5
PROXY INDICATORS OF CLIMATE



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Natural Climate Variability on Decade-to-Century Time Scales 5 PROXY INDICATORS OF CLIMATE

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Natural Climate Variability on Decade-to-Century Time Scales Proxy Indicators of Climate: An Essay SOROOSH SOROOSHIAN AND DOUGLAS G. MARTINSON INTRODUCTION Establishing a baseline of natural climate variability over decade-to-century time scales requires a perspective that can be obtained only from a better knowledge of past variability, particularly that which precedes the pre-industrial era. Information revealing these past climate conditions is contained in historical records and "proxy" indicators. The historical records of climate (other than systematic weather observations, which began in the late 1800s), while invaluable because of their scope and often uniquely relevant perspective, are usually limited to the last several hundred years (see Chapter 2). The proxy indicators represent any piece of evidence that can be used to infer climate. Typically, proxy evidence includes the characteristics and constituent compositions of annual layers in polar ice caps, trees, and corals; material stored in ocean and lake sediments (including biological, chemical, and mineral constituents); records of lake levels; and certain historical documents. Such proxy indicators can provide a wealth of information on past atmospheric compositions, tropospheric aerosol loads, volcanic eruptions, air and sea temperatures, precipitation and drought patterns, ocean chemistry and productivity, sea-level changes, former ice-sheet extent and thickness, and variations in solar activity—among other things. These records are particularly appropriate for detecting three manifestations of climate variability: Periodic or near-periodic variations (the latter are those that become evident only after examination of considerable data through which a clear statistical signal stubbornly emerges); Large and pronounced climate signals, such as severe and sustained droughts, drastically altered precipitation patterns, anomalously warm or cold periods, or floods; and Gradual trends, infrequent shifts, or other characteristics of natural variability that are difficult to recognize without the benefit of a long, continuous (or near-continuous) record. Because the bulk of these proxy indicators are recorded naturally, their time span is potentially unlimited; their resolution and accuracy are limited only by the fidelity of the recorder itself. These "natural archives" are simply there for the taking—awaiting discovery, recovery, means of extraction, and interpretation. Consequently, proxy indicators represent a potential treasure trove of unique past climate information. The use of these data can present difficult problems of interpretation, particularly in light of the scanty spatial and temporal sampling, but can enhance our ability to reconstruct global changes. This chapter provides a state-of-the-art look at several aspects of this relatively new topic with respect to the climate of the last few millennia. HISTORY OF PROXY INDICATORS The use of proxy indicators in the study of modern climate draws on a broad range of disciplines and techniques. The potential of tree rings was recognized in the

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Natural Climate Variability on Decade-to-Century Time Scales early 1900s, when the 11 -year sunspot cycle was found to be recorded in the rings of trees in the southwestern United States. Major advances have been made since then, particularly within the last few decades. Many problems associated with interpretation of the rings (e.g., their causal relationship to climate) have been overcome, and the records have been extended from several hundred years to several thousand years or longer. Tree rings are now invaluable proxy indicators, because of their continuity and remarkable precision. In a similar vein, though more recently, the study of annual rings in massive corals is now yielding information regarding marine climates. Many of the other proxy indicators, such as isotopes, fossil assemblages, and lake levels, had been used in the past to provide geological evidence in paleoclimate and paleoceanographic studies; they represent one of the standard tools in the geologist's arsenal. The relatively low resolution of the typical geological record had restricted their use chiefly to the study of long-time scale phenomena, such as glacial cycles. However, the desire to answer questions demanding higher-resolution data (for instance, whether the rapid climate excursions observed during the last deglaciation were anomalous, or were characteristic of a major climate transition) spurred improvements in the methods of recovery, analysis, and interpretation, which have yielded higher-resolution data. Improved collection techniques, such as long coring and better drilling methods, have made it possible to acquire the larger samples needed with the higher-resolution deposits; more natural data banks, such as ice caps and high deposition sediments, have been identified; and our understanding of both natural recorders and appropriate extraction techniques has increased, permitting higher precision with smaller samples. All these developments are beginning to contribute to our knowledge of natural climate variability on decade-to-century time scales. Indeed, proxy indicators are now producing some of the most exciting and valuable records of variability to date. Furthermore, given the relatively embryonic state of the science, they have great potential for contributing to our understanding of the modern climate, particularly over longer time scales. New indicators are constantly being evaluated. For example, the use of biological ecosystems as proxy indicators of climate is demonstrated by McGowan (1995) and Reifsnyder (1995), both of which appear in this chapter. Numerous other proxy recorders—lake sediments, cave deposits, marshes—can now provide significant new insights. These are reviewed more thoroughly in Bradley and Jones (1992). In addition, the tremendous quantity of material in the historic records that is pertinent to climate is becoming increasingly useful, thanks to recent cataloguing that includes important metadata. OCEANIC PROXY INSIGHTS Numerous proxy indicators exist in the ocean, representing a wide spectrum of different oceanic and climate variables and spanning a wide range of time scales. Indicators residing in the sea-floor sediments may provide a nearly continuous record, often spanning tens of millions of years. These include pollen, faunal, and floral assemblages that have settled and accumulated, the isotopic compositions of skeletal material and tests from bottom-dwelling or floating organisms, and certain geological deposits and sediment types or compositions. These ocean proxy records are accessible through sediment coring, drilling, and, in the case of certain bottom dwellers such as isolated corals, dredging. While the degrees of accuracy and resolution available vary, the information that can be extracted is staggering. For example (see the papers of Ruddiman and McIntyre, 1973; CLIMAP, 1981; and Imbrie et al., 1992 for more details), ocean proxy indicators have yielded regional information on the following characteristics: sea-surface and bottom temperature (from fossil assemblage composition), continental and landlocked ice volume and sea-level height (from oxygen isotope ratios), the partitioning of carbon between the land and oceans (from carbon isotope ratios), alkalinity of local water (from fossil preservation indices), deep-water circulation (from relative isotope compositions), surface productivity (from vertical isotope gradients), water-column stability (from radiolarian abundances), major front locations (from fossil, ice-rafted debris, and sediment-type distributions), deep-water temperature changes and surface salinity (from relative isotope compositions), vertical gradients of water-mass properties like temperature or salinity or of water-mass distributions (from analyses of sediments from different depths), deepwater velocity changes and source information (from sediment distributions near restricted passages), and predominant wind directions and intensities (from sediment compositions). At typical sedimentation rates in the deep ocean basins (about 3 cm per 1000 years), sediment mixing serves as a low-pass filter, limiting the resolution of these proxy indicators to time scales of thousands of years. Regions in which the sediments accumulate faster offer the potential for resolving variations over significantly shorter time scales, but such areas often occur along continental margins where the interpretation of the sediment column is notoriously difficult, due to processes such as mass wasting. Consequently, until recently, these oceanic proxy indicators were used primarily to document and study climate variability on millennial or greater time scales. However, as Lehman (1995) explains in this chapter, areas with high deposition rates and relatively clean sediment records have now been located and sampled. These sample data are providing useful infor-

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Natural Climate Variability on Decade-to-Century Time Scales mation about natural variability and rapid climate change on time scales as short as decadal. The paper in this chapter by Cole et al. (1995) describes the shorter, but often high-quality, records that can be found in the chemical composition of corals. Coral records spanning hundreds of years can be pieced together from a single region to provide information regarding sea surface temperature, upwelling, rainfall, and winds. Because corals often grow at rates three orders of magnitude greater than the rate of sediment accumulation in the deep sea, their temporal resolution is exceptionally good, providing high-fidelity records of natural variability on time scales as short as seasonal. Finally, in addition to the oceanic proxy indicators preserved through time that are discussed above, some researchers have proposed that the spatial and temporal distributions of plankton and fish populations are intimately linked to ocean and climate conditions. The paper in this chapter by McGowan (1995) reviews the current state of our knowledge and discusses the problems in using such data as proxies for ocean/climate variabilities. McGowan's view of the relationships between climate and various proxies contrasts with the findings of Dickson (1995) that appear in Chapter 3. Their differences highlight the uncertainties surrounding this relatively new endeavor and the challenges associated with using highly complex biologic indicators. ATMOSPHERIC PROXY INSIGHTS Scientists have been examining a number of noninstrumental atmospheric proxy data sources in search of clues to past climate conditions. Each type of record contains information on one or more aspects of climate. Among these are historical documents (almost all aspects of climate), tree rings (temperature, precipitation, pressure patterns, drought, and runoff), ice cores (temperature, precipitation, atmospheric aerosols, atmospheric composition, and more), lake levels (runoff and drought patterns), and varved sediments (temperature, precipitation, and solar radiation). With the exception of the historical documents, perhaps, the climate information in these proxy indicators must be isolated from the nonclimate portion of the signal and any accompanying noise. Of the various sources of atmospheric proxy indicators, the greatest attention has so far been given to the use of historical documents, tree rings, and ice cores. (Of these, tree rings have been subjected to the most rigorous testing as sources of information on past climate.) A number of papers in this volume make reference to non-instrumental records, primarily tree rings (Jones and Briffa, 1995; Cook et al., 1995b, and Reifsnyder, 1995, all in this chapter; and Diaz and Bradley, 1995, in Chapter 2.) The uses of ice cores (Grootes, 1995, in this chapter), which reflect past temperatures, and of lake levels (Street-Perrott, 1995, in this chapter), which reflect overall net moisture supply to the landscape, are presented as well. The paper in this chapter by Jones and Briffa (1995) sets out to show the potential value of proxy records by looking at dendroclimate reconstructions of summer temperatures of four regions: northern Fennoscandia, the northern Urals, Tasmania, and northern Patagonia. In none of the thousand-year reconstructions did the twentieth century stand out as the warmest century, although it was among the warmest. Jones and Briffa also provide a useful discussion of the potential limitations of single-site dendroclimate reconstruction, and of the difficulties and uncertainties associated with comparison of multi-site records. Similar limitations can be expected to apply to other site-specific proxy indicators, so they represent a general caution about the interpretation of climate proxy records. Despite the limitations, these long tree-ring records clearly document large and pronounced climate signals, such as the century-long cold period beginning in 1550. Tree-ring evidence is of critical importance in establishing the magnitude and duration of natural climate anomalies, since the instrumental records are too short to do so and also may reflect anthropogenic contamination. In addition, the tree-ring data can provide key information for the interpretation of gradual trends in climate. For example, they show that the period from 1880 to 1910 was anomalously warm throughout much of the Northern Hemisphere. Thus, the apparent magnitude of the present warming trend will depend on when the selected period begins. The contribution to this chapter of Cook et al. (1995b) examines the power spectrum of a 2290-year reconstruction of warm-season Tasmanian temperatures to detect signs of periodic decadal-scale fluctuations. The paper suggests that the decadal-scale temperature anomalies over Tasmania during the twentieth century, both warm and cold, have been driven in part by long-term climate oscillations. It neither supports nor eliminates greenhouse warming as a possible contributor to the recent temperature increase in Tasmania. However, their study also investigates mechanisms of climate change that are testable (e.g., internal forcing related to deep-water formation, or external forcing by long-term solar variability) and provides specific explanations of synoptic-scale variability (e.g., the expansion and contraction of the circumpolar vortex). Grootes (1995, in this chapter) discusses the potential role of ice-core records in reconstructing decade-to-century-scale climate variations. He seems confident that the new ice-core records being obtained from the summit of the Greenland ice sheet provide an accurate and remarkably detailed history of changes in climate over the North Atlantic basin as far back as 200,000 years ago. Indeed, there is little doubt as to the potential utility of the high-resolution, fast-response climate records from ice cores. At present, however, they can be obtained only from high-latitude or

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Natural Climate Variability on Decade-to-Century Time Scales high-altitude locations, and few of the ice-core reconstructions of temperature have been explicitly calibrated against instrumental time series. The differences between ice-core records from various sites also make it clear that there is a pressing need to identify and differentiate the relative influences of local mesoscale and hemispheric atmospheric factors. In her paper in this chapter, Street-Perrott (1995) discusses how fluctuations in the water level and surface area of relatively undisturbed lakes (i.e, there has been no human-induced change in the water budget) can provide a measure of climate variability on time scales of months to millions of years. The paper provides examples of the response of tropical lakes to variations in ocean-atmosphere interactions over the Pacific (Lake Pátzcuaro), the Indian Ocean (Lake Victoria), and the Atlantic (Lake Chad, Lake Malawi), and points to paleolimnological evidence for century-scale droughts in southern Africa and the tropical Americas. Finally, Reifsnyder (1995, in this chapter) analyzes observational and paleoclimate records of temperature in an attempt to determine the realism of models' predicted global-warming rates. Some models predict a rate of warming that is 10 to 40 times faster than the natural warming that followed the last ice age. On the basis of his analysis, however, Reifsnyder argues that the climate change between now and the end of the next century, for a "business as usual" scenario, should take place at no more than twice the maximum rate estimated for changes since the last ice age. He further argues that, because models are known to overpredict by a factor of two the rate of change over the past century in response to the CO2 increase, the rate of global warming over the next century is in fact unlikely to be higher than any estimated post-ice-age maximum rate— a position which certainly could generate a lot of debate, given the other forcings likely to be operating. Soil and varved sediments, and historical documents, are two sources of atmospheric proxy data that are not covered in this volume. Sedimentary and soil cores can reveal significant paleoclimate information. For example, in a recently published paper in Science, Weiss et al. (1993) attributed the demise of the Akkadian culture (fl. 3000 B.C.) of southern Mesopotamia (present-day northeast Syria) in 2200 B.C. to an abrupt change in climate. The combined archaeological and soil-stratigraphic data for the area point to a shift in climate toward the arid, and a dry period persisting for about 300 years. A variety of historical sources, such as ancient inscriptions, personal diaries and correspondence, scientific and quasi-scientific writings, government records, and public and private chronicles and annals record climate events that were seen as having some significance at the time. The information may include observations of weather phenomena, such as the occurrence of extreme rain- and snowfalls, droughts, floods, or lake and river freeze-ups and breakups. Historical records, unfortunately, fail to give a complete picture of former climate conditions. They are often discontinuous observations, biased towards extreme events. The important long-term trends tend to go unremarked. A good source of discussions of documentary records of past climate is the recently published book by Bradley and Jones (1992). CURRENT LIMITATIONS While proxy indicators are extremely important, even crucial, to climate reconstruction, their limitations should be kept in mind. The most significant are: It is not always clear whether the signal they record reflects only local conditions, or is representative of regional or global conditions. Their accuracy is often unknown or untested. They often reflect more than one variable, making interpretation difficult. The absolute, and even the relative, timing of the record is often not certain. Bradley and Eddy (1989) stressed the importance of the last of these limitations, and noted that without accurate dating it is impossible to determine whether certain events occurred synchronously or not. In the Vostok ice core of central East Antarctica, for example, variations in air temperature (Jouzel et al., 1987) and atmospheric CO2 concentration (Barnola et al., 1987) have been reconstructed for the last 160,000 years. Within the dating uncertainties, these records show striking correspondence between high CO2 concentrations and warm temperatures. However, whether the increases in CO2 concentration precede or follow the temperature increases cannot be assessed accurately from these records. Accurate dating is also a problem in attempting to overcome the first limitation listed above. That is, in order to reconstruct a global picture of climate at a given time, it is necessary to reconstruct climate variables from a number of different locations for that time. The accurate dating needed to establish the relationships of the various records is not always available. This problem plagues most of the ocean records, because distinct annual varves are rarely present. Dating is mostly accomplished through other techniques, again of limited accuracy, which constrains the degree to which comparisons can be made. Sowers et al. (1993) attempted to relate the variations in the Vostok ice record to those in the ocean by correlating supposedly globally synchronous changes in oxygen isotope concentrations. This technique has met with some success, but the range of error precludes comparisons over time scales less than millennial. Other limitations arise because recording is often discontinuous. For instance, the recording mechanism may be disturbed during or after the recording, or the recovery may

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Natural Climate Variability on Decade-to-Century Time Scales be incomplete. This introduces gaps or perturbations that disguise or break the general patterns. An excellent example of the type of inconsistencies that may occur from the different recording properties of different climate proxies is provided by Grootes's (1995) paper in this chapter. However, the promising news in this area is that more researchers are reporting long-term reconstructions from different proxy sources and different geographic locations. The latest is the study by Lara and Villalba (1993) of a 3620-year reconstructed temperature record from alerce trees (Fitzroya cupressoides) from southern Chile. Their work, which has shown the alerce tree as being the second longest-living tree after the bristlecone pine, expands the availability of long-term climate records for the Southern Hemisphere. In Chapter 2 Diaz and Bradley (1995) provide a useful discussion of a number of potential sources of uncertainties and biases associated with climate variable reconstruction, even for the relatively well-understood tree rings. They warn that changes over time in the composition of the tree-ring network used for reconstruction are likely to affect the high-frequency variance and, to some extent, the low-frequency variance as well. Considerable effort has been made to reduce the uncertainties associated with the long-term reconstruction of climate variables from the composition of tree-ring samples. For instance, an index value (temperature, for instance) for a given site is obtained from samples of trees that may vary considerably in age or even be dead. Attention must therefore be given during the reconstruction analysis to getting the chronology right for every tree; removing the biological growth trend; identifying the nature of the climate signal and its strength; replication; and the way in which the analysis method deals with changes in site conditions over the recorded interval. As a result of this careful scrutiny, tree-ring records exhibit exceptionally high fidelity, with comparatively minor interpretational problems. Similar sets of considerations must be addressed for other proxy indicators, no doubt including some not yet identified. THE FUTURE OF PROXY INDICATORS It is clear that accurate dating is required to assess the rate at which past climate changes have occurred and to reconstruct globally synchronous records of climate, particularly for changes on time scales of less than a century. The duration of high-frequency, short-term events may be less than the normal error associated with most methods used in dating the proxy records available. The issue of dating thus merits continued attention. Similarly, a sustained effort must be made to identify problems and uncertainties in reconstructions based on proxy indicators and to assess the associated errors; this should come naturally with increased experience in these relatively novel techniques. Last. combining records that have been drawn from different areas and that use different types of indicators into a consistent picture will be crucial for the study and reconstruction of global climate variations. Finally, as with climate modeling, the reconstruction of past climates requires the employment of a full range of different proxy indicators. These must be developed more fully so that they can be used to intercalibrate the reconstructed climates and cross-check the reconstructions; to provide multiple images of climate through different climate indicators; and to reduce the influence of noise through averaging. In this early stage, we are seeing only the tip of the iceberg; the results of the use of proxy indicators only hint at the climate insights yet to be won by means of these invaluable resources.

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Natural Climate Variability on Decade-to-Century Time Scales Monitoring the Tropical Ocean and Atmosphere Using Chemical Records from Long-Lived Corals JULIA E. COLE1, RICHARD G. FAIRBANKS2, AND GLEN T. SHEN3 ABSTRACT The tropical ocean and atmosphere constitute an active component of the global climate system on interannual-to-century time scales, yet instrumental climate records from this extensive region are scarce, short, and unevenly distributed. Geochemical records from corals offer a promising means of extending the record of tropical climate variability. Corals grow rapidly (more than 1 cm/yr), may live for several centuries, are datable by several independent means, and incorporate the signatures of key atmospheric and oceanic processes in their skeletal chemistry. Efforts to extend coral records beyond the instrumental period and to recover Holocene and deglacial-age samples are under way throughout the tropics. Coral-based reconstructions of the El Niño/Southern Oscillation (ENSO) phenomenon demonstrate how this approach can contribute to understanding past variability in tropical climate dynamics. The abundance of atolls and coral reefs in the tropical Pacific enables coral studies to target specific features of ENSO, including variability in sea surface temperatures, upwelling, rainfall, and winds. Short records from three sites spanning the Pacific show coherent variations in these parameters associated with warm and cool ENSO extremes. In the Galapagos Islands, a tracer intercomparison study from a single coral head demonstrates high correlations with measured ENSO-related variability between 1936 and 1982, especially for the interannual periods characteristic of ENSO. Farther west, a coral record from Tarawa Atoll closely monitors central-western Pacific rainfall over the past century. Evolutionary spectral analysis of this record suggests a mode of variation between about 1930 and 1950 that differs from preceding or subsequent periods. Over the span of this coral record, variance at the annual cycle has been weakest during the past few decades. Between 1930 and 1950, however, the annual cycle reaches a magnitude comparable to that of the interannual variations, and a strong 3-year component virtually disappears. This shift occurs concurrently with an observed weakening of the Southern Oscillation and its teleconnections. 1   Institute for Arctic and Alpine Research, University of Colorado, Boulder, Colorado 2   Lamont-Doherty Earth Observatory and Department of Geological Sciences, Columbia University, New York, New York 3   School of Oceanography, University of Washington, Seattle, Washington

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Natural Climate Variability on Decade-to-Century Time Scales INTRODUCTION The tropical ocean and atmosphere constitute an active component of the global climate system, especially on sub-millennial time scales. Understanding the climatic processes that act on these time scales across this extensive region requires that we characterize their past variability. Yet instrumental records from the sparsely inhabited tropical oceans are scarce and short. Those records that extend beyond the past few decades often owe their locations to opportunistic, rather than climatic, rationales. Chemical records from the skeletons of long-lived corals provide one of the few high-quality means for extending the length of the climatic record in the tropical oceans beyond the brief period of instrumental coverage. Coral records offer an unusual opportunity to monitor the history of key tropical climate systems, especially since sites and tracers can be chosen explicitly for their climatic relevance. The development of proxy climate records from coral archives promises to offer important insights into the natural patterns and causes of tropical climate change, and into the sensitivity of low-latitude oceans to shifts in the global boundary conditions of climate. In the tropical Pacific, we have found that short records of isotopic and trace-metal variability in coral skeletons closely track specific ocean-atmosphere processes over the past few decades. An examination of multiple tracers within a single coral specimen demonstrates the varied utility of isotopic and trace-metal variations in tracking interannual sea surface temperature (SST) and upwelling changes. A 96-year record from the equatorial region west of the date line provides an index of ENSO variability comparable in quality to climatological records, and suggests that the spectrum of climate variability in this region has varied over the course of the present century. Tropical climate variability may take many forms; describing variations in tropical ocean-atmosphere systems thus requires a multivariate approach capable of resolving specific processes. SST changes dramatically over annual-to-interannual periods in regions of upwelling such as the eastern Pacific, eastern Atlantic, and northern Indian oceans, while SST in the western basins of tropical oceans tends to vary much less. Changes in upwelling have important consequences for surface ocean chemistry as well as for temperature. Surface levels of nutrients and other dissolved species enriched in deep water also vary in accordance with the upwelling of deep water to the surface. Advective changes have site-dependent effects on both chemical and thermal distributions. Patterns of rainfall and wind may shift in response to changed SST distribution; they in turn alter the thickness and salinity of the ocean's surface mixed layer, which affect the degree of ocean-atmosphere interaction (Godfrey and Lindstrom, 1989; Lukas and Lindstrom, 1991). Significant changes in SST, surface upwelling, currents, rainfall, and winds may result from interactions within the tropical ocean-atmosphere system. Alternatively, such changes may be forced by continental systems, such as the Asian monsoon. Many chemical tracers in coral skeletons possess distinct sensitivities to specific environmental variables such as SST, rainfall, winds, and upwelling; as with instrumental data, many coral records monitor specific oceanic or atmospheric processes. Reconstructing the behavior of large-scale, multivariate climate systems such as the El Niño/Southern Oscillation (ENSO) will require the integration of many such coral indices over extensive regions (Figure 1). CORAL RECORDS Individual coral heads may live for several centuries, offering the potential for climate reconstructions from remote sites that predate even the longest instrumental records. Because they grow at rates of 1 to 2 cm/yr and can be sampled at intervals less than I mm, records at sub-monthly resolution are obtainable. Interpretation of such long, high-resolution records demands precise chronologies. Figure 1 Map showing the distribution of sites where living coral heads have been cored for paleoclimatic analysis (after Dunbar and Cole, 1993). Sites with long records are mentioned in the text.

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Natural Climate Variability on Decade-to-Century Time Scales Corals can be dated over a wide range of time scales (from seasons to periods greater than 100,000 years) by a variety of independent means, including density and fluorescent banding, 230Th/234U ratio, 14C content, stable-isotope fluctuations, and amino acid racemization (Dodge et al., 1974; Isdale, 1984; Edwards et al., 1988; Fairbanks, 1989, 1990; Bard et al., 1990; Cole and Fairbanks, 1990; Goodfriend et al., 1993). Recent work on monthly and even daily coral banding offers the promise for increasingly refined chronological determinations (Barnes and Lough, 1989; Risk and Pearce, 1992). Corals precipitate a skeleton of aragonite (CaCO3) that can incorporate several independent chemical tracers used to monitor variability in oceanic and atmospheric processes. The most consistently useful of these are the isotopic ratios of carbon and of oxygen in particular, and the concentration of certain lattice-bound trace metals that appear to substitute for skeletal calcium (expressed as Cd/Ca, Ba/Ca, Mn/Ca, and Sr/Ca ratios). In this section, we present an overview of the many tracers now in use for coral-based paleoclimatic reconstruction. Subsequent sections discuss examples of both short records from a variety of sites and longer records that yield insight into the recent history of tropical climate variability. Isotopic Indicators The stable isotopic content of coral aragonite is expressed in parts per thousand () as d180 and d13C, where the d notation is defined in terms of the isotopic ratios R (either 13C/12C or 18O/16O) in the sample relative to a standard: d = [(Rsample - Rstandard)/Rstandard] × 1000 For the oxygen and carbon isotopic content of coralline aragonite, the conventional reference standard is the Pee Dee Belemnite, or PDB. Coral skeletal d18O reflects a combination of local SST and the d18 O of ambient seawater. The d18O of biogenic calcium carbonate precipitated in equilibrium with seawater decreases by about 0.22 for every 1°C rise in water temperature (Epstein et al., 1953). In corals, this effect is biologically mediated such that the d18O of the coral skeleton is offset below seawater d18O. This offset is constant within a coral genus (Weber and Woodhead, 1972) for the rapidly growing central axis of the skeleton (Land et al., 1975; McConnaughey, 1989). Coral d18O records taken along the axis of maximum growth thus track ambient temperatures at subseasonal resolution (Fairbanks and Dodge, 1979; Dunbar and Wellington, 1981; Pätzold, 1984; McConnaughey, 1989). Coral skeletal d18O also records any variations in the d18O of the seawater (Epstein et al., 1953; Fairbanks and Matthews, 1979; Swart and Coleman, 1980). Such variations are small in most regions of the tropical ocean, but in some locations changes in evaporation, precipitation, or runoff may cause pronounced d18O variability (Dunbar and Wellington, 1981; Cole and Fairbanks, 1990). In regions with fairly constant or well-known temperature histories, coral d18O provides a record of past variations in the hydrologic balance (Cole and Fairbanks, 1990). In many cases, coral skeletal d18O reflects a combination of thermal and hydrologic factors. The interannual d13C signal in coral skeletons is often difficult to decipher in environmental terms, because of complicated interactions with biological processes that involve strong isotopic fractionation. Environment-related controls on skeletal d13C include (1) the isotopic composition of the ambient seawater (Nozaki et al., 1978), (2) coral geometry and growth rate (e.g., apex versus side of coral head) (Land et al., 1975; McConnaughey, 1989), and (3) photosynthesis of endosymbiotic dinoflagellates (Weber, 1974; Goreau, 1977; Fairbanks and Dodge, 1979; Swart, 1983; McConnaughey, 1989). This last parameter depends upon ambient light levels, as regulated by water depth and insolation. Coral skeletal d13C correlates positively with insolation in many contexts, from depth-dependent variation (Weber and Woodhead, 1970; Fairbanks and Dodge, 1979; McConnaughey, 1989) to annual cycles that reflect rainy (i.e., cloudy) seasons (Fairbanks and Dodge, 1979; Pätzold, 1984; McConnaughey, 1989; Cole and Fairbanks, 1990). However, shallow corals may experience reduced photosynthesis during brighter periods, while deeper corals may respond to increased light by increasing photosynthesis (Erez, 1978; McConnaughey, 1989). These responses can produce opposite skeletal d13C signatures in deep and shallow corals (McConnaughey, 1989). Environmental reconstruction from coral d13C records requires more information about growth conditions and physiological responses than is usually available in a paleoceanographic context. Trace Metals Specific environmental processes, including upwelling, advection, aeolian transport, and runoff, influence the surface-water concentrations of certain trace elements (Boyle, 1988; Martin et al., 1976; Shen and Boyle, 1988; Lea et al., 1989; Shen and Sanford, 1990; Shen et al., 1991, 1992b). Several such metals appear to substitute readily for Ca in the aragonite lattice of coral skeletons. Estimated distribution coefficients between corals and seawater allow the reconstruction of ambient seawater metal concentrations from metal concentrations in the coral skeleton. Trace metal records from corals thus yield a history of the processes that control the local distribution of trace metals. The most useful metals for coral reconstructions of ENSO variability include Cd, Ba, Mn, and Sr. Many authors (Shen and Boyle, 1988; Lea et al., 1989; Linn et al., 1990; Shen and Sanford, 1990; Shen et al., 1987, 1991; Beck et al., 1992; de Villiers et al., 1993) describe these applications

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Natural Climate Variability on Decade-to-Century Time Scales in greater detail, including specific techniques for sample cleaning and analysis. Studies of growth rate and species influences show very limited evidence of biological mediation of metal incorporation (de Villiers et al., 1993; G.T. Shen, unpublished results). However, SST may play a role in the incorporation of Ba (Lea et al., 1989), and the dependence of Sr incorporation on ambient SST (Smith et al., 1979) makes possible precise reconstruction of SST from coral Sr records (Beck et al., 1992). Cadmium The modern distribution of cadmium follows that of marine nutrients. Low levels in surface waters reflect biological removal, while higher levels at depth result from the regeneration of organic matter (Boyle et al., 1976; Martin et al., 1976). In coral records, the skeletal Cd content usually depends on the balance between Cd-rich upwelled deep water and Cd-poor oligotrophic surface waters. In Galapagos corals, Cd/Ca ratios directly reflect upwelling variations associated with both seasonal cycles (Linn et al., 1990; Shen and Sanford, 1990) and interannual ENSO variability (Shen et al., 1987, 1992a). Barium This trace metal exhibits nutrient-like behavior akin to cadmium's (Chan et al., 1977). Higher concentrations in both seawater and corals render coral Ba records less susceptible to contamination. Seasonal-resolution records of Ba/Ca from Galapagos corals reflect regional upwelling variability (Lea et al., 1989). Relative to Cd, Ba may exhibit greater sensitivity to periods of weak upwelling, possibly because the biological uptake of Cd occurs at rates comparable to the slow rate of supply during these times. Ba is also enriched in continental runoff waters, and Ba/Ca records from corals near continental margins may reflect this input (Shen and Sanford, 1990). However, a slight temperature effect may occur upon incorporation of Ba into the coral skeleton, which would complicate paleoclimatic reconstruction from coral Ba/Ca records (Lea et al., 1989). Manganese Unlike Cd and Ba, Mn reaches high concentrations in surface waters. A mid-depth maximum coincides with the local O2-minimum zone, where particulate Mn oxides are reduced and solubilized; concentrations then diminish with increasing depth. Aeolian and fluvial input provide important Mn sources, as do reducing environments where particulate Mn is degraded, such as the O2-minimum zone, shelf sediments, and lagoons. The interpretation of coral Mn/Ca records is thus usually site-specific, reflecting transport and mixing of water masses with varying Mn levels. In the Galapagos, for example, seawater Mn levels reflect a combination of aerosol deposition on the surface waters and long-range advection of Mn-enriched surface water from the continental shelf (Shen et al., 1991). In localized reef settings, dissolved Mn in the water column can be augmented by local sediment fluxes, producing very high skeletal concentrations of Mn in corals from the Gulf of Panama and some Caribbean islands (Shen et al., 1991). Diagenetic Mn fluxes from lagoonal sediments offer useful indicators of climate variability at certain Pacific atoll sites far removed from continental sources of Mn (Shen et al., 1992b). Strontium Work by Smith et al. (1979) demonstrated that the Sr/ Ca ratio of coral skeletons may provide a monitor of past temperature changes. However, the measurement technique used in that study was not sufficiently precise to offer detailed SST reconstructions at a useful resolution for the tropics. The recent application of thermal ionization mass spectrometry (TIMS) to these measurements has greatly improved the precision of Sr/Ca determinations (Beck et al., 1992); recent results indicate that monthly SST can be reconstructed with an apparent accuracy of better than 0.5°C (Beck et al., 1992; de Villiers et al., 1993). SST reconstructions based on skeletal Sr determinations are less susceptible to artifacts associated with hydrologic changes than d18O-based reconstructions. The paired application of d18O and Sr/Ca data may thus allow the separation of SST changes from seawater d18O variability. Paleoceanographic Applications Many studies have documented the paleoclimatic utility of isotopic and trace-metal measurements by developing short (2-20 yr) data sets that correlate with nearby instrumental records (e.g., Fairbanks and Dodge, 1979; Pätzold, 1984; Shen et al., 1987, 1992b; Carriquiry et al., 1988; Lea et al., 1989; Cole and Fairbanks, 1990; Winter et al., 1991; Cole et al., 1992; Beck et al., 1992). With the growing interest in long records of natural climate variability, many groups are applying these techniques to the development of proxy records that extend beyond the period of local instrumental coverage. Efforts to develop long, high-resolution reconstructions of climate are in progress using coral records from sites throughout the low-latitude oceans. Several studies have focused explicitly on reconstructing interannual-to-decadal variability in the ENSO system; we detail current results from the equatorial Pacific in the following section. Other sites from which coral records extending beyond the present century have been developed include Florida Bay (Kramer et al., 1991), Cebu Island, Philippines (Pätzold and Wefer, 1986), Espiritu Santo Island (Quinn et al., 1992), the southern Great Barrier Reef (Druffel and

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Natural Climate Variability on Decade-to-Century Time Scales Griffin, 1992), Bermuda (Pätzold and Wefer, 1992), and Puerto Rico (A. Winter, pers. commun., 1992). Records from sites spanning the tropics are in progress at many institutions (Figure 1; see also Dunbar and Cole, 1993). CORAL MONITORS OF ENSO VARIABILITY The ENSO phenomenon consists of the large-scale oscillation of the tropical Pacific ocean-atmosphere system between two extreme states, characterized by alternately warm and cool SST anomalies in the eastern Pacific (Philander, 1990; Deser and Wallace, 1990). Both warm- and cool-phase ENSO conditions exhibit coherent patterns of coupled oceanic and atmospheric anomalies that leave distinct signatures in the isotopic and trace-metal content of coral skeletons (Figure 2). The influence of ENSO dominates interannual climate variability throughout the equatorial Pacific and the global tropics, and ENSO-related climate anomalies propagate to higher latitudes via mechanisms that include the displacement of upper atmospheric pressure patterns and the generation of troughs that penetrate into southwestern North America (van Loon and Rogers, 1981; Rasmusson and Wallace, 1983; Horel and Wallace, 1981). The cool phase of ENSO is characterized by strong easterly trade winds at the surface, a convective maximum (the Indonesian Low) over Indonesia and northern Australia, and return flow aloft that brings dry subsiding air over most of the eastern and central Pacific. This zonal atmospheric pattern that spans the tropical Pacific is known as the Walker circulation. The strong trade winds during this phase of Figure 2 Schematic cross-section of equatorial Pacific showing major features of the warm and cool phases of ENSO in relation to selected sites where coral paleoclimatic reconstructions of ENSO are under way: Bali, Sulawesi, Tarawa, Kanton, and the Galapagos (after Cole, 1992). ENSO intensify the upwelling of cool, nutrient-rich waters in the eastern Pacific and transport surface waters westward, generating a strong zonal SST gradient and an eastward slope in sea level. Transition to the warm phase of ENSO may occur when the trade winds relax or reverse west of the date line, depending on surface ocean conditions. As the warm phase of ENSO begins, the western Pacific warm pool spreads eastward (Lukas et al., 1984; McPhaden and Picaut, 1990), and the Indonesian Low convective system migrates to the region near the date line and the equator. In the eastern Pacific, surface waters become warm and oligotrophic, as the thermocline deepens and warm waters move in from the west. Trade winds remain weak and variable as a result of the diminished zonal SST gradient. Dramatic shifts in precipitation patterns occur across the entire tropical Pacific, from South America to southeast Asia, in response to Indonesian Low migration and the development of convection over newly warmed ocean regions. Frequency-domain analyses of climate data that span the past four decades indicate that ENSO operates on three fundamental time scales. The annual cycle sets the phasing of ENSO anomalies; the evolution of ENSO extremes appears phase-locked to the annual cycle (Rasmusson and Carpenter, 1982). Several aspects of ENSO variability, including SST, winds, rainfall, and sea level pressure (SLP), also possess a quasi-biennial pulse that varies in intensity throughout the instrumental record (Trenberth, 1980; Rasmusson et al., 1990; Barnett, 1991; Ropelewski et al., 1992). Finally, ENSO warm extremes recur approximately every 3-7 years, lending a ''low-frequency" beat to the spectrum of ENSO variability (Rasmusson and Carpenter, 1982; Rasmusson et al., 1990; Barnett, 1991; Ropelewski et al., 1992). Recent analyses of multi-decadal records of Pacific climate suggest that ENSO variability may also experience significant decadal-scale shifts (Elliott and Angell, 1988; Cooper et al., 1989; Trenberth, 1990). Such a shift is evident over the interval between 1977 and 1988, which exhibited no strong cool anomalies (Trenberth, 1990; Kerr, 1992). The impacts of the Southern Oscillation both within and beyond the tropical Pacific fluctuate on a multi-decadal time scale (Trenberth and Shea, 1987; Elliott and Angell, 1988; Cole et al., 1993; Michaelsen and Thompson, 1992; Rasmusson et al., 1995, in this volume). During the first 20 years of this century, for example, the interannual pulse of the Southern Oscillation was strong and highly correlated with climatic records both within the tropical Pacific and in sensitive teleconnected sites. The subsequent three decades, however, witnessed a general decline in the correlations among Pacific climate variables and between Pacific climate and teleconnected sites. Elliott and Angell (1988) propose that these fluctuations may result from the migration of the centers of strongest coherent variability in the Southern Oscillation over the course of the present century.

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