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Earth System History and Modeling COORDINATOR: BERR1EN MOORE ITI Because of the immense complexity of the earth system, we must employ modelsreduced description of reality to describe the system or its components. Modeling is, in a sense, simply the formulation of working hypotheses of how the system is structured. In the context of understanding and predicting global change the continued development of a variety of earth system and subsystem models is clearly needed in light of the underlying complexity. Model development generally places great demands on available contempo- rary data, and unfortunately, little independent information about the present is available for mode! testing. In addition, the ultimate objective of the IGBP to predict changes in the global environment places added burdens on mode! validation. As models of the earth system and its components emerge, they will generally be based upon the current state of the system and reflect processes and rates asso- ciated with the present environment. Thus reproduction of current dynamics is a basic but sometimes limited test of the models. To test them over a wide range of conditions, models must be exercised against the record of past environments. This paper has been compiled from discussions on earth system history at the workshops on ecological systems and dynamics and biogeochemical cycling, and further discussions within the committee. 201

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202 The geologic record contains information about the earth's envi- ronment extending back as far as 3.S billion years. Although incom- plete and hard to interpret, this record becomes progressively more complete toward the present and is most complete in the Quaternary (<2 minion years), the later Pleistocene (S400,000 yr), and especially in the Holocene (<10,000 yr). The geologic record is particularly useful as it shows the range and direction of excursions in the terrestrial environment, for exam- ple, major glaciations between 450 and 250 minion years and since 40 minion years and global warming between 140 and 65 million years. An important aspect of this record is the evidence of rapid change from one mode of global environment to another. In some cases there is evidence that the rapid change may have been related to specific identifiable triggering events. For example, a change to what is basically the present pattern of oceanic circulation originated about 15 million years ago and appears to have been related to the closing of part of the Tethys Ocean in {ran (Woodruff and Savin, in press), which stopped a southward flow of hot saline waters into the Indian Ocean. Paleoclimatic and paleohydrological research reveals numerous climatic events and trends that characterize the past few million years. These include histories of glacier extent, global ice volume, surface ocean temperature, abundances of CO2, CH4, and other trace gases in the atmosphere, extent of forests and arid zones, and sea and lake levels. This information is provided by a global network of fairly continuous records that contain quantifiable environmental and proxy-climate indicators such as pollen, ratios of stable isotopes, and chemical and particulate concentrations. These natural "diaries" provide different sets of insights into the history of the earth, covering a variety of characteristic spans of time and space. The detailed records of the past 25,000 years, with a focus upon the past 1,000, should be particularly useful in providing specific tests for models of global change on time scales of decades to centuries. RECONSTRUCTION OF THE ENVIRONMENTAL HISTORY OF THE EARTH The reconstruction of the earth's paleoenvironmental history began in earnest with the development of new geochemical tools de- veloped in the mi(ldle of the twentieth century. The essential charac- teristics of environments over the last million years emerged from the

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203 sedimentary records preserved within the ocean floors. Long-term ice core records bridge the gap between the longer records available from ocean sediment cores and the shorter, high temporally resolved histories available from pollen sequences in lake sediments, tree rings, corals, insect remains, en cl speleothems. Additionally, higher tem- poral resolution ice core records from high latitudes and carefully selected high elevation ice caps in both middle and Tow latitudes offer critically important shorter records with fine temporal detail. The marine sediment record reveals that the warm global cli- mates that characterized the past 10,000 years are but the interglacial phase of an ongoing glacial-interglacial cycle. Only 1S,000 years ago, ice sheets covered most of Canada, part of the United States, and much of Northern Europe, with sea level some 60 to as much as 140 m Tower, and with a climate much cooler and (on a global basis) drier than today. Analysis of various marine sediment properties (such as cadmium, i3C and i4C to i2C ratios, and species variability) provides evidence of circulation patterns, temperature, salinity, bio- Togical processes, and distribution of nutrients, carbon, and oxygen. Sediment records on land provide evidence that the pattern of ice-age vegetation in temperate North America was very different from that of today, and a clear picture of how the vegetation responded during the past 1S,000 years to a continually changing climate is emerging. Insights into climates of the past few tens of thousands of years have been derived from cores driDed in the Greenland and the antarc- tic ice caps (Dansgaard et al., 1984~. The oxygen-isotope and deu- terium ratios of the ice reveal the temperature at which the water in the snow evaporated from the surface of the ocean, modified to some extent by the condensation temperature and by global ice vol- ume. Therefore, changes in these ratios with depth provide a record highly correlated with changes in polar temperature between glacial and interglacial epochs. The record suggests that very rapid changes in climate may have occurred during glacial times, and that the glacial to interglacial transition may be relatively rapid (Berger and Labeyrie, 1987~. Records of dust in ice and oceanic sediment suggest altered patterns of arid zones and atmospheric circulation. As discussed in the background paper on biogeochemical dynam- ics concentrations of atmospheric trace gases have been measured in the entrapped air within ice cores. From such studies, we know that CH4 concentrations have doubled in the air since A.D. 1600 and were much Tower during the last ice age, and that the CO2 content of the

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204 air was 30 percent lower during the time of glacial maximum than over the last 10,000 years. The acidity of ice cores reflects the temporal history of the atmo- spheric concentration of acid aerosols at high latitudes and hence the volcanic flux of sulfate particles, which interact with solar radiation. This quantitative record of explosive vuIcanism can be compared with other geologic records of climate in order to assess the role of such events in the alteration of climate. Further, the record of anthropogenic alteration of the sulfur and nitrogen burden of the atmosphere is most clearly captured within the ice core records. Pollen and other microfossils e.g., diatoms, and insect remains trapped in lake and bog sediments reveal past patterns of biota in the surrounding region and physical properties of the lake. These proxy indicators, through the use of transfer functions calibrated to modern species (distributions, can be used to infer changes of seasonal temperature or precipitation. Although time lags may be introduced in lateral migration by the slow dispersal of seeds of certain species, lake and bog cores do allow inference of both spatial and temporal changes of climate over much of the earth's land area. This field is relatively undeveloped considering its potential. The thickness of wood in an annual tree ring where growth is limited by climate provides a direct measure of growth in that year, and hence of local climate. For some applications, seasonal resolution can be obtained by analysis of early and late wood in a single ring. In many areas of the world, such records can be extended back several hundreds of years, and in some cases several thousand years, providing unique insights into the history of our environment. Furthermore, the tree ring records contain important information about the isotopic character of the past atmosphere and hence about valuable biogeochemical histories. Finally, past glacier fluctuations can be inferred from moraine limits, stream terraces, and other geomorphic indicators. Lake level data can be obtained directly by age-dating materials (gastropods, tufa) that grew in shallow water or may be obtained indirectly by examination of the organic or inorganic content of age-dated cores taken from lake basins. Groundwater recharge events can also be age-dated and the carbon and oxygen stable isotope ratios used to infer change in temperature or moisture source region.

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205 MODELS Models are needed to synthesize our understanding of the interac- tions of the various components of the biogeochemical, hydrological, and physical-climate systems, including the coupling with vegetation systems. Models provide a framework for analyzing the impacts of human activities on the earth system. Another related synthetic role of models is to optimally assimilate observations of complex fields of related variables, as is now done in global weather prediction sys- tems. Such assimilation constrains the observations through known physical laws and uses these physical laws to extrapolate and inter- polate from the observations to data-poor locations. For example, important land properties such as soil moisture cannot be measured directly on the regional scale, but can be tightly constrained from estimates of rainfall patterns and evapotranspiration, which would result from assimilation of observations into a four-dimensional mode] of the system. Moclels of the physical-cTimate system are ideal tools for synthesis of paleoclimate and hydrological data and, conversely, are dependent on such data for validation of their capabilities to predict future global change. However, it is necessary to improve the coordination of paleo-observations and their synthesis by climate modeling studies, on both global and regional scales. Emphasis must be placed on the verification of model sensitivity to various climate forcings, including the present seasonal cycle, volcanic dust veils, and factors driving paTeocTimatic episodes. These are all potential "natural experiments" that provide tests of model performance. In addition, the behavior of the interacting climatic subsystems must be understood, and the data must be synthesized and enhanced. Global climate models are in part an outgrowth of weather fore- casting activities. More emphasis is needed on the linkages to other parts of the climate/hydrological systems and on the wide range of temporal and spatial scales over which these occur. The scientific community at large needs to understand the underlined physical relationships of the parameterizations and, the limitations of the models in order to use them appropriately and more effectively in the development on the models. Tools and concepts that have been developed by other communities, such as geographical information systems and hierarchy theory, may prove to be valuable for the stiffly of climate problems. In addition to global approaches, a focus on regional climate change processes will be especially valuable. Global simulations can

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206 be extended through the use of mesoscale models embedded in the global model to provide required detail. Only with such an approach can topographically complex regions such as the western United States and western China be adequately treated. Models of the earth's biogeochemical system are, in compari- son with climate models, less advanced. In part, this is due perhaps to a greater level of complexity. There has been progress, partic- ularly in the area of the global carbon cycle. From the simplest perspective, one can consider the earth biogeochemical system as a three-box model: an atmosphere, a terrestrial biosphere, and an ocean including the marine biosphere. In this context, the questions are as follows: what is the flux of various biogeochemical compounds (CH4, CO2, CO, N20, NOR, NH3, COS, DMS, and so on) between the boxes, what controls these fluxes, and how are they affected by anthropogenic activity? Obviously, in order to give even first-order consideration to these questions, the heterogeneity of the "boxes," (the atmosphere, the terrestrial biosphere, and the ocean) must be considered explicitly. Models of the chemistry of the atmosphere that reflect a range of spatial and chemical complexity are under development. In terres- trial systems, serious methodological questions regarding scale partly reflect past traditions of ecosystem modeling at a relatively small scale (hectares to square kilometers) as well as the difficulties inher- ent in the system. New work, however, is emerging at the regional to global scale that has been encouraged by advances in the inte- gration of models with geographical information systems and remote sensing. Major physical oceanographic programs such as The World Ocean Circulation Experiment and complementary biogeochemical programs such as Joint Global Ocean Flux Study set the stage for rapid advances in ocean biogeochemical modeling. But in all of these areas, we are faced with extremely difficult problems of methodology and data availability, as wed as a multiplicity of feedbacks at varying spatial and temporal scales that connect the physical-climate system to the biogeochemical system. DECODING THE PAST: CHALLENGING GLOBAL MODELS Tests of models of global change will come from slate from the past, as will insights into fundamental processes that operate on time scales of many decades to centuries. We are a long way from a

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207 fully satisfactory model of the causes of past major global changes. The recent records of CO2 and t8O in the Vostok, Antarctica, ice core (Barnola et al., 1987; Lorius et al., 1985) reveal a close linear relationship between temperature and CO2 abundance, as previ- ously reported in the Dye 3, Greenland, core (Stauffer et al., 1985~. However, the Vostok core provides the first detailed look at an in- terglacial/glacial transition about 120,000 years ago. The CO2 con- centration remains high for nearly 10,000 years as the bi8O proxy for temperature exhibits a rapid decline into fuD glacial conditions. Either we understand very little about the physical and chemical relationships between climate (temperature) and the biogenic and oceanic cycling of CO2 and/or there are problems in the bi8O tem- perature interpretations. The CH4 record poses similar problems. On shorter temporal scales problems also arise. What is the cause of the linear rise in CO2 from the middle of the eighteenth century to the middle of the nineteenth century? Is it anthropogenic forcing, or is it related to the end of the Little Ice Age? The accurate recon- struction of these histories requires improving and synthesizing our understanding of the physical, chemical, and biological processes. The modeling and data analysis efforts could be usefully focused on two key temporal periods: the past 25,000 years and the most recent 1,000 years. In both cases, special consideration could be given to periods of rapid change since large abrupt changes in the global system (Younger Dryas, Little Ice Age, major episodes of volcanic activity, and El Nino events) offer special challenges and tests of both model capabilities and our understanding of the causes of climate change. The Past 25,000 Years Emphasis should be placed upon the reconstruction of the earth's environmental history over the past 25,000 years. This period en- compasses the range of conditions from fud glacial stage to full inter- glacial. It is easily dated and is well preserved; numerous deep ocean cores have been retrieved, and there are extensive ice core and pollen records. Over this period, the global climate system went from the coldest extremes of the last ice age to the present interglacial with ac- companying large changes in patterns of temperature, precipitation, ice cover, and distributions of ecosystems. Over the past 25,000 years, there were also large changes in

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208 biogeochemical cycles reflected in large changes in global concentra- tions of CH4 and CO2. Isotopic tracers such as i8O and deuterium can be used to track the global climate and geochemical changes that occurred over this period. The first extensive look at a global reconstruction of full glacial conditions resulted from the Climap (Climate Long Range Investigation and Mapping) Program (CTimap Project Members, 1981~. Land records directly describe variations in vegetation cover and lake levels. For instance, global vegetation maps are emerging from regional compilations of pollen data. As the geographical coverage increases, these maps will provide reconstruc- tions of increasing global biomass as CO2 concentrations increased at the glacial-interglacial transition. Measurements of the t3C con- tent of CO2 and CH4 can provide important clues to the processes responsible for the changing concentrations of these gases in the atmosphere. Postglacial conditions have been mapped by the Coop- erative Holocene Mapping Project (COHMAP Members, 1988~. One suggested modeling thrust toward interpreting the paleo- record over the past 25,000 years is the application of mesoscaTe models coupled to global models. Such mesoscaTe models can pro- vide simulated weather data as input to runoff-infiItration models, and can couple to lake thermal-evaporation models as wed. Thus simulations of lake-lever variation could be compared to the actual record. On the biological side, the mode! climate can be used via response-surface transform techniques to recreate aspects of the veg- etation history of a region. The simulated pattern of vegetation in space and time might then be compared to a proxy-data network, e.g., pollen and microfossi! and macrofossiT records. Such a mesoscale model would need accurate representations of surface physics, and a grid fine enough to resolve important variations in topography and surface characteristics over large watersheds. Its hydrologic com- ponents would require careful development and validation. Lake modifications of regional climate may have a significant impact on the growth of large lakes. Proxy data sets on several time scales are needed over the region of interest including that provided by a coring program both onshore and offshore in multiple lake basins. If methods to adjust apparent groundwater ages are perfected, the timing of lake-level rise in one area could be compared to the timing of groundwater recharge in the same or other areas.

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209 The Past 1000 Years Special emphasis should be placed in global change research on the past 1000 years, which incorporates the Little Ice Age and the entire industrial period. Within this time frame, numerous proxy records can be incorporated with archeological, historical, and in- strumental records. The records for the past 1000 years are often annually, and in some instances seasonally, resolvable. The research focus should be the history of the climate (particularly temperature), atmospheric chemistry, terrestrial vegetation, and patterns of oceanic circulation and production. This focus offers an excellent opportu- nity to exercise models of the planet's biogeochemical system as it interacts with the physical-climate system on time scales of decades to centuries, which are particularly relevant to global change. This intensive study of the naturally archived records of the past 1000 years should inclu(le comparisons with available "ground truth" data contained in direct historical accounts such as weather and sea records. While this sort of comparison is routinely done in the course of sharply focused studies of specific environmental parametersas in calibrating tree ring widths in a given location in terms of soil moisture or other meteorological parameters such a study has never been organized for multiple parameters focused on an extended test period. A more organized study of this type would serve several purposes that would benefit the IGBP. Specifically, it would . illuminate more fully the transfer functions needed to inter- pret natural archives by this use of direct, historical data as "ground truth"; establish the potential and limits of reliability of an organized, multiparameter study in which naturally archived data from different sources are combined to gain a deeper knowledge of a specific period or of specific events such as volcanic eruptions; and maximize what is known of global change in the past 1000 years a period that includes an increase of a factor of 30 in world population (with accompanying changes in land use), the indus- triaTization of much of the world (with accompanying changes in atmospheric chemistry), the onset of modern worldwide intensive agriculture, and the fuD span of the most recent distinguishable fea- ture of global climate change (the Little Ice Age, cat 1450 to 1850 A.D.~. The naturally archived data that should be employed include

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210 i ace cores from Greenland and the Antarctic (hydrogen and oxygen isotope ratios; trace gases, including CO2 and CH4 and their iso- topic compositions; sulfate and nitrate concentrations; and pollen, atmospheric aerosols, and volcanic dust at seasonal resolution within distinct annual layers); ice cores from carefully selected tempo- ral latitude glaciers (hydrogen, oxygen, and carbon isotope ratios, other chemical species, aerosols and pollen, and volcanic dust at seasonal resolution in easily distinguished annual layers); tree ring data, including ring width and hydrogen, oxygen, and carbon iso- tope ratios (annual resolution with the potential of discriminating spring/summer seasons in early and late wood); high-resolution ter- restrial sediment sequences (pollen, runnoff at decadal resolution); lacustrine sediments (pollen, lake level at annual resolution); and cores from coral reefs (hydrogen, oxygen, cadmium, and carbon iso- tope ratios with annual resolution of evidence of humic acid). The period of truly extensive "ground truth" data will belimited to at most the last 100 years of this 1000-year period, and for most parameters of interest, to an even shorter, more recent period. It is the most recent epoch- say the past 50 years that will provide the most meaningful tests and illuminate most usefully the trans- fer functions and their limits. Less extensive and far more regional historical data are nevertheless available and should be gathered to- gether for a systematic study of the longer 1000-year span. Real-time studies would provide a strong focal point for better understanding the relationship between processes and the resulting record. In this study the more extensive suite of meteorological and environmental parameters taken during the IGBP would be compared, for example, with real-time samples of precipitation in the glaciated areas where ice cores are drawn. This step would make a substantial contribution to the establishment of the real significance of measured parameters such as i80:~60 and H:D ratios that are routinely measured in ice, sediment, and coral cores. Thus these relatively detailed data from ice, ocean, and ter- restrial cores reflecting the changes occurring during this period wiD provide a rigorous test of earth system models and thus wiD be a step toward validating these models for use in projecting future change. An obvious difficulty, however, remains the anticipated future rates of change will likely be far greater, and the forcing functions wit! be different. The challenge to the modeling community is to construct models of the biogeochemical and physical-climate systems (or components

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211 thereof) that are consistent with the multiparameter observation sets in the historic record and that are also consistent with the very recently observed transients in the system such as bomb i4C or fossil fuel CO2. This is a major challenge; in seeking to meet it, it may be particularly useful to focus specifically upon periods of rapid change within either the l.()OO-vear record or the 25,000-year period. . . ~ ~ Abrupt Change Ice core results from both the polar regions and the high-altitude tropics contain evidence of rapid transitions from one mode of climate to another. The dust, sulfate, oxygen isotope, and CO2 records change from fuD glacial to interglacial conditions in less than one century in the Dye 3 core (Hammer et al., 1985; Herron et al., 1985~. The dust concentrations in the Camp Century, Greenland, core show an equally abrupt transition (Thompson, 1975~. The transition from Neoglacial Little Ice Age to current conditions in the Peruvian Andes occurred in 3 years as reflected in the particulate (soluble and insoluble) and oxygen isotopic records (Thompson et al., 1986; Thompson and MosTey-Thompson, 1987~. The Younger Dryas, a brief cool period interrupting the Wisconsin (Wurm)/Holocene transition, is another example of an abrupt transition from one set of conditions to another. Fine-scale sampling of peats and laminated lake sediments pro- vides detailed studies of vegetation change. High priority should be given to studies of vegetation dynamics during periods when inde- pendent data from ice cores record rapid changes in the environment. In this way lags in vegetation or ecosystem response can be measured, and the extent to which response lags blur the record in proxy records of rapid environmental change can be better understood. RESEARCH CHALLENGES As discussed above, research efforts should focus on reconstruc- tion of earth system history over the past 25,000 years to encompass the range of conditions from full glacial to full interglacial stages, with special emphasis on the past 1,000 years to cover the period of intensive human interactions with the system. Provisions for se- lecting specific events or eras within these time frames need to be established. In particular, high-resolution studies are important for documenting the rate of change of the earth system in the past. For

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212 example, the resolution of marine cores at present is too coarse to distinguish between instantaneous changes caused by threshold ef- fects from a nonlinear response to steady changes, such as a switch from one mode of ocean circulation to another. New studies must be initiated to focus on periods of rapid change to document the time scale of change, as well as to determine rates of changes of different responding variables in the system. It will be important to com- pare marine and terrestrial sites to identify differences in temporal responses in different parts of the earth system. Models need to be developed to interpret the response of the earth system to past changes. Specifically, models of global biogeochemical cycling that consider nutrient interactions and of the hydrologic cycle in specific geographic regions need to be developed. Moreover, efforts need to focus on linking global models of the biogeochemical system to global climate models. Physical and geological models are required to develop and test hypotheses about causes for changes in biological systems observed in the paleorecord, such as Cretaceous-Tertiary and late Pleistocene extinctions. Combining paleoecologica] records with geomorphic evidence and ice core records is necessary for determining cause-effect relationships. In particular, investigations of the causes of transitions from one ecosystem type to another are essential for predicting future changes. Finally, an accessible `data and information system, with the aim of assembling various proxy historical and instrumental data into a coordinated and validated archive, is an essential component of a research effort on earth system history. REFERENCES Barnola, J. M., D. Raynaud, Y. S. Korotkevich, and C. Lorius. 1987. Vostok ice core provides 160,00~year record of atmospheric CO2. Nature 329:40~413. Berger, W. H., and L. D. Labeyrie. 1987. Abrupt Climatic Change. Evidence and Implications. Dordrecht, Netherlands: Reidel. 425 pp. Climap Project Members. 1981. Seasonal Reconstruction of the Earth's Surface at the Last Glacial Maximum. Geological Society of America, Map and Chart Series MC-36. Cooperative Holocene Mapping Project Members. 1988. Climatic changes of the last 18,000 years: Observations and model simulations. Science 241:1045-1052. Dansgaard, W., S. J. Johnsen, H. B. Clausen, D. Dahl-Jensen, N. Gundestrup, C. U. Hammer, and H. Oeschger. 1984. North Atlantic climatic oscillations revealed by deep Greenland ice cores. Pp. 288-298 in Climate Processes and Climate Sensitivity. American Geophysical Union, Geophysical Monograph 29.

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213 Hammer, C. U., H. B. Clausen, W. Dansgaard, A. Neftel, P. Kristinsdottir, and E. Johnson. 1985. Continuous impurity analysis along the Dye 3 deep core. Pp. 9~94 in Greenland Ice Core: Geophysics, Geochemistry, and the Environment, C. C. Langway, Jr., H. Oeschger, and W. Dansgaard, eds. American Geophysical Union, Geophysical Monograph No. 33. Herron, M. M., S. L. Herron, and C. C. Langway, Jr. 1985. Chloride, nitrate, and sulfate in the Dye 3 and Camp Century, Greenland ice cores. Pp. 77-84 in Greenland Ice Core: Geophysics, Geochemistry, and the Envirorunent, C. C. Langway, Jr., H. Oeschger, and W. Dansgaard, eds. American Geophysical Union, Geophysical Monograph No. 33. Jouzel, J., C. Lorius, J. R. Petit, C. Genthon, N. I. Barkov, V. M. Kotlyakov, and V. M. Petrov. 1987. Vostok ice core: A continuous isotope temperature record over the last climatic cycle (160,000 years). Nature 329:403 407. Lorius, C., L. Merlivat, J. Jouzel, and M. Pourchet. 1979. A 30,000 year isotope climatic record from antarctic ice. Nature 280:644-648. Lorius, C., J. Jouzel, C. Ritz, L. Merlivat, N. I. Barkov, Y. S. Korotkevich, and V. M. Kotlyakov. 1985. A 150,00() year climatic record from antarctic ice. Nature 316:591-596. Stauffer, B., A. Neftel, H. Oeschger, and J. S6hw~der. 1985. CO2 concentration in air extracted from Greenland ice samples. Pp. 85-89 in Greenland Ice Core: Geophysics, Geochemistry, and the Environment, C. C. Langway, Jr., H. Oeschger, and W. Dansgaard, eds. American Geophysical Union, Geophysical Monograph No. 33. Thompson, L. G. 1975. Variations in microparticle concentration, size distribution and elemental constituents found in the Camp Century, Greenland, and the Byrd Station, Antarctica, deep ice cores. Pp. 351-364, in Isotopes and Impurities in Snow and Ice. Proceedings of the Grenoble Symposium, 1975. IAHS-AISH Publication No. 188. Thompson, L. G., and E. Mosley-Thompson. 1987. Evidence of abrupt climatic change during the last 1,5Q0 years recorded in ice cores from the tropical Quelccaya ice cap, Peru. Pp. 99-110 in Abrupt Climatic ChangeEvidence and Implications, W. H. Berger and L. D. Labeyrie, eds. 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. Woodruff, F., and J. Savin. In press. Miocene deep water oceanography. Paleoceanog- raphy.

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