. The cooling was accompanied by increasing aridity in the tropics and had a profound effect on life in the vicinity of the North Atlantic (Stanley, 1986~. The emergence of the Isthmus of Panama at this time or slightly earlier separated marine organisms in the Atlantic and Pacific and allowed extensive interchange of mammals between North and South America. The interval spanning the Eocene-Oligocene boundary marks the transi- tion from the relatively warm periods of the early Cenozoic to the cold periods of the middle and late Cenozoic. The transition is marked by at least three relatively abrupt steps- the middle to late Eocene (40 Ma), the Eocene-Oligocene boundary (34 Ma), and the mid-Oligocene (30 Ma). These transitions are marked by some significant climate steps (Figure 3.6~. There is evidence for ice sheet development as early as 40 Ma on Antarctica, and oxygen isotope evidence clearly records an additional large change at 34 Ma. There is additional evidence for glacial expansion on Antarctica about

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

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

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

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

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.

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.

EARTH SYSTEM HISI ORY AND MODELING 103 Cane, M.A., S.C. Dolan, and S.E. Zebiak. 1986. Experimental forecasts of the 1982/83 El Nina. Nature 321:827-832. Charlson, R.J., J.E. Lovelock, M.O. Andreae, and S.G. Warren. 1987. Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature 326:655- 661. CLIMAP Project Members. 1976. The surface of the ice-age earn. Science 191:1131- 1144. CLIMAP Project Members. 1981. Seasonal reconstruction of the earth's surface at the last glacial maximum. Map and Chart Series 36. Geological Society of America, Boulder, Colo. CLIMAP Project Members. 1984. The last interglacial ocean. Quat. Res. 21:123- 124. COHMAP Members. 1988. Climatic changes of the last 18,000 years: Observa- tionsandmodelsimulations. Science241:1043-1052. Corliss, B.H., and L.D. Keigwin, Jr. 1986. Eocene-Oligocene paleoceanography. Pp. 101-118 in K.J. Hsu (ed.), Mesozoic and Cenozoic Oceans. AGU Geodynamics Series 15:101-118. Crowley, T.J. 1990. Are there any satisfactory geological analogs for a future greenhouse warming? Journal of Climate, in press. Crowley, T.J., and G.R. North. 1990. Paleoclimatology. Oxford University Press, New York, in press. Crowley, T.J., D.A. Short, J.G. Mengel, and G.R. North. 1986. Role of seasonality in the evolution of climate over the last 100 million years. Science 231:579- 584. Dansgaard, W., J.W.C. White, and S.J. Johnsen. 1989. The abrupt termination of the Younger Dry as climate event. Nature 339:532-534. Delaney, M.L. 1990. Miocene benthic foraminiferal Ed/C a records: South Atlantic and western equatorial Pacific. Palaeoceanography, in press. Denton, G.H., and W. Karlen. 1973. Holocene climatic variations their pattern and possible cause. Quat. Res. 3: 155-205. Dodge, R.E., R.G. Fairbanks, L.K. Benninger, and F. Maurrasse. 1983. Pleistocene sea levels from raised coral reefs of Haiti. Science 219:1423-1425. Douglas, R.G., and F. Woodruff. 1981. Deep sea benthic foraminifera. Pp. 1233- 1327 in C. Emiliani (ed.), The Sea, 7. Wiley-Interscience, New York. Elmore, D., et al. 1988. Geoscience Research with New and Improved AMS Instrumentation. A Report of the Accelerator Mass Spectrometry Advisory Committee (AMSAC). 10 pp. Fairbanks, R.G. 1989. A 17,000-year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation. Nature 342:637-642. Frenzel, B., and W. Bludau. 1987. On the duration of the interglacial to glacial transition at the end of the Eemian Interglacial (Deep Sea Stage 5 e): Botanical and sedimentological evidence. Pp. 151-162 in W.H. B erg er and L.D. Labeyrie (eds.), Abrupt Cl~matic Change. D. Reidel, Dordrecht, The Netherlands. Gaffin, S.R., M.I. Hoffert, and T. yolk. 1986. Nonlinear coupling between surface temperature and ocean upwelling as an agent in historical climate variations. J. Geophys. Res. 91:3944-3950.

104 RESEARCH STRATEGIES FOR THE USGCRP Graumlich, L.J., and L.B. Brubaker. 1986. Reconstruction of annual temperature (1590-1979) for Longmire, Washington, derived from tree rings. Quat. Res. 25:223-234. Guiot, J., A. Pons, J.L. de Beaulieu, and M. Reille. 1989. A 140,0)0-year continental climate reconstruction from two European pollen records. Nature 338:309- 313. Hansen, J., and S. Lebedeff. 1987. Global trends of measured surface air temperatures. J. Geophys. Res. 92(D11~:13345-13372. Harvey, D.L.D. 1988. Climatic impact of ice-age aerosols. Nature 334:333-335. Hays, J.D., J. Imbrie, and N.J. Shackleton. 1976. Variations in the earth's orbit: Pacemaker of the ice ages. Science 194:1121 -1132. Hovan, S.A., D.K. Rea, N.G. Pisais, and N.J. Shackleton. 1989. A direct link between China loess and marine ALSO records: Aeolian flux to the north Pa- cific. Nature 349:296-298. Huntley, B., and I.C. Prentice. 1988. July temperatures in Europe from pollen data, 6000 years before present. Science 241:687-690. Huntley,B.,andT.Webb. 1989. Migration: Speciest response to climatic variations caused by changes in the earth's orbit. J. Biogeogr. 16:5-19. Imbrie, J., et al. 1984. The orbital theory of Pleistocene climate: Support from a revised chronology of the marine ALSO record. Pp. 269-305 in A. Berger, J. Imbrie, J. Hays, G. Kukla, and B. Saltzman (eds.), Milankovitch and Climate. D. Reidel, Dordrecht, The Netherlands. Imbrie, J., A. McIntyre, and A. Mix. 1989. Oceanic response to orbital forcing in the late Quaternary: Observational and experimental strategies. Pp. 121-164 in A. Berger, S.H. Schneider, and J.-C. Duplessy (eds.), Climate and Geosciences. Kluwer, Dordrecht, The Netherlands. International Geosphere-Biosphere Programme (IGBP). 1989. Global Changes of the Past. Report No. 6. IGBP, Stockholm, Sweden. 39 pp. Jacobson, G.L., Jr., T. Webb III, and E.C. Grimm. 1987. Pattems and rates of vegetation change during the deglaciation of eastern North America. Pp. 277- 288 in W.F. Ruddiman and H.E. Wright, Jr. (eds.), North America and Adja- cent Oceans During the Last Deglaciation. DNAG Vol. K-3. Geological Society of America, Boulder, Colo. Jouzel, J., et al. 1987. Vostok ice core: A continuous isotope temperature record over the last climatic cycle (160,000 years). Nature 329:403-418. Keeling, C.C., et al. 1989. A Three Dimensional Model of Atmospheric CO2 Transport Based on Observed Winds: Observational Data and Preliminary Analysis. Appendix A in Aspects of Climate Variability in the Pacific and the Western Americas. Geophysical Monograph 55. American Geophysical Union, Washington, D.C. Keigwin, L.D., Jr., and E.A. Boyle. 1985. Carbon isotopes in deep-sea benthic foraminifera: Precession and changes in low-latitude biomass. Pp. 319-328 in E.T. Sundquist and W.S. Broecker (eds.), The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present. Geophysical Monograph 32. American Geophysical Union, Washington, D.C. Keig win, L.D., Jr., and G.A. Jones. 1989. Glacial-Holocene stratigraphy, chronology, and paleoceanographic observations on some North Atlantic sediment drifts. Deep Sea Research 36:845-867.

EARTH SYSIEM HISTORY AND MODELING 105 Kennett, J.P., et al. 1974. Development of the Circum-Antarctic current. Science 186: 144-147. Knox, F., and M. McElroy. 1984. Changes in atmospheric CO2: Influence of biota at high latitudes. J. Geophys. Res. 89:4629-4637. Koerner, R.M. 1989. Ice core evidence for extensive melting of the Greenland ice sheet in the last interglacial. Science 244:964-968. Kukla, G. (ed.~. 1989. Long continental records of climate. Palaeogeogr. Palaeoclimatol. Palaeoecol. 72:1-225. Kutzbach, J.E., and F.A. Street-Perrott. 1985. Milankovitch forcing of fluctuations in the level of tropical lakes from 18-O kyr BP. Nature 317:130-134. LaMarche, V.C. 1974. Paleoclimatic inferences from long tree-ring records. Sci- ence 183:1043-1048. LaMarche, V.C., and K.K. Hirschboeck. 1984. Frost rings in trees as records of major volcanic eruptions. Nature 307:121-126. Legrand, M.R., R.J. Delmas, and R.J. Charlson. 1988. Climate forcing implications from Vostok ice-core sulphate data. Nature 334:418-420. Luther, M.E., J.J. O'Brien, and W.L. Prell. 1990. Variability in upwelling fields in the northwestern Indian Ocean, 1, Model experiments for the past 18,000 years. Palaeoceanography 5:433-445. Maier-Reimer, E., and U. Mikolajewicz. 1989. Experiments with an ocean GCM on the cause of the Younger Dryas. Report No. 39. Max-Planck-Institut fur Meteorologie, Hamburg, FRG. Maier-Reimer, E., U. Mikolajewicz, and T. Crowley. 1990. Ocean GCM sensitivity experiment with an open Central American isthmus. Palaeoceanography 5:349- 366. Manabe, S., and R.J. Stouffer. 1988. Two stable equilibria of a coupled ocean- atmosphere model. Journal of Climate 1:841 -866. Martinson, D.G., N.G. Pisias, J.D. Hays, J. Imbrie, T.C. Moore, and N.J. Shackleton. 1987. Age dating and orbital theory of the ice ages: Development of a high- resolution 0-300,000-year chronostratigraphy. Quat. Res. 27:1-29. Mayewski, P.A., G.H. Denton, and T.J. Hughes. 1981. Late Wisconsin ice sheets in North America. Pp. 67-178 in G.H. Denton and T.J. Hughes (eds.), The Last Great Ice Sheets. Wiley-Interscience, New York. Mercer, J.H. 1978. West Antarctic ice sheet and CO2 greenhouse effect: A threat of disaster. Nature 271 :321-325. Mesolella, K.J., R.K. Matd~ews, W.S. Broecker, and D.L. Thurber. 1969. The as~onomical theory of climatic change: Barbados data. J. Geol. 77:250-274. Mitchell, J.F.B., N.S. Grahame, and K.H. Needham. 1988. Climate simulation for 9000 years before present: Seasonal variations and the effect of the Laurentide Ice Sheet. J. Geophys. Res. 93:8282-8303. Mosley-Thompson, E., L.G. Thompson, P.M. Grootes and N. Gundestrup. 1990. Little Ice Age (Neoglacial) paleoenvironmental conditions at Siple Station, Antarctica. Ann. Glaciol. 14: 199-204. National Research Council (NRC). 1986. Recommendations for a U.S. Ice Coring Program. National Academy Press, Washington, D.C. 67 pp. Neftel, A., H. Oeschger, T. Staffelbach, and B. Stauffer. 1988. CO2 record in the Byrd ice core 50,000-5,000 years BP. Nature 331:609-611.

106 RESE~CH STRATEGIES FOR THE USGCRP North, G.R., and T.J. Crowley. 1985. Application of a seasonal climate model to Cenozoic glaciation. J. Geol. Sac. London 142:475482. Oeschger, H. 1985. The contribution of ice core studies to the understanding of environmental processes. Pp. 9-17 in C.C. Langway, Jr., H. Oeschger, and W. Dansgaard (eds.), Greenland Ice Core: Geophysics, Geochemistry, and the Environment. Geophysical Monograph 33. American Geophysical Union, Washington, D.C. Oeschger, H., B. Stauffer, R. Finkel, and C.C. Langway, Jr. 1985. Variations of the CO2 concentration of occluded air and of anions and dust in polar ice cores. Pp. 132-142 in E.T. Sundquist and W.S. Broecker (eds.), The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present. Geophysical Monograph 32. American Geophysical Union, Washington, D.C. Peterson, W.H. 1979. A steady thermohaline convection model. Technical Report TR-79-4. Rosenstiel School of Marine and Atmospheric Science, University of Miami, Coral Gables, Fla. 160 pp. Petit, J.R., L. Mounier, J. Jonzel, Y.S. Korotkevich, V.I. Kotylakov, and C. Lorius. 1990. Palaeoclimatological and chronological implications of the Vostok core dust record. Nature 343:56-58. Pisias, N.G., J.P. Dauphin, and C. Sancetta. 1973. Spectral analysis of late Pleistocene- Holocene sediments. Quat. Res. 3:3-9. Pokras, E.M., and A.C. Mix. 1987. Earth's precession cycle and Quaternary climatic changes in tropical Africa. Nature 326:486487. Prell, W.L. 1984. Monsoonal climate of the Arabian Sea during the late Quaternary: A response to changing solar radiation. Pp. 349-366 in A. Berger, J. Imbrie, J. Hays, G. Kukla, and B. Saltzman (eds.), Milankovitch and Climate. D. Reidel, Dordrecht, The Netherlands. Prell, W.L., and J.E. Kutzbach. 1987. Monsoon variability over the past 150,000 years. J. Geophys. Res. 92:8411-8425. Rind, D., and D. Peteet. 1985. Terrestrial conditions at the last glacial maximum and CLIMAP sea-surface temperature estimates: Are they consistent? Quat. Res. 24:1-22. Rind, D., D. Peteet, W. Broecker, A. McIntyre, and W. Ruddiman. 1986. The impact of cold North Atlantic sea surface temperatures on climate: Implica- tions for the Younger Dryas cooling (ll-lOk). Climate Dynamics 1:3-33. Rind, D., D. Peteet, and G. Kukla. 1989. Can Milankovitch orbital variations initiate the growth of ice sheets in a general circulation model? J. Geophys. Res. 94:12851-12871. Rothlisberger, F. 1986. 10,000 Jahre Gletschergeschichte der Erde. Aarau, Verlag, Sauerlender. Ruddiman, W.F., and A. McIntyre. 1981. The North Atlantic Ocean during the last glaciation. Palaeogeogr. Palaeoclimatol. Palaeoecol. 35:145-214. Ruddiman, W.F., and M.E. Raymo. 1988. Northern Hemisphere climate regimes during the past 3 Ma: Possible tectonic connections. Philos. Trans. R. Soc. London B318:411430. Saltzman, B., and A. Sutera. 1984. A model of the internal feedback system involved in late Quaternary climatic variations. J. Atmos. Sci. 41:736-745.

EARTH SYSTEM HISTORY AND MODELING 107 Sarmiento, J.L., and J.R. Toggweiler. 1984. A new model for the role of the oceans in determining atmospheric pCO2. Nature 308:621-624. Siegenthaler, U., and T. Wenk. 1984. Rapid atmospheric CO2 variations and ocean circulation. Nature 308:624-625. Stanley, S.M. 1986. Anatomy of a regional mass extinction: Plio-Pleistocene decimation of the western Atlantic bivalve fauna. Palaios 1:17-36. Street-Perrott, F.A., and S.P. Harrison. 1984. Temporal variations in lake levels since 30,000 yr BP an index of the global hydrological cycle. Pp. 118-129 in J.E. Hansen and T. Takahashi (eds.), Climate Processes and Climate Sensitivity. Geophysical Monograph 29. American Geophysical Union, Washington, D.C. Stuiver, M., and T.F. Braziunas. 1988. The solar component of the atmospheric ]4C record. Pp. 245-266 in F.R. Stephenson and A.W. Wolfendale (eds.), Secular Solar and Geomagnetic Variations in the Last 10,000 Years. Kluwer, Dordrecht, The Netherlands. Thompson, L.G., and E. Mosley-Thompson. 1987. Evidence of abrupt climatic change during the last 1,500 years recorded in ice cores from the tropical Quelccaya ice cap, Peru. Pp. 99-110 in W.H. Berger and L.D. Labeyrie (eds.), Abrupt Climatic Change. D. Reidel, Dordrecht, The Netherlands. Thompson, L.G., E. Mosley-Thompson, W. Dansgaard, and P.M. Grootes. 1986. The Little Ice Age as recorded in the stratigraphy of the tropical Quelccaya ice cap. Science 234:361-364. Thompson, L.G., et al. 1989. Holocene-Late Pleistocene climatic ice core records from Qinghai-Tibetan Plateau. Science 246:474477. Washington, W.M., and G.A. Meehl. 1989. Climate sensitivity due to increased CO2: Experiments with a coupled atmosphere and ocean general circulation model. Climate Dynamics 4:1 -38. Webb, T., III, P.J. Bartlein, and J.E. Kutzbach. 1987. Climatic change in eastem North America during the past 18,000 years; Comparisons of pollen data wi~ model results. Pp. 447-462 in W.F. Ruddiman and H.E. Wright, Jr. (eds.), North America and Adjacent Oceans During the Last Deglaciation. DNAG Vol. K-3. Geological Society of America, Boulder, Colo. Webster, P.N., and N. Streten. 1978. Late Quaternary ice age climates of tropical Australia, interpretation and reconstruction. Quat. Res. 10:279-309. Woodruff, F., and S.M. Savin. 1989. Miocene deep water oceanography. Palaeoceanography 4:87-140. Zhang, J., and T.J. Crowley. 1989. Historical climate records in China and recon- struction of past climates. Journal of Climate 2:833-849.

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This book recommends research priorities and scientific approaches for global change research. It addresses the scientific approaches for documenting global change, developing integrated earth system models, and conducting focused studies to improve understanding of global change on topics such as earth system history and human sources of global change.

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