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Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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6

Paleoclimate Overview

SUMMARY

The task of understanding climate change and predicting future change would be complex enough if only natural forcing mechanisms were involved. It is significantly more daunting because of the introduction of anthropogenic forcing and even more so considering the limitations in available records. Earth history provides a unique opportunity to assess the temporal and spatial characteristics of climate variability prior to any anthropogenic forcing; assess the natural rates of change associated with the evolution of the Earth system to understand how physical and biospheric systems interact across multiple time- and space scales; define the nature of the sensitivity of the Earth' s climate and biosphere to a large number of forcing factors; examine the integrated climatic, chemical, and biological response of the Earth system to a variety of perturbations; and test the predictions of numerical models for conditions significantly different from the present day. In effect, the paleoclimate record provides a series of cases and lessons upon which our understanding of climate change can be constructed and tested.

The paleo perspective has provided some significant surprises concerning climate change, changes in atmospheric chemistry, and the response of natural systems to climate change. The most recent dramatic new discovery is the verification that rapid and massive reorganizations in the ocean-atmosphere system—rapid climate change events—have occurred at frequent intervals throughout at least the last glacial cycle (the past ~100,000 years). The largest of these events are characterized by changes in climate that are close to the order of glacial/interglacial cycles. Perhaps most surprising is the demonstration that these rapid climate change events turn on and off in decades or less and may last centuries to millennia. Furthermore, these events are globally distributed and

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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found in a variety of paleoenvironments (ocean, atmosphere, and land). Several potential causes for these events have been proposed, but without a more detailed understanding of the relative phasing of these events from region to region, definitive causal mechanisms cannot be constructed.

Of greatest consequence to humans is the fact that subdued versions of these events are documented during our current interglacial (the Holocene, which began ~11,500 years agoa). While subdued relative to earlier events, they are still sufficient to significantly perturb natural systems and still operate at rapid rates (years to decades). Thus, one of the most important tasks for paleoclimatologists is to improve our understanding of Holocene climate, for it is within the Holocene that the boundary conditions for modern natural climate variability can be identified and from which the relative importance of natural versus anthropogenic climate forcing can be assessed.

Patterns in climate variability can be identified on the interannual to millennial scale. This finding is particularly encouraging since one of the end goals of climate change research is predictability. However, deconvolving predictable patterns at the regional scale and determining the temporal baseline from which predictability can be assessed will require more dense spacing of paleodata.

Few instrumental records precede the era of anthropogenic involvement; thus, it is necessary to supplement and hindcast these data with paleoclimate records. The intended meaning of hindcast is to extend instrumental time series back prior to their onset date using proxy records. The assumption is made that a transfer function of some type links the instrumental and proxy records allowing this process. Fortunately, many paleodata series afford detailed views of pertinent climate indicators (e.g., temperature, precipitation, El Niño-Southern Oscillation (ENSO), monsoon). On the other hand, since there are no true analogs in the paleoclimate record for modern or future climate, it is essential to utilize both modern observational and paleoclimate records to solve this complex problem.

New advances in paleoclimate research reaffirm the necessity to view climate change over varying timescales; utilize a variety of globally distributed paleoclimate records that monitor change throughout the Earth system; and focus attention on well-dated, highly resolved multivariate paleoclimate records. These paleodata are essential for understanding global environmental change and its potential impact on humans, assessing human influence on the global environment and for the evaluation of predictive climate models.

The research imperatives for paleoclimate are to:

  • Document how the global climate and the Earth's environment have changed in the past and determine the factors that caused these changes. Explore how this knowledge can be applied to understand future climate and environmental change.

a

Assumed format is calendar years unless specified as 14C years.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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  • Document how the activities of humans have affected the global environment and climate and determine how these effects can be differentiated from natural variability. Describe what constitutes the natural environment prior to human intervention.

  • Explore the question of what the natural limits (e.g., in the frequency of events, trends, extremes) are of the global environment and determine how changes in the boundary conditions (e.g., greenhouse gases, ocean circulation, ice extent) for this natural environment are manifested.

  • Document the important forcing factors (e.g., greenhouse gases, solar variability, ocean circulation, volcanic aerosols) that are and will control climate change on societal timescales (season to century). Determine what the causes were of the rapid climate change events and rapid transitions in climate state.

INTRODUCTION

Since ancient times humans have modified their local and regional environments, but only since the Industrial Revolution has human activity had a significant measured effect at the planetary scale. Human impact on the composition of the global atmosphere is now without question. Human disturbance of biogeochemical cycles may now be approaching a critical level. Over the past few decades concentrations of atmospheric gases (e.g., CO2, CH4, N2O) have been increasing dramatically and have moved into a range unprecedented for the past million years. This increase has produced serious concern regarding the heat balance of the global atmosphere. Greenhouse gases are, however, only part of the human problem. For example, sulfur gases and dusts can reinforce or counteract greenhouse gas effects on local to regional scales. While remarkable efforts are under way to resolve the history and significance of the human influences on climate, pollution, and resource depletion, our understanding of climate change is still hampered by a lack of knowledge of the processes that underlie natural climate variations.

The importance of understanding natural climate variability has been clearly articulated in previous National Research Council (NRC) reports. In a 1975 report prepared by the U.S. Committee for the Global Atmospheric Research Program, documentation is provided for the presence of seasonal to millennial scales of natural climate variability and for regularities in climatic series. In the 1990 report the Committee on Global Change summarized several important contributions to the understanding of natural climate variability made by a variety of major scientific efforts that had emerged since the 1975 report. For example, the CLIMAP (Climate Mapping, Analysis, and Prediction) group produced the first comprehensive reconstructions of the Earth's climate during the last glacial maximum; the COHMAP (Cooperative Holocene Mapping Project) group extended paleoclimatic reconstructions to the post-glacial era and demonstrated the

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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emergence of the African-Asian monsoon system; and the SPECMAP (Spectral Mapping Project) group verified the strong relationship between the Earth's orbitally induced cycles of insolation and major fluctuations in climate.1 Since the 1990 NRC report several important discoveries have been made that have focused even more attention on the paleoclimate record.

The most dramatic of these new discoveries is the verification that rapid and massive reorganizations in the ocean-atmosphere system —rapid climate change events—have occurred at frequent intervals throughout at least the last glacial cycle (the past ~100,000 years). The largest of these events are characterized by changes in climate that are close to the order of glacial/interglacial cycles. Perhaps most surprising is the demonstration that these events initiate and terminate in decades or less and may last centuries to millennia. Of greatest consequence, however, is the fact that subdued versions of these events are documented during our current interglacial (the Holocene, which began ~11,500 years ago). Thus, these rapid climate change events have immense significance to our understanding of both natural climate variability and modern climate.

While the causes of rapid climate change events and natural climate variability, in general, are still not fully understood, evidence continues to accumulate emphasizing the significance of a variety of climate processes, such as changes in thermohaline circulation of the world's oceans, Earth's orbitally induced (Milankovitch) cycles of insolation, solar variability, greenhouse gases, volcanic activity, and ice sheet dynamics.

CASE STUDIES

This report focuses on five case studies chosen to demonstrate the potential wealth of information available from the paleorecord. The first three are presented in specific time domains (the last glacial cycle to onset of the Holocene; the Holocene; the past 2,000 years of the Holocene). The last two focus on subject areas that draw on a wide range of Earth history—namely, climate-vegetation interactions and warm climates.

The Last Glacial Cycle to the Onset of the Holocene (~11,500 years ago)
Summary of Previous Work

A variety of paleoclimate records demonstrate that the Earth's climate has varied significantly throughout the past 1 million years. This natural climate variability ranges from glacial to interglacial states, in approximately 100,000-year cycles that terminate as ~10,000-year-long interglacials, characterized by relatively ice free and warm conditions. 2

Knowledge of the low-frequency component of the Earth's climate variabil-

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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FIGURE 6.1 Comparison of three high-latitude records from the southern hemisphere showing the overall good agreement between CO2 and temperature changes (inferred from δD). Taken from Crowley and North (1990). Data sources: the Vostok δD record (Jouzel et al., 1987) and the CO2 record (Barnola et al., 1987) are plotted according to the revised chronology of Petit et al. (1990).SOURCE: Crowley and North (1990). Courtesy of Oxford University Press.

ity, resulting from changes in the Earth's orbital cycles, pioneered by the CLIMAP project and described by Imbrie et al. (1992, 1993), has been verified and further elucidated by the SPECMAP project. Orbitally induced variations in insolation at the Milankovitch periods (primarily 100,000, 41,000 and 23,000 years) explain much of the change in global ice volume throughout the late Pleistocene and have been identified in a variety of paleoclimate records (e.g., marine and ice cores and loess sequences). CO2 and CH4 figure prominently in climate change over the last glacial/interglacial cycle, as demonstrated by the close association between Vostok (Antarctica) ice core CO2 and temperature (see Figure 6.1).3 This dramatic demonstration of the long-term association between temperature and CO2 has had a profound effect on the implications of anthropogenically induced greenhouse gas warming. However, the fact that CO2 lags temperature at major climate transitions (e.g., the end of the last interglacial) suggests that the system response may be complex.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
×
New Developments

The most dramatic recent contributions to our understanding of paleoclimate during the last glacial cycle have come in the millennial-scale range of climate variability. Unprecedented swings in the Earth's climate have now been recorded in two ice cores from central Greenland, instigating new higher-resolution investigations of land and marine paleoclimate records.

In 1993 the Greenland Ice Sheet Project Two (GISP2) successfully completed drilling to the base of the Greenland ice sheet in central Greenland. In so doing, GISP2, along with its European companion project GRIP (Greenland Ice Core Program), developed the longest high-resolution continuous paleoenvironmental record (>250,000 years) available from the northern hemisphere. Based on the comparison of electrical conductivity and oxygen isotope series between the two cores,4 at least the upper 90 percent displays extremely similar if not absolutely equivalent records.

The central Greenland ice cores provide a framework for other paleoclimate records because of their relatively precise dating. The current best estimate of the age at ~2,800 m is ~110,000 years, based on a combination of multiparameter annual layer counting combined with measurements of the d18O of atmospheric O2 calibrated with the Vostok ice core in Antarctica.5 Error estimates in the dating are quite remarkable, from 2 percent for 0 to 11,640 years ago to 10 percent for over 40,000 years.6 Agreement between the GISP2 and GRIP ice cores (separated by 30 km or ~10 ice thicknesses) over the record period of the past ~110,000 years provides strong support for the climatic origin of even the minor features of these records and implies that investigations of subtle environmental signals can be rigorously pursued. The climatic significance of the deeper part of these ice cores (>110,000 years in age) is a matter of considerable controversy. Without additional records, the evidence for rapid climate change in Greenland during the last interglacial remains equivocal.

The millennial-scale events recorded in the upper 110,000 years of the two central Greenland ice cores are, however, unequivocally climate events. They represent large climate deviations (massive reorganizations of the ocean-atmosphere system) that occur over decades or less and during which ice-age temperatures in central Greenland may have been as much as 20°C colder than today (see Figure 6.2).7 These events have their greatest magnitude during the glacial portion of the record, prior to ~14,500 years ago), when large northern hemisphere ice sheets provided positive climate feedbacks.8

Examination of one of these events, the Younger Dryas (a near return to glacial conditions during the last deglaciation, previously identified in a variety of paleoclimate records), demonstrates the importance of conducting multiparameter high-resolution paleoclimate investigations on well-dated records. During this event lowered temperatures were accompanied by up to twofold and greater changes in snow accumulation, order-of-magnitude changes in the amount

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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FIGURE 6.2 The central Greenland δ18O history for the most recent 40,000 years. The smooth curve results when this history is filtered to mimic the thermal averaging in the ice sheet. All temperature histories that give this same curve when filtered are indistinguishable to borehole thermometry. The right axis shows the calibrated temperature scale. SOURCE: Cuffey et al. (1995). Courtesy of the American Association for the Advancement of Science.

of wind-blown dust and sea salt in the atmosphere, and large changes in methane concentration, with cold, dry, dusty, conditions correlated with low-methane (see Figure 6.3).9 Annually resolved sampling over early and late stages of the Younger Dryas indicates that this ~1,300-year duration event began and ended in less than 5 to 20 years.10

The identification of rapid climate change event style variations in the GRIP CH4 record11 (see Figure 6.4) prompted considerable interest in the identification of such events in other regions since the source areas for CH4 during the last glaciation may have been in the middle to lower latitudes. In addition, several rapid climate change events recorded in Greenland are in the isotopic temperature record of the Vostok ice core from central East Antarctica, although with apparently smaller amplitude than in Greenland (see Figure 6.5).12

Paleoclimate records from North Atlantic marine sediment cores also contain notable millennial-scale variability,13 although the exact timing of these events is less precisely known than for the Greenland ice cores. Several of the marine cores reveal evidence that the formation of NADW (North Atlantic deep water; warm, saline, nutrient-depleted deep return flow water), and thus the oce-

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
×

FIGURE 6.3 Composite figure. Above: the Younger Dryas was an abrupt return to near-glacial conditions (about 7°C lower, decreased accumulation rate, decreased methane, increased atmospheric dust) that lasted approximately 1,300 years and punctuated the transition from glacial to interglacial climates. Figure modified from Alley et al. (1993), Grootes et al., (1993), and Brook et al. (1996). Right: This high-resolution calcium record from the GISP2 ice core indicates the relative amount of dust in the atmosphere over Greenland and thus documents other abrupt, frequent, and massive changes in climate that characterize the glacial portion of the ice core record. SOURCE: Adapted from Mayewski et al. (1994, 1997). Courtesy of the American Association for the Advancement of Science.

anic thermohaline circulation, fluctuated dramatically in the past. 14 NADW diminished greatly during the last glaciation and was relatively strong during the interglacials. Recent studies confirm that NADW fluctuates on millennial timescales and correlates with sea surface and atmospheric temperatures.15

Changes in the flux of ice-rafted detritus, d18O of foraminifera shells, and the abundance of climate-sensitive foraminifera indicate that during the last glaciation the North Atlantic was punctuated by iceberg discharge events potentially produced in response to changes in ice sheet dynamics.16 The largest of these (Heinrich events) have a characteristic recurrence in the marine record on the order of 5,000 to 10,000 years. They are also associated with similar events of shorter-timescale variability described above (on the order of 1,000 to 3,000 years long, termed Dansgaard/Oeschger rapid climate change events) that correlate with the stadial/interstadial changes observed in ice core records from central Greenland (see Figure 6.6).17

Evidence for the presence of millennial-scale climate fluctuations has been

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
×

FIGURE 6.4 Top: GRIP ice core values for methane. The thick line runs through the mean concentration (black dots), and the two accompanying thin lines correspond to the experimental uncertainty (2 sigma). Bottom: Mean δ18O record along 2.2-m sections of the core (Dansgaard et al., 1993; Johnsen et al., 1992). The significant climatic events are noted by name or suggested numbering (Dansgaard et al., 1993). The timescale applies to both records. The depth scale applies only to the CH4 curve (top) because of the difference in age between trapped air and ice at a given depth. SOURCE: Chappellaz et al. (1993). Courtesy of Macmillan Magazines Ltd.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
×

FIGURE 6.5 Greenland (GISP2) and Antarctic (Vostok) climate records covering the last glacial/interglacial cycle. Top: Plot shows the close correlation between GISP2 and Vostok δ18O of O2 in air in these ice cores. Bottom: Curves show close correlation between two proxies for temperature, δDice (Vostok) and δ18O (GISP2) in the ice. SOURCE: Adapted from Bender et al. (1994). Courtesy of Macmillan Magazines Ltd.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
×

FIGURE 6.6 Comparison of the δ18O record and age model for the GRIP ice core, Summit, Greenland (Dansgaard et al., 1993), with measurements of lithic concentrations and percentages of the planktonic foraminifera Neogloboquardrina pachyderma (left coiling), a proxy for surface water temperatures, in WM23-81 (Bond et al., 1993). That foraminifera today lives in waters <10°C and comprises about 95 percent of the fauna at summer temperatures of less than 5°C. Age model for the marine record is from Bond et al. (1992, 1993). Cycles between the Heinrich events are given letters to aid their description in the text. A good match exists between the lithic concentration cycles and the temperature cycles in the ice core; the match of the lithic cycles to the ocean surface temperatures, however, is much poorer. The GISP2 timescale derived from layer counting to 41,000 years is included for comparison with the GRIP timescale and the 14C timescale. The GISP2 timescale was transferred into the GRIP record at the sharp interstadial boundaries, which are precisely located in both ice core records (Dansgaard et al., 1993; Mayewski et al., 1994), and then ages were interpolated between these boundaries. The progressive difference in ages, reaching about 10 percent at 40,000 calendar years ago, is consistent with the error estimated for ice core dating. SOURCE: Bond and Lotti (1995). Courtesy of the American Association for the Advancement of Science.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
×

extended outside the North Atlantic and polar regions. Marine cores from the Santa Barbara basin reveal highly sensitive perturbations in the ocean circulation patterns of the East Pacific region 18 and ice-rafted debris events in the North Pacific that correlate with the Greenland ice core records. Abrupt changes in atmospheric circulation patterns and precipitation regime also are recorded over eastern Asia in a thick sequence of wind-deposited loess from central China. 19 Records of alpine glacier fluctuations, mountain snowlines, and paleovegetation in the Andes reveal climate fluctuations that are similar in regularity to events in the Greenland ice cores.20

While the exact phasing of rapid climate change events from region to region is still being examined, new advances in age-dating correlation techniques have provided insight into the bipolar phasing of major climate events close to the last glacial maximum. Measurements of the d18O of atmospheric O2 from the Byrd and Vostok ice cores in Antarctica and the GISP2 ice core suggest that the transition from glacial maximum to deglaciation began in Antarctica approximately 3,000 years before the onset of warming in Greenland.21 This view creates a more complex event phasing than that suggested by previous correlations of marine, coral reef, and ice extent records, which suggested that during the last termination nearly synchronous temperature changes affected ice masses from the poles to the equator. 22

New advances in paleoclimate reconstruction also come from the tropics. For example, a 30,000-year-long paleotemperature record from lowland Brazil, based on noble gas concentrations in groundwater23 and an Andean ice core24 suggests a cooling of 5 to 8 degress, contrasted with earlier estimates from marine cores that limit cooling to >3 degrees.25 Implications of this change in temperature to the hydrological cycle and consequently to climate are intriguing.26 These new findings have stimulated examination of other tropical paleoclimate records and renewed investigations into climate forcing that is tied to changes in the tropics.

Causal mechanisms for glacial-age climate fluctuations appear to be complex, and phasing of these events is not understood from region to region. However, evidence for the identification of regularity in the timing of some climate events is building. Studies ranging from the North Atlantic (GISP2) to the subtropics demonstrate 1,450-to 1,800-year periodicities for rapid climate change events.27 In addition, the cumulative effect of multiple climate forcings can now be demonstrated. As an example, ~90 percent of the variance in the GISP2 paleoatmospheric circulation series is related to insolation changes induced by the Earth's orbital cycles, which operate in concert with faster periodic climate forcings such as changes in ice sheet dynamics, thermohaline ocean circulation, and solar variability (see Figure 6.7).28 Additional climate forcing mechanisms are, undoubtedly, also involved, such as changes in CO2, CH4, water vapor, volcanism, biogenic source cloud condensation nuclei, and dusts.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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FIGURE 6.7 Top curve: [NRC9] The PCI (Polar Circulation Index) is a time series describing the dynamics (i.e., increase and decrease from mean values) of the well-mixed atmosphere represented by the dominant EOF of the major ions in the GISP2 ice core (Mayewski et al., 1994). The PCI provides a relative measure of the average size and intensity of polar atmospheric circulation. In general terms PCI values increase (e.g., more continental dusts and marine contributions) during colder portions of the record (stadials) and decrease during warmer periods (interstadials and interglacials; Mayewski et al., (1994)). The PCI is contrasted with the sum of the bandpass components (>99 percent significance) estimated from this series. The sum represents about 90 percent of the variance in the original PCI series. Lower curves: Major bandpass components derived from the PCI series, including those with periodicities close to elliptical precession, axial precession, precession of the equinoxes, lower-order harmonics of the preceding, and periodicities potentially related to ice sheet dynamics, internal ocean oscillations (including changes in thermohaline circulation), and solar variability. The 6,100-year bandpass component describes the timing of H (Heinrich events) and the 1,450-year bandpass component of the timing of the rapid climate change events (Dansgaard/Oeschger) events in the GISP2 record. SOURCE: Adapted from Mayewski et al. (1997). Courtesy of the American Geophysical Union.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
×
The Holocene
Summary of Previous Work

One of the most important tasks for paleoclimatologists is improving our understanding of Holocene climate, for it is within the Holocene that the boundary conditions for modern natural climate variability can be identified and from which the relative importance of natural versus anthropogenic climate forcing can be assessed. Understanding modern climate and predicting future climate will require a detailed understanding of Holocene climate forcing and response.

Millennial-scale and finer Holocene climate fluctuations have been identified for more than two decades in a variety of Holocene records. 29 In general, however, Holocene climate variability is significantly more subdued in magnitude than that recorded during the last glaciation, and significantly less attention has been paid to this portion of the paleoclimate record.

New Developments

Environmental response to climate change since the last glacial maximum has been considerable. COHMAP and numerous smaller research efforts have characterized and modeled the effect of changes in land and sea ice extent, sea surface temperature, vegetation, and extent of arid regions during selected periods. Fossil pollen data keyed to the distribution of modern analogs have been used to develop paleovegetation maps for regions such as eastern North America (see Plate 6).30 Pollen and tree ring data have been used to reconstruct the spatial variations of temperature and precipitation over northern North America for the past ~6000 years.31

Several primary conclusions can be drawn from paleovegetation reconstructions. 32 Climate change events of the magnitude captured in these studies result in

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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dramatic changes in vegetation over regions. Even individual taxa respond sensitively to climate change, and vegetation produced under conditions that lack modern analogs may not be found under modern climate conditions. Some attempts have been made to simulate vegetation patterns that could exist in 2 × CO2 eastern North America, utilizing general circulation models (GCMs) coupled with paleoclimate-vegetation distributions.33

Annually resolved continuous paleoclimate records from the GISP2 ice core demonstrate that Holocene climate is characterized by annual-to millennial-scale variability and that Holocene climate is significantly more complex than glacialage climate.34 Time series for the major ions dissolved in the atmosphere, utilized as tracers for major atmospheric circulation systems, reveal a strong association between expansions of northern hemisphere polar atmospheric circulation systems and a variety of discontinuous paleoclimate records that record worldwide coolings (see Figure 6.4).35 These events have a quasi-periodicity of 2,600 years in phase with previously defined ~2,500- year variations in d14C, suggesting perhaps a solar variability-climate connection.36

Complexities in Holocene climate are noted in a comparison of several environmental parameters recorded in the Summit, Greenland, ice cores. During major Holocene coolings recorded in the GISP2 paleoatmospheric circulation series (see Figure 6.8), the climate response system operated similarly to pre-Holocene cooling events (Figure 6.3). Namely, cooler temperatures (more negative stable isotopes), reduced methane, reduced accumulation rate, and intensification of polar atmospheric circulation (expanded polar circulation index) all vary, in general, together. However, the coherence between these variables weakens as the events get younger and is particularly poor during the periods between events, suggesting increased regionalization of climate from early to late Holocene. This progressive regionalization of climate may be the manifestation of the varying influence of a variety of climate-forcing mechanisms, such as changes in total and season to season insolation, ice sheet and sea ice extent, solar variability, and volcanism.37

Several paleoclimate records document specific periods of climate reorganization. For example, African lake level records, developed in 1990,38 suggest that a major period of ocean-atmosphere reorganization occurred between some 7,000 to 8,000 years ago. A similarly timed reorganization in climate has recently been documented by comparing records from tropical Africa with those from Greenland and Antarctica.39 Other lake level records from Africa plus Dead Sea records suggest that both the tropics and the midlatitudes experienced a series of major changes in hydrological balance.40

Analysis of Arabian Sea sediment records spanning the past 24,000 years reveals that the response of the southwest monsoon over this region to long-term changes in insolation occurred in several distinct events of less than 300-year duration between 14,300 and 7,300 14C years ago.41 Arabian Sea sediment records also document changes in the strength and frequency of the Indian monsoon (see Figure 6.9), confirming earlier reports that the monsoon strengthened in stages

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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FIGURE 6.8 From top to bottom: GISP2 annually dated Holocene EOF1, a proxy for northern hemisphere polar cell intensity (PCI; described in Figure 6.8) smoothed with a robust spline (equivalent to a 100-year smooth)) with a quasi-2,600-year periodicity (O'Brien et al., 1996); GISP2 accumulation rate (Meese et al., 1994a,b); GRIP methane (Blunier et al., 1995); GRIP δ18O record (Dansgaard et al., 1993); worldwide glacial expansions and their relative magnitude (Denton and Karlen, 1973); synthesis of various climate proxy records from Europe, Greenland, North America, and the southern hemisphere showing cold periods (Harvey, 1980); the Cockburn Stade (Andrews and Ives, 1972; Alley et al., 1997); and the Younger Dryas event (Alley et al., 1993; Mayewski et al., 1993). Letters specify major cold periods with A equal to the Little Ice Age. Courtesy of the American Association for the Advancement of Science, Macmillan Magazines Ltd., the University of Oulu, Geological Society of America, and Academic Press.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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FIGURE 6.9 Summary figure illustrating the hypothesized insolation forcing and the lagged response of post-glacial southwest Indian monsoon intensification: (a) June and July insolation for 40°N, (b) RC27-23 surface isotopic record, (c) RC27-23 G. bulloides abundance (proxy for monsoon strength), and (d) histograms of abrupt monsoon-strength increases and decreases at sites across East Africa and southwestern Asia. Mid-Holocene decreases in monsoon strength appear to have been more time transgressive than the earlier abrupt increases and also more in phase with insolation forcing. The figure shows the timing of the first abrupt increase in monsoon strength (mean, 11,400 years), and the date at which the period of maximum monsoon strength apparently ended at each site (mean 5,500 years). Note that the period of maximum monsoon strength (at 11,000 to 5,000 years) lags peak insolation (at 14,000 to 8,000 years). SOURCE: Adapted from Overpeck et al. (1996). Courtesy of Springer-Verlag.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
×

over the deglaciation.42 The former study also identified a 3,000-year lag between monsoon intensity and insolation that lasted from about 9,500 to 5,500 years ago. By the end of this period, when northern hemisphere glacial boundary conditions had disappeared, monsoon behavior responded more linearly to insolation. Further significant centennial-scale decreases in monsoon intensity occurred prior to ~6,000 years ago, when monsoon strength was enhanced relative to the present. Since abrupt changes in Arabian Sea sediment monsoon records occurred when northern hemisphere summers were significantly warmer than present,43 some researchers have speculated that future greenhouse-warmed summers 44 may offer “surprises” in monsoon behavior.

Major reorganizations in Holocene climate plus finer-scale climate fluctuations such as abrupt shifts in drought and flood frequency may be explained by a combination of climate forcings.45 For the Holocene such forcings may include (1) changes in thermohaline circulation; (2) changes in insolation, notably precession that may generate long-period El Niño-type reorganizations in moisture and temperature and changes in marine and land ice cover; (3) changes in solar output; and (4) changes in the concentrations of volcanic aerosols and dusts.46 A variety of paleorecords are available to test the impact of these forcing mechanisms, including, for example, potential proxies for solar variability derived from d14C series in tree rings and 10Be series from ice cores, CO2 from ice cores, CH4 from ice cores, and volcanic sulfate from ice cores.47

The Late Holocene (~2,000 years ago to present)
Summary of Previous Work

Although the exact timing and geographic distribution of Holocene climate change events are complex, the past 1,000 to 2,000 years offer important opportunities for unraveling the decadal- to centennial-scale and finer climate variability that influences modern climate. There is general agreement that glaciers around the world expanded during at least parts of the thirteenth through the nineteenth centuries, a period called the Little Ice Age (LIA), and that warming occurred for several centuries prior to this period,48 at least in some regions, during what is controversially called the Medieval Warm Period (MWP).

Similarities in decadal- to centennial-scale variability over the past 1,000 years are observed in a variety of paleoclimate records from, for example, China (e.g., temperature, drought, rain frequency, dust events), although differences in the timing of peak cooling differ by region.49 Furthermore, broad similarities exist between the Chinese records and records covering a wide geographic range.

New Developments

Although spatially and temporally incomplete at present, paleoclimate records provide unique environmental reconstructions for the most recent millennia. The

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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LIA appears to play an important role in understanding modern climate. Based on the GISP2 atmospheric circulation record (see Figure 6.8), the LIA had the most abrupt onset (AD 1400 to 1420) of any of the Holocene rapid climate change events.50 This extends findings from a 1,500-year-long ice core record in the Andes which suggests that entrance into and out of the LIA was abrupt.51

Previous research summarized by Lamb (1995) demonstrates changes in climate such as increased severity of winter storms and sea ice extent, plus accompanying changes in food harvests during the LIA and contrasting milder conditions during the MWP. Recently developed marine sediment records from the Sargasso Sea52 suggest that sea surface temperatures in the Bermuda Rise region were ~1 degree cooler than today ~400 years ago (during the LIA) and ~1,700 years ago, and ~1 degree warmer than today 1,000 years ago (during the MWP). On the basis of this work,53 it is suggested that part of the general climate warming of the past few decades54 could be natural. During the MWP extreme and persistent drought characterized such regions as California and Patagonia,55 implying potential “surprises” during warmer-than-present climates.

Composite time series for El Niño recurrence (see Figure 6.10) suggest that fewer such events occurred during the MWP than during colder intervals prior to and following this time.56 Studies conducted over only the past 500 years, which do not include the LIA/MWP transition, suggest that El Niño recurrence rate is stationary over the long term but that strong El Niño events are nonstationary over centennial scales.57

Analysis of records covering the past 500 years suggests the presence of persistent natural interdecadal and century-scale climate oscillations. A compilation of paleoclimate records representative of the past 400 years of circum-Arctic climate variability indicates that the highest temperatures over this period have occurred since 1840, demonstrating the role of natural climate variability and, as of 1920, the added climate influence of atmospheric trace gases.58 Multidecadal modes and step-function changes in precipitation, temperature, and wind regimes have been identified in a number of regions, ranging from the Intertropical Convergence Zone (ITCZ) to both northern and southern midlatitudes. Recent attempts to match decadal-scale climate change events from region to region do not, however, necessarily reveal synchronous behavior over the past few centuries.59

Although their relative importance is still debated, several mechanisms have been proposed for the natural changes in climate of the past millennium, including changes in solar output, an increase in volcanic aerosols during specific periods, an increase in long-term average atmospheric aerosol loading, variations in thermohaline circulation, and changes in greenhouse gases.60

The paleoclimate record offers the potential to deconvolve the region-to-region climate variability that characterizes the Holocene on millennial to decadal plus finer timescales. Although few such records are available at present, these records offer immense potential (see Box 6.1).

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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FIGURE 6.10 Composite time series for recurrence of El Niño events since AD 622. Linearly weighted 19-year running mean. Data from Quinn et al. (1987) and Quinn (1992). Segment C (Nile), 622 to 1525; segment B, 1525 to 1800; segment A, 1800 to 1984.SOURCE: Anderson (1992). Courtesy of Cambridge University Press.

Climate-Vegetation Interaction
Summary of Previous Work

Early climate studies of modern tropical deforestation61 have focused attention on the importance of climate-vegetation interactions in governing the surface energy and moisture fluxes and hence the importance of climate-vegetation interactions. More recently,62 it was proposed that boreal forests, in addition to tropical vegetation, have an important climatic effect. A climate model was used to demonstrate that expansion of the boreal forest, associated with greenhouse warming, would enhance this warming through albedo feedback because trees mask the high reflectance of the high-latitude snow cover. In particular, tree cover, even with underlying snow cover, promotes springtime warming. These contributions introduce the potential of climate-vegetation feedbacks to substantially modify the sensitivity of the Earth's climate to a large number of different forcing factors. Earth history enables an assessment of the importance of vegetation-climate feedbacks for a diverse number of forcing factors (carbon dioxide, solar variations, orbital variations, sea level, changes in continental geometry). But most importantly, the combination of knowledge about the nature of the forcing factors, the paleobotanical record of vegetation distribution, and the record of temperatures presents a unique opportunity to critically assess climate-biosphere sensitivity.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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BOX 6.1 Available Climate Records of the Past Millennium

Climate Patterns

Temperature and Drought. Tree ring studies offer the greatest potential for developing broad spatial arrays of relatively recent, well-dated proxies for temperature and drought. Filtered tree ring series from, for example, Tasmania clearly reveal interdecadal patterns in temperature (see Figure 6.11).63 In addition, 250- to 300-year reconstructions of spring/summer temperatures are available for western North America and western Europe,64 winter precipitation and annual temperature in western North America, 65 and drought in the conterminous United States.66

ENSO. There are proxy ENSO time series available from a variety of records, including, for example, historical records, tropical corals, tropical ice cores, and polar ice cores. 67 Of particular note are the newly emerging annually resolved coral records that through calibration with the instrumental record provide proxies for zonal winds and precipitation (see Figure 6.12).68

North Atlantic Oscillation (NAO). GISP2/GRIP d18O series have significant correlations with the NAO series.69

Sea Ice. Proxies for sea ice variability are available from Antarctic and Arctic ice cores.70

Climate Forcing

CO2. Detailed records of naturally and anthrepogenically induced changes in atmospheric CO2 from an Antarctic ice core (see Figure 6.13) reveal preindustrial carbon dioxide mixing ratios in the range of 275 to 284 ppm, over the time period of the LIA and MWP.71

Sulfate and Nitrate. High-resolution time series for sulfate and nitrate from a south Greenland ice core, covering the past two centuries, demonstrate the difference between natural pre-1900 levels of these two acidic species versus post-1900 values (see Figure 6.14).72 In the preanthropogenic atmosphere over Greenland, nitrate levels were approximately two times the sulfate levels. Increased levels of sulfate during the anthrepogenic era mask volcanic sulfate levels, indicating the large rise over natural background during this period. Volcanic events recorded in Greenland ice core sulfate series correlate with annual changes in atmospheric temperature, providing evidence for sulfate aerosol shielding.73

Solar Variability. d14C series from tree rings and 10Be series from a Greenland ice core provide potential proxies for solar variability.7410Be series vary inversely with sunspot activity, and 10Be production varies inversely with temperature (see Figure 6.15).75

New Developments

Recent applications of biome and simple vegetation-climate models coupled with GCMs and applied to the study of Earth history have resulted in a number of

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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FIGURE 6.11 The median aggregate waveform since AD 1800 with its approximate 95 percent confidence limits compared to the low-pass filtered series developed from Tasmanian tree ring series. The aggregate appears to explain much of the interdecadal warming in the reconstruction since AD 1965. SOURCE: Cook et al. (1996a, b). Courtesy of Springer-Verlag.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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FIGURE 6.12 Instrumental and coral records of environmental conditions at Tarawa Atoll. From top: zonal winds, coral Mn/Ca (Shen et al., 1992), the Southern Oscillation Index (SOI; Wright, 1989); composite coral δ18O (plotted reverse; Cole and Fairbanks, 1990) and monthly rainfall (from Monthly Climatic Data for World and New Zealand Meteorological Service). Shaded periods reflect ENSO warm-phase conditions as noted by Quinn et al. (1978, 1987), which involve the weakening and reversal of the trade winds and dramatically increased rainfall at this site. SOURCE: Cole et al. (1992). Courtesy of Cambridge University Press.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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FIGURE 6.13 Top: CO2 mixing ratios from the DEO8, DEO8-2, and DSS ice cores, Law Dome, East Antarctica, and modern South Pole atmospheric record (Keeling, 1991). Bottom: CO2 mixing ratios since early last century from DEO8 and DEO8-2 Law Dome ice cores and the South Pole record (Keeling, 1991). The thick line is a smoothing spline fit to the DEO8 and DEO8-2 data. The degree of smoothing has been set such that an attenuation of 50 percent occurs for CO2 variations of 20 years' duration. Such smoothing was found to best attenuate the shorter-frequency variations that, because of the averaging effect of the air enclosure process, are unlikely to be real atmospheric features and are assumed to be measurement errors. SOURCE: Etheridge et al. (1996). Courtesy of the American Geophysical Union.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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FIGURE 6.14 Time series of the nonseasalt sulfate (nss; dashed line) and nitrate (solid line) concentrations at ice core site 20D, southern Greenland. Data have been smoothed (using a gaussian function) to periods of one to two years from the original five to eight samples per year to remove seasonal signals. Examples of volcanic events recorded in nss sulfate spikes are (1) Laki (1783), (2) Tambora (1815), and (3) Katmai (1912). SOURCE: Adapted from Mayewski et al. (1990). Courtesy of Macmillan Magazines Ltd.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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FIGURE 6.15 Top: 10Be measurements derived from the Dye 3 ice core vary inversely with sunspot activity. Bottom: Observed inverse relationship of 10Be production and temperature. SOURCE: Beer et al. (1994). Courtesy of Cambridge University Press.

remarkable conclusions. Foley et al. (1994) have shown that mid-Holocene warming associated with changes in the Earth's orbit was enhanced, particularly in springtime, by an associated poleward expansion of the boreal forests. Gallimore and Kutzbach (1996) have suggested that the onset of the last glaciation was accentuated by the observed equatorward retreat of the boreal forests associated with the cooling. Deriving sufficient climate model sensitivity from changes in the Earth's orbit to produce the observed glacial-interglacial cycles has been a long-standing problem. Evidently, climate-vegetation feedbacks are a

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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likely candidate to solve this problem, at least partially. In a similar vein, Dutton and Barron (1996, 1997) have suggested that the development of widespread grasses in the Miocene and the retreat of the boreal forests observed to be associated with the development of widespread ice caps could have substantially enhanced the Late Cenozoic cooling. They suggest that both biological innovation (development of a new widespread flora) and climate-vegetation feedbacks (e.g., climate-boreal forest-albedo relationship) may play an important role in governing climate.

More than 20 parameterizations for the land surface are now included in the World Climate Research Programme's Project for Intercomparison of Land Surface Parameterization Schemes.76 Intercomparison of these parameterizations provides a strong foundation for understanding the role of vegetation in governing moisture and energy fluxes. With this foundation we can expect a much greater understanding of the importance of vegetation character and distribution in governing climate.

A primary objective for future research must be to include dynamic vegetation, which evolves with climate change and allows explicit incorporation of climate-vegetation feedbacks. A number of different vegetation models are available now or are under development. Biome models, in which vegetation functional form and species distribution are governed by key climatic variables, provide the foundation for other models.77 The life-form model (EVE—Equilibrium Vegetation Ecology model)78 provides an alternative, in which the net phenology and physical attributes of the vegetation community are determined for each climate model grid point based on the climatic relationships of 110 life forms. Again, pathfinder studies guide research on the application of these models to Earth history. A prescribed change in climate to assess vegetation change has been applied using the Prentice biome model.79 A “dynamic” Holdridge-type vegetation scheme80 has been used to assess climate-vegetation change for a transient doubled carbon dioxide experiment. Again, each of these pioneering studies lacks a capability to verify the results based on climates very different from the present day. In contrast, Earth history presents a considerable record of vegetation character and distribution. Extensive published summaries of the vegetation distribution and history for the Last Glacial Maximum have been provided.81 Global Tertiary (past 65 million years) vegetation distribution maps and recent updates are available.82 Extensive references are given for the Eocene climate,83 and both phytogeographic maps and extensive references for the mid-Cretaceous are available.84

However, actual applications of climate-vegetation models for past climates have been limited. The ability of three different biome models to predict present-day vegetation distribution85 has been assessed using the GENESIS86 GCM; these biome models were then applied to the simulation of the Younger Dryas and the Eocene. In these cases the vegetation was not coupled to the GCM but rather was calculated “off-line” after the GCM produced a climate simulation.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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Hence, these models predict a change in vegetation that can be compared to observations but do not include vegetation-climate interaction. EVE was utilized to perform a dynamically coupled experiment to incorporate the role of a changing vegetation in assessing climate sensitivity. 87 Results for the Late Cretaceous suggest that vegetation-climate feedbacks could be a contributing explanation of Cretaceous polar warmth.

The application of ecological models, coupled with climate models, is obviously in its infancy. However, efforts to date have several implications. First, climate-vegetation interaction has the potential to substantially alter the sensitivity of climate models. Second, an intercomparison of several different landatmosphere interaction parameterizations and the spectrum of ecological models is essential. Third, vegetation characteristics and distributions from Earth history have considerable potential as a means of assessing the importance of climate-vegetation feedbacks and in validating coupled climate-vegetation simulations. These data are a unique opportunity to investigate a critical aspect of global change.

Climate-vegetation interactions extend beyond the effects associated with the expansion and contraction of specific biomes due to warming or cooling. For example, carbon dioxide concentrations are evidently correlated with stomatal parameters (frequency and size) on the leaves of land plants.88 It has also been suggested that C4 plants are adapted to conditions of water and CO2 stress and that their widespread expansion 5 million to 7 million years ago was related to lower carbon dioxide levels.89 These vegetation responses are related specifically to the nature of the climate forcing and introduce the potential of utilizing the floral record with proxies of past atmospheric carbon dioxide levels to examine plant responses.

Warm Climates
Summary of Previous Work

Earth history is characterized by time periods in which the climate was significantly cooler and significantly warmer than the present day. Much of the past 150 million years has been substantially warmer than the present day, although a long-term climatic cooling has dominated over the past 60 million years. The extreme warm climates of the past provide an unique set of case studies of global change. They challenge our ability to describe, model, and understand how various elements of the Earth's climate operate and interact. These time periods also provide a critical test of the models used to predict the global climate response to increases in atmospheric greenhouse gases. Although a wide variety of climatic forcing factors have operated over Earth history, variations in atmospheric carbon dioxide concentrations are now linked to the record of climate change on almost every timescale, from abrupt events, to glacial to interglacial

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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cycles, to the Cenozoic global cooling trend, to the major glaciations, and to the climate of the Archean (early Earth). Substantial evidence is available that links many of the extreme warm periods of the past 100 million years to higher carbon dioxide levels.90 The most recent occurrences of these climatic extremes tend to be the best documented, with sufficient information to provide major challenges to climate models.

There is abundant evidence for global warmth during the mid-Cretaceous (about 100 millions years ago). There is no convincing evidence for permanent polar ice during this time. Arctic Ocean cores recover abundant phytoplankton,91 suggesting at least a seasonally ice-free Arctic. More importantly, more than 400 species of land plants have been recorded from land well within the Arctic Circle.92 Vegetation analysis93 indicates mean annual temperatures from the North Slope of Alaska to be 10 to 13 degrees. Cretaceous deep-water temperatures, derived from the oxygen isotopic composition of benthic foraminifera, are as warm as 15 to 17 degrees. Evidence from the Cretaceous tropics is more problematic, but evidently temperatures were similar to or somewhat warmer than the present day. Equator-to-pole surface temperature gradients were therefore substantially lower than at present, and the Cretaceous Earth was substantially warmer, perhaps at all latitudes. Continental geometries were substantially different from the present day, and carbon dioxide levels were two to eight times present-day values.94

The Eocene (approximately 40 million to 50 million years ago) was also substantially warmer than at present. Oxygen isotopic measurements suggest that the high-latitude oceans were quite warm (perhaps as much as 10 degees warmer than at present at 60 degees N), but the tropics were as much as 5 degrees cooler.95 The interpretation of polar warmth is supported by abundant floral and faunal data.96 The remains of ectotherms, like alligators, above the Arctic Circle have been used to suggest frostless winters in coastal regions above 70 degrees N. In addition, floral data from the Eocene continental interior, such as the presence of palms at the latitude of present-day Chicago, indicate very mild winters even in continental interiors. 97The nature of the Eocene climate is also deduced from measurements of the size of aeolian dust transported through the atmosphere and deposited in the deep sea. Eocene grain sizes are substantially smaller, on a global basis, than earlier records or modern records. The Eocene record is also one of a substantially smaller equator-to-pole gradient, but in this case the poles are substantially warmer and the tropics cooler. This suggests a very different climate mode from the warm Cretaceous. Temperature gradients similar to the Eocene pattern are characteristic of much of the Cenozoic preceding the development of permanent ice caps on Antarctica. According to one report,98 Eocene carbon dioxide levels may have been moderately higher than at present.

The Cretaceous and Cenozoic warm climates present a substantial challenge to climate models used to predict future climate change. They are the basis for exploring a number of different forcing factors and for determining the sensitivity

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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of the climate system to these factors. Each provides an interesting and valuable opportunity to validate climate models and to assess the nature of climate change and climate sensitivity. If major discrepancies exist between model predictions and the geological record, this analysis will focus attention on areas of needed research.

New Developments

Past warm climates have become a focal point for climate model study. Two results from these studies are particularly interesting. First, modern atmospheric GCMs appear to be incapable of achieving the equator-to-pole temperature gradients for the Eocene or the Cretaceous, although with higher carbon dioxide concentrations they can achieve reasonable globally averaged surface temperatures. Second, the record suggests that the mid- to upper range of Intergovernmental Panel on Climate Change (IPCC) estimates of climate sensitivity to increases in carbon dioxide better fits the geological data.

The first challenge introduced by the record of past warm climates is the reduced equator-to-pole surface temperature gradients. A universal response of atmospheric GCMs to increases in carbon dioxide concentrations is a small increase in tropical temperatures and a greater sensitivity at higher latitudes. To achieve the polar warmth of the Cretaceous (e.g., by increasing atmospheric carbon dioxide levels and incorporating Cretaceous continental geometries), GCMs produce an overheated tropics. 99 The nature of the problem is even greater for the Eocene, as tropical temperatures are lower compared to the present day while high-latitude regions are considerably warmer.100 If the sea surface temperature data are correctly interpreted, the solution must lie in a redistribution of energy. The choices are limited. Either the ocean-atmosphere system is capable of more efficient transport of energy poleward, cloud amounts and character change systematically with latitude in warmer climates, or the energy input to the Earth system is dramatically different. The latter point appears to be excluded based on celestial mechanics. A cloud hypothesis can be made to work but is difficult to justify. The more efficient energy transport idea also can be made to work but presents an enigma.

If the mechanisms of poleward heat transport are examined, there are few opportunities to promote increases in the efficiency of transport during conditions like that of the Eocene. With cooler tropics and warmer poles, both atmospheric sensible and latent heat transport should decline. The aeolian record described above, in fact, is evidence for a weaker atmospheric circulation. If the atmospheric circulation is weaker, the wind-driven ocean surface circulation should be weaker. The only apparent opportunity to increase poleward heat transport is through the deep thermohaline circulation of the ocean. The role of ocean heat transport changes in explaining climate change has become a topic of considerable interest.101 One possibility is a reversal of the thermohaline circula-

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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tion and the production of warm highly saline deep water in the evaporative subtropics as the dominant bottom water source in times of warm polar temperatures. The implications of such a reversal in circulation on the characteristics of the ocean are considerable. A number of studies102suggest that modest increases in ocean heat transport—on the order of one or two times the amount of heat transported by North Atlantic deep water—would be sufficient to explain much of the paleoclimatic data.

The conclusions from such studies are of considerable interest. These studies introduce the possibility that the role of the ocean under climatic conditions very different from the present day may be very different from the modern ocean. GCMs without coupled oceans may be unable to simulate climatic sensitivity or the distribution of temperatures over the surface to the Earth. Furthermore, the challenge is to incorporate an adequate thermohaline circulation into coupled ocean GCMs. What is clear is that modern atmospheric GCMs are incapable of reproducing the temperature gradients of much of the warm climate periods of the past 100 million years.

The geological record also can be utilized to calibrate model sensitivity to increases in carbon dioxide concentrations in the atmosphere. 103 For example, one study uses global, mean annual, equator-to-pole temperature gradients, ranges in atmospheric carbon dioxide, and assumptions about the nature of other forcing factors to suggest that the midrange of the IPCC estimates (1.5 to 4.5 degees) for a doubling of carbon dioxide is appropriate for much of the Earth' s history.104 It was further noted that the uncertainties in some of these factors probably mean that the paleoestimates of climate sensitivity to increases in atmospheric carbon dioxide have about the same error range as the IPCC estimates.105 In more detailed studies using GCM results106 the actual distribution of surface temperatures was the basis for comparing model simulation results for differing levels of carbon dioxide. The GCM that was used yielded a 2.75 degrees warming for a doubling of atmospheric carbon dioxide. With this model the upper level of carbon dioxide estimates from the Cretaceous based on a variety of geochemical estimates107 was required to achieve Cretaceous warmth. This implies that either unknown forcing factors were operating during the period or that the middle to upper range of IPCC estimates of climate sensitivity is required to explain past climates.

Caution must be applied in using the Earth's history to assess climate sensitivity. There are simply too many potential errors, in the interpretations of the observations, in knowledge of the forcing factors involved, and in the veracity and capability of the climate models used. It is critical to focus on better proxy information at all timescales. However, it is interesting to note that in the more than 100 simulations of past climates with GCMs we are yet to see a simulation that did not underestimate the record from past climates, unless specific assumptions were made in order to achieve past climatic conditions. This statement applies to both past cold and past warm extremes. This aspect of paleoclimate

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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simulations has been the basis for suggesting that past climates imply a climate sensitivity that is within or exceeds IPCC estimates.

A focus on past extremes in climate obviously presents a challenge to the climate models used to assess future climates. The record of past warm climates appears to be impossible to assess adequately without taking into account the potential role of the ocean in global change. Furthermore, these time periods are suggestive of a very different role for the oceans during warm climates. In fact, it may be inadequate to have a coupled ocean-atmosphere model if the ocean model does not have a credible thermohaline circulation. Importantly, this is just one example of a potential inadequacy of current climate models as illustrated by paleoclimate investigations. Other factors and model systems also may be important, such as incorporation of dynamic vegetation. Past climates also present an opportunity to assess the nature of climate sensitivity. This type of application is imperfect but appears to suggest that past climates will be very difficult to simulate if climate sensitivity to carbon dioxide increases is in the lower range of IPCC estimates.

KEY SCIENTIFIC QUESTIONS AND ISSUES

The key scientific questions facing paleoclimate researchers have been articulated in a series of international projects, including PAGES (Past Global Changes) of the International Geosphere-Biosphere Programme, CLIVAR (Climate Variability) of the World Climate Research Program, and GLOCHANT (Global Changes in the Antarctic) of the Scientific Committee on Antarctic Research. Through the integration of ice, ocean, and terrestrial paleorecords, these international efforts seek to develop a basis for understanding the characteristics of natural global environmental change, notably climate change. These paleodata are essential for assessing human influence on the global environment and for evaluating predictive climate models.

Several questions and issues have evolved as foci for the paleocommunity. These consensus views have been expressed in several documents108 which form the basis for the specific scientific questions that follow.b

Focus on the Past 2,000 Years

Paleoclimate records demonstrate that Holocene climate has been by comparison with glacial climates both warm and relatively stable. Furthermore, it is within the confines of Holocene climate that modern civilization has emerged and prospered, which is suggestive of some relatively benign climate influence.

b

Following internationally accepted PAGES guidelines, these issues are divided into two streams—the past 2,000 years and the past 250,000 years.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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While correct in the context of glacial/interglacial cycles, annually resolved ice core records from central Greenland demonstrate increased complexity in climate through the Holocene—as, for example, cold climate feedbacks produced by the presence of ice sheets dissipated during the early Holocene. Moreover, detailed examination of these and other paleoclimate records suggests that subdued versions of glacial-age rapid climate change events regularly punctuated the Holocene and that major atmospheric phenomena such as ENSO and monsoons varied markedly in frequency and magnitude throughout the Holocene. In fact, closer examination of Holocene climate records reveals more variability than that typically observed in the instrumental records covering the past century. Paleoclimate reconstructions, while not generally as accurate as the instrumental record, can uncover information from times and regions not covered by the instrumental record, thus supplementing it.

The past 2,000 years of Holocene climate offer examples of climates both warmer than modern (the MWP) and colder (LIA). While these climate events do not serve as strict analogs for future warmer or colder climates because they predate the industrial era, they do offer important “climate opportunities.” Furthermore, despite the fact that instrumental records are most commonly less than a century in length, abundant and relatively untapped records in the form of historical journals and natural archives (e.g., tree rings, ice cores, corals) are relatively abundant for this period. Embedded in these records is evidence of both climate response (e.g., changes in temperature, precipitation, circulation strength) and climate forcing (e.g., solar variability, biogenic emissions, volcanism).

The most recent major climate event of the 0Holocene—the Little Ice Age—was a period of regionally lowered temperatures, increased atmospheric circulation intensity, and significant decadal-scale variability. While Holocene climatic shifts larger than those of the LIA are recorded in several proxy records, the LIA appears to have started more abruptly (within several decades) than other Holocene climate change events. The significance of this climatic “surprise” is not fully understood.

Despite general agreement that temperatures characteristic of the LIA have not characterized the twentieth century, there is considerable debate over the timing and geographic extent of this event and little discussion of the other climate parameters (e.g., precipitation, atmospheric circulation intensity) that characterized it. While climate forcings such as solar variability and volcanism explain some degree of LIA climate, more complex multiple forcings and nonlinear responses to these forcings must be investigated. Therefore, while the LIA is the most recent example of a naturally forced cooler climate, our knowledge of this event is not sufficient to understand its significance or explain its causes. Less is known about the MWP. Understanding of modern climate is, therefore, hampered not only by questions concerning the influence of humanly induced forcing through emissions of radiatively important trace gases and aerosols but as

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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fundamentally through a lack of understanding of the natural climate variability that underpins modern climate.

While significant progress has been made in the short-term prediction of important atmospheric phenomena such as ENSO, relatively little is understood about potential changes in the frequency and magnitude of this and other important climate systems (e.g., monsoons, North Atlantic hurricanes) on decadal and greater scales, despite evidence that such events did vary in frequency and magnitude over such periods as the LIA.

Holocene climate and environmental response have been sufficient to create significant stresses for emerging civilizations. Human impact on the chemistry of the atmosphere and on the land-ocean environment and climate has also been extremely significant. However, the complex interplay of human activity and environmental and climatic change still holds many unanswered questions (see Box 6.2).

Focus on the Past 250,000 Years

The interglacial climate we currently enjoy is known from paleoclimate records that cover the past 1 million years to be relatively rare. These records also demonstrate that interglacial climates appear coincident with the relatively rapid dissipation (several hundred to thousands of years) of glacial-age ice sheets and end more gradually as ice sheets re-encroach. Pioneering studies link the cadence of these glacial/interglacial cycles to insolation changes produced by changes in the Earth's orbit (Milankovitch cycles), yet not all details of event phasing or event frequency can be explained by these theoretically calculated insolation changes.

Ice core records from Antarctica have demonstrated the close linkage between temperature and the greenhouse gases CO2 and CH4. However, issues of phasing, particularly for CO2, remain less well understood. Furthermore, despite their importance in climate forcing, changes in the concentration of greenhouse gases cannot fully explain documented changes in temperature.

Rapid climate change events recently developed from the central Greenland ice cores and since found in marine and terrestrial sediments punctuate the slower pattern of ice sheet growth and decay. From the highly resolved and well-dated Greenland ice cores comes evidence that many of these rapid climate change events occur in relatively predictable cycles of ~6,000 years (Heinrich events, first revealed from marine sediments as massive discharges of icebergs) or as massive atmosphere-ocean reorganizations that occur with ~1,500-year frequency (Dansgaard/Oeschger cycles).

Although globally distributed and dramatic in magnitude and timing, important characteristics of the rapid climate change events are still not clearly understood. These events were documented through the investigation of well-dated continuous records; to understand the phasing of these events from region to

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
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BOX 6.2 Key Scientific Questions: Stream One—The Past 2,000 Years

  • Is the warming experienced during the twentieth century unusual?

  • Are there major modes of subdecadal-, decadal-, and centennial-scale variability?

  • Do certain regions of the Earth play leading roles in global climate change by either driving or responding to climate change (e.g., North Atlantic, Southern Ocean, tropics)?

  • Are climate change events (e.g., LIA, MWP) synchronous in magnitude and timing in both hemispheres or do some display regional differences?

  • Have major atmospheric circulation systems such as ENSO, Asian-Australian-African monsoon, and westerlies shifted over time? Is there a recognizable pattern to these changes? Why have these changes occurred? Are there regularities, synchronisms, or teleconnections that can be used for developing predictive model capability? Have these changes been in response to changes in other components of the climate system (e.g., sea surface temperature, snow cover, cloud cover)?

  • How are atmospheric teleconnections manifested at regional to global scales and over decadal to century timescales?

  • How has the hydrology of the planet changed over the past two millennia?

  • How has the record of ENSO and its climate teleconnections changed over the past two millennia?

  • How has the record of explosive volcanism changed over the past two millennia? How does this record relate to climate change?

  • How do natural feedbacks (dusts, biogenic trace gases) operate and affect the climate system?

  • How has solar irradiance varied over the past two millennia? What are the mechanisms by which changes in solar irradiance cause climate change? How has the environment responded to these variations?

  • How has sea level changed over the past two millennia? How do ice sheets, mountain glaciers, and other changes in the hydrological cycle contribute to this change?

  • How have ecosystems (e.g., equatorial rain forests, tundra, forest, steppe, glacial) responded to environmental change over the past two millennia?

  • How has human activity impacted the environment?

  • How have humans responded to environmental change?

region more such records will be required. Phasing information will be crucial in determining the cause of these events.

Changes in thermohaline circulation of the ocean are believed to play a major role in the production of rapid climate change events, but the causes of such changes are not well understood. Multiple nonlinear forcings that invoke internal ocean oscillations, changes in ocean-continent boundary conditions, solar variability, changes in the concentration of greenhouse gases and biogenic

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
×

BOX 6.3 Key Scientific Questions: Stream Two—The Past 250,000 Years

  • Are there major modes of centennial- to millennial-scale variability?

  • What is the phasing of climate evolution between the two hemispheres?

  • How have changes in Milankovitch insolation cycles, thermohaline circulation, trace gases, aerosols, and solar variability affected climate evolution in the two hemispheres?

  • Are the rapid climate change events recorded in the Greenland ice sheet found in the southern hemisphere? Are these events also found in marine and terrestrial records? Are these events synchronous in timing and magnitude?

  • How have boundary conditions changed in specific regions (e.g., south Asian “Warm pool,” Tibetan Plateau) and have these changes caused responses in major atmospheric circulation systems such as ENSO, Asian-Australian monsoon, ITCZ, and jet streams?

  • How are hydrological changes in the tropics related to climate change in extratropical regions? What causes these changes?

  • How are changes in biomass productivity linked to changes in trace gases in the atmosphere?

  • How has monsoonal circulation varied in the past and are these changes synchronous in different regions?

emissions, changes in the hydrological cycle, volcanism, and ice sheet dynamics superimposed on insolation cycles must be investigated through both more well-resolved and well-dated paleoclimate records and modeling efforts to understand rapid climate change events.

Glacial/interglacial cycles and rapid climate change events provide dramatic examples of the dynamic range of natural climate variability. As such these climate events offer excellent opportunities for examining global to regional-scale changes in atmospheric circulation patterns (e.g., ENSO, Asian-Australian monsoon, ITCZ, jet streams) (see Box 6.3).

LESSONS LEARNED

The task of understanding climate change and predicting future climate change is particularly complex. However, the paleoclimate record provides a series of lessons upon which our understanding can be constructed and tested. An understanding of climate change and an understanding of the consequences of this change are possible if the lessons learned from the paleoclimate record are considered.

The first lesson is that, while there are no true analogs for modern or future climate in the paleorecord, the modern instrumental record is too short and too

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
×

undersampled to understand and predict the full range of climate change. There are, in fact, parts of the Earth (e.g., Antarctica, high-altitude sites) for which no multidecadal instrumental series are available. Yet these sites are often ideal for the recovery of paleodata (e.g., ice cores, lacustrine sediments, tree rings). The most robust paleodata are those that can be calibrated with instrumental records. A classic example is the perfect fit between modern atmospheric CO2 and the CO2trapped in ice cores.109 While paleodata may come from a variety of mediums and reveal a wide range of proxies to direct environmental indicators, such data are all we have short of a time machine. Unfortunately, the paleorecord is also severely undersampled in time and space.

The second lesson is that only from the perspective of paleoclimate records can patterns, trends, thresholds, and ranges of natural climate variability (both unforced and naturally forced) be assessed. Recognition of potential “surprises” in natural climate variability, such as rapid climate change events, ENSO, and drought recurrence, require long backward glances to assure proper perspective. As a corollary to this it is clear that the paleoclimate record is essential as a basis for quantifying signal to noise ratios in shorter observational records. It will not be possible to assess interannual- to decadal-scale signal to noise in expensive satellite-based measurements until well into the twenty-first century, at which time too much will have been invested to make a mistake. It will be impossible to assess decadal- to centennial-scale variability without data collections that extend well into the twenty-second century.

The third lesson is that regular patterns of climate variability can be identified on the decadal-to-millennial scales. This finding is particularly encouraging since one of the end goals of climate change research is predictability. It is also apparent that there are significant temporal and spatial differences in climate variability. Deconvolving predictable patterns at the regional scale and determining the temporal baseline from which predictably can be assessed will require a considerably more dense spacing of paleodata.

The fourth lesson is that it is not possible to utilize modern instrumental records, which characteristically cover a century or less, to understand natural climate variability because these data series are too short and record mixed responses to natural and anthropogenic forcing of climate. However, natural climate variability is a major component of modern and future climate. Only paleodata covering periods prior to the past century can be considered to reflect natural responses to natural forcing. The paleorecord provides a unique testbed for assessing relative differences in the significance of natural climate forcings (e.g., solar variability, greenhouse gases, sulfate aerosols) over time and space. Only the paleoview provides the recognition and understanding that climate change is the cumulative effect of causal mechanisms that operate over short to long timescales. Furthermore, it is not likely that absolutely unambiguous determinations of the natural background state for atmospheric chemistry can be determined without examining paleodata.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
×

The fifth lesson is that the value of high-resolution (subannual to annual), annually resolved, continuous, and multivariate paleodata provide the best opportunity for step function advances. A new standard for this type of record has been set by the Greenland ice cores, but too few such datasets exist. These records provide a new framework for examining shorter, less well resolved, discontinuous paleodata. Not all paleorecords allow annually resolved dating. Some depend on other dating techniques that include, for example, modeling, 14C dating, and stratigraphic markers. Extensive efforts will be required to remove limitations in these techniques. For example, 14C reservoir effects in oceans and lakes are not well understood. The value of less well resolved, even discontinuous, records should not, however, be underestimated as a tool for exploration and as a stimulus for new ideas. A classic example comes from a 1987 examination of beetle remains in Britain, which demonstrated the potential for rapid onset and decay of the Younger Dryas event.110

The sixth lesson is that, while spatially dense networks of paleodata are preferable in some cases, single sites can provide significant results if the records are well dated, continuous, of high resolution, and multivariate. Such records may prove to be of immense value because they are close to major environmental transitions or because they allow measurement of variables that reflect local- to regional- to global-scale change depending on the measurements.

The seventh lesson is that paleodata provide a wide range of case studies for assessing human, animal, and plant responses to a variety of environmental changes, such as shifts in atmospheric circulation, temperature, and moisture availability.

RESEARCH IMPERATIVES

During the past decade paleoclimate research has made dramatic advances that built on a strong background of previous research. New records have emerged that reveal rapid changes in climate that operate on scales and at magnitudes that are of crucial relevance to human society. The paleoclimate record has revealed the complexity of both climate response and climate forcing and variations of both in time and space. While complex, the identification of recurring patterns in some climate records offers promise for future understanding and prediction. Furthermore, considerable advances have been made in our understanding of environmental response to climate. Paleoclimate studies literally stand on a threshold of understanding that will continue to expand if fueled by future research. The research imperatives are as follows:

  • Document how the global climate and the Earth's environment have changed in the past and determine the factors that caused these changes. Explore how this knowledge can be applied to understand future climate and environmental change.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
×
  • Document how the activities of humans have affected the global environment and climate and determine how these effects can be differentiated from natural variability. Describe what constitutes the natural environment prior to human intervention.

  • Explore the question of what the natural limits are of the global environment and determine how changes in the boundary conditions for this natural environment are manifested.

  • Document the important forcing factors that are and will control climate change on societal timescales (season to century). Determine what the causes were of the rapid climate change events and rapid transitions in climate state.

Paleoclimate records come from a variety of archives (e.g., tree rings, ice cores, marine and lake sediments, corals, historical documents) and are available at a variety of resolutions and age ranges. Paleoclimate records also provide different types of information ranging from proxy to direct. Furthermore, they contain evidence of both environmental response to change and the potential causes of change. A broad range of types of records will be required to understand climate change. No one type of record will suffice, since no single record type can provide the temporal, spatial, proxy, and direct measurements required to develop the global array required to understand past changes in climate.

Our most detailed view of climate comes from the past few decades of instrumented observations. To take the fullest advantage of paleoclimate records, calibration between paleoclimate and instrumented records is essential. Landmark examples already exist, notably the calibration between CO2 measurements in ice cores and CO2 observations in the atmosphere. Preliminary comparisons between coral, tree ring, and ice core paleoclimate series and instrumental series of, for example, ENSO and the NAO are extremely encouraging.

Paleoclimate studies calibrated to instrumental series offer the “missing years” prior to the introduction of Earth-observing satellites and instrumented observations. These missing years cannot be produced by any other known methodology. Furthermore, through the hindsight offered by paleoclimate records, observing programs will be able to more accurately assess natural climate variability, patterns, trends, and signal to noise. Finally, paleoclimate records offer the climate modeling community a bold opportunity for testing climate response and forcing on a variety of timescales and resolutions heretofore untouched.

Nine guiding principles for future research are as follows:

  1. Development of a global array of highly resolved, continuous, precisely dated, multivariate paleoclimate records that sample the atmosphere, ocean, cryosphere, and land. For the identification of environmental change over the past two millennia, annual to decadal resolution will be required, and for longer timescales decadal- to centennial-scale resolu-

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
×

tion will be needed. Continuously sampled and precisely dated records are essential. Tree ring and more recently ice core studies have set a standard of annually resolved dating that should be adhered to wherever possible. Multiproxy paleorecords should be developed wherever possible to maximize interpretations. The primary purpose of this global array should be to specify change over critical regions and during critical periods. For example, a major focus should include investigation of the frequency and extent of rapid climate change events (millennial to ENSO range) and identification of the controls on such events.

  1. Long paleodata series (centennial- to millennial-scale) should be complemented by spatial arrays of shorter records (decades) to enhance record interpretations and allow differentiation of local versus regional and wider environmental signals.

  2. Integration and detailed calibration of paleodata and observational series will be essential. This approach will allow hindcasting of the relatively short timescale observational series. With coupled instrumental/paleodata series, it will be possible to specify the frequency and magnitude of variability for major atmospheric circulation systems (e.g., ENSO, NAO, North Pacific Oscillation) and extreme events (e.g., droughts, floods).

  3. Paleoclimate and observational series should be coupled with process studies to specify controls on climate behavior. Ground-based and remote observing systems afford a unique tool for studying processes. Such studies should be closely integrated with existing observational sites and regions from which valuable paleodata series may be collected. Future ground-based stations and satellite observations should be planned with paleodata in mind.

  4. A clear demonstration of the “natural state of environmental variability” is needed as a baseline for assessing the influence of anthropogenic forcing on climate change. This will require the collection and interpretation of a broad array of paleodata series that capture variability in the physical, chemical, and biological boundary conditions down to regional and in some cases locally specific areas over several timescales.

  5. Time-dependent modeling of climate behavior is required that is explicitly designed to test paleoclimate event histories, address boundary conditions present during periods of purely natural climate forcing and boundary conditions for the modern climate era where complex interactions of an anthropogenically forced climate are superimposed on a system of naturally varying climate.

  6. Improved dating techniques are required for paleodata. While annually resolved records are available from tree rings and some ice cores, most paleoseries are dated by models, spot relative and absolute dating, and spot marker horizons. Great advances have been made in 14C dating of small-volume materials and the identification of unique events such as globally distributed volcanic events, but more work is needed.

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
×
  1. More unique tracers are needed to reconstruct ocean and air mass trajectories. Unique chemical signatures have proven to be valuable tracers for atmospheric circulation systems, but more work is needed.

  2. Free and open exchange of paleodata and other data (e.g., instrumental, satellite, ground-based) is essential if continued progress is to be made in global environmental change.

Over the past few decades science has demonstrated the impact of human activity on the environment and has realized the importance of natural variability in climate. As a consequence of both natural forces and human activities, climate change and environmental change are now known to be inevitable. What is not known are the rates, frequency, and magnitude of these changes and the exact controls on them.

As inextricably as our understanding of the Earth is dependent on viewing it as a complex system from space, thus assuring a view of the whole system, so too is it inextricably true that this understanding must include the evolution over years to millennia of the Earth system as a major goal. Paleoclimate records offer the temporal perspective of years to millennia. Within the paleoclimate record are the answers to principal scientific questions. Does climate evolve or is it chaotic? Is climate predictable and at what resolutions?

Many major advances in our understanding of climate would not have come without paleoclimate research. Although paleoclimate reconstructions are not absolute truth, when based on high-quality records they are the best information available for understanding past climate changes. Paleoclimate reconstructions provide fertile ground for modeling, and those involving multiple records that have broad geographic coverage and multiple-proxy capability should be encouraged as input to models. The results of these model experiments undertaken in an iterative mode and incorporated into paleoclimate reconstructions hold great promise for increasing our understanding of climate change.

Based on paleoclimate research we now realize the following:

  • The Earth's orbital cycles of insolation provide the cadence for glacial/ interglacial cycles.

  • Greenhouse gas concentrations and biogenic emissions are closely linked to temperature.

  • Rapid climate change events that operate at magnitudes and rates significant to humans have operated throughout at least the past glacial/interglacial cycle.

  • Human influences on the chemistry of the atmosphere (e.g., trace gases, aerosols, trace metals) exceed rates and magnitudes measured over the past few million years.

  • Climate change is produced by single forcings (e.g., volcanism) nd multiple forcings that can act nonlinearly to produce climate “surprises. ”

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
×

NOTES

1. CLIMAP Project Members (1976, 1981), COHMAP Members (1988), Hays et al. (1976), Imbrie et al. (1984, 1989).

2. E.g., Hays et al. (1969), Imbrie et al. (1973).

3. Jouzel et al. (1987), Barnola et al. (1987).

4. Taylor et al. (1993), Grootes et al. (1993).

5. Annual layer counting, Alley et al. (1993), Meese et al. (1994a); ice core, Bender et al. (1994).

6. Alley et al. (1993), Meese et al. (1994a, b), Sowers et al. (1993).

7. Cuffey et al. (1995).

8. E.g., Birchfield and Weertman (1983), LeTreut and Ghil (1983), Budd and Smith (1987).

9. Snow accumulation, Alley et al. (1993); magnitude changes, Mayewski et al. (1993); methane, Brook et al. (1996).

10. Alley et al. (1993), Mayewski et al. (1993), Taylor et al. (1993).

11. Chappellaz et al. (1993).

12. Bender et al. (1994).

13. E.g., Heinrich, (1988), Broecker et al. (1992), Bond et al. (1992), Andrews and Tedesco (1992), Lehman and Keigwin (1992).

14. E.g., Ruddiman and McIntyre (1981), Boyle and Keigwin (1987).

15. Oppo and Lehman (1995).

16. Discharge events, Heinrich (1988); ice sheet dynamics, MacAyeal (1993).

17. Bond et al. (1993), Mayewski et al. (1994), Cortijo et al. (1995), Bond and Lotti (1995).

18. Kennett and Ingram (1995), Behl and Kennett (1996), and Kotlainen and Shackleton (1995).

19. Porter and An (1995).

20. Lowell et al. (1995).

21. Sowers and Bender (1995).

22. Broecker and Denton (1989).

23. Stute et al. (1995).

24. Thompson et al. (1995).

25. CLIMAP Project Members (1981).

26. Broecker (1996).

27. Cortijo et al.(1995), Sirocko et al. (1996), Mayewski et al. (1997).

28. Mayewski et al. (1997).

29. E.g., Denton and Karlen (1973), Harvey (1980).

30. E.g., Overpeck et al. (1992).

31. Diaz et al. (1989).

32. Delcourt and Delcourt (1987), Huntley and Webb (1988), Davis (1990), Prentice et al. (1991), Overpeck (1993).

33. Overpeck et al. (1991).

34. E.g., Meese et al. (1994b), O'Brien et al. (1996).

35. Circulation systems, Mayewski et al. (1994, 1997); paleoclimate records, Denton and Karlen, (1973), Harvey (1980), Alley et al. (1997); coolings, O'Brien et al. (1996).

36. E.g., Denton and Karlen (1973), Stuiver and Braziunas (1989), 'Brien et al. (1995).

37. O'Brien et al. (1996).

38. Street-Perrott and Perrott (1990).

39. Stager and Mayewski (1997).

40. African lake levels, e.g., Gasse and Van Campo (1994), Stager et al. 1997), Lamb et al. (1995); Dead Sea levels, Klein (1986), Frumkin et al. (1991).

41. Sirocko et al. (1993).

42. Indian monsoon, Overpeck et al. (1996); earlier reports, Kutzbach (1987), lemens et al. (1991).

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
×

43. COHMAP members (1988), Overpeck et al. (1996).

44. Houghton et al. (1992).

45. E.g., Street-Perrott and Perrott (1990), Hodell et al. (1991, 1995), Ely et al. (1993), Knox (1993).

46. (1) E.g., Broecker (1987), Broecker et al. (1992); (2) insolation, Thomson (1995); moisture and temperature, McIntyre and Molfino, (1996); (3) e.g., Denton and Karlen (1973), Damon et al. (1989), Damon and Jirikowic (1992), Stuiver and Braziunas (1993).

47. Solar variability, Stuiver and Braziunas (1993); 10Be series from ice cores, Finkel and Nishiizumi (1997); CO2 from ice cores, e.g., Barnola et al. (1987), Wahlen et al. (1991), Etheridge et al. (1996); CH4 from ice cores, Chappellaz et al. (1993), Blunier et al. (1995), Brook et al. (1996); volcanic sulfate from ice cores, Zielinski et al. (1994, 1996).

48. Grove (1988).

49. Zhang and Crowley (1989).

50. O'Brien et al. (1996).

51. Thompson and Mosley-Thompson (1987).

52.Keigwin (1996).

53. Ibid.

54. Houghton et al. (1992).

55. Stine (1994).

56. Anderson (1992).

57. Enfield and Cid (1991).

58. Regions, Ebbesmeyer et al. (1991); ITCZ, e.g., Mann et al. (1995), Linsley et al. (1994), Cole et al. (1993), Dunbar et al. (1994); midlatitudes, e.g., Boninsegna (1992).

59. E.g., Bradley and Jones (1993), Mosley-Thompson et al. (1993), Hughes and Diaz (1994).

60. Solar output, e.g., Eddy (1976, 1977), Jirikowic and Damon (1994), Lean et al. (1995); aerosols, e.g., Stothers (1984), Bradley (1988), Scuderi (1990); aerosol loading, Porter (1981, 1986); thermohaline circulation, Weyl (1968), Watts (1985).

61. E.g., Dickinson and Henderson-Sellers (1988), Henderson-Sellers and Gornitz (1984).

62. Bonan et al. (1992).

63. E.g., Cook et al. (1996a).

64. Briffa et al. (1992).

65. Fritts (1991).

66. Meko et al. (1993), Cook et al. (1996b).

67. Historical records, Quinn et al. (1978, 1987), Quinn (1992); tropical corals, Cole et al. (1992); tropical ice cores, Thompson et al. (1992); polar ice cores, Legrand and Feniet-Saigne (1991).

68. Cole et al. (1992).

69. Barlow et al. (1993), White et al. (1997).

70. Antarctic, Welch et al. (1993); Arctic, Grumet et al. (1998).

71. Etheridge et al. (1996).

72. Mayewski et al. (1990).

73. Lyons et al. (1990).

74. Tree rings, Stuiver and Braziunas (1993); Greenland ice core, Beer et al. (1994).

75. Beer et al. (1994).

76. Henderson-Sellers et al. (1995a, 1995b).

77. Holdridge (1947), Prentice et al. (1992).

78. Bergengren (1994), Bergengren et al. (1997).

79. Prentice et al. (1993).

80. Henderson-Sellers and McGuffie (1995).

81. Huntley and Webb (1988).

82. Vegetation maps, Wolfe (1985); updates, Berhensmeyer and Potts (1992).

Suggested Citation:"6 Paleoclimate Overview." National Research Council. 1999. Global Environmental Change: Research Pathways for the Next Decade. Washington, DC: The National Academies Press. doi: 10.17226/5992.
×

83. Wing and Greenwood (1993).

84. Spicer and Corfield (1992), Spicer et al. (1993).

85. Mangan (1996).

86. Pollard and Thompson (1995a, 1995b).

87. DeConto et al. (1997).

88. E.g., Woodward (1987), Kurschner (1996), Kurschner et al. (1996).

89. Cerling et al. (1993).

90. E.g., Berner et al. (1983), Berner (1991), Freeman and Hayes (1992), Cerling (1991).

91. Clark (1977).

92. Smiley (1967).

93. Spicer and Parrish (1990), Spicer and Corfield (1992).

94. Berner (1991), Freeman and Hayes (1992), Cerling (1991).

95. Shackleton and Boersma (1981), Keigwin and Corliss (1986), Zachos et al. (1994).

96. E.g., Estes and Hutchinson (1980), McKenna (1980), Wolfe (1980).

97. Wing and Greenwood (1993).

98. Berner (1991).

99. Barron et al. (1995).

100. Shackleton and Boersma (1981), Sloan and Barron (1990).

101. Spelman and Manabe (1984), Covey and Thompson (1989), Rind and Chandler (1991), Barron and Peterson (1990, 1991), Barron t al. (1995).

102. Rind and Chandler (1991), Barron et al. (1995).

103. E.g., Hansen and Lacis (1990), Lorius et al. (1990), Hoffert and Covey (1992), Crowley (1993).

104. Hoffert and Covey (1992).

105. Crowley (1993).

106. Barron et al. (1995).

107. Berner (1991), Freeman and Hayes (1992), Cerling (1991).

108. PANASH (1995), CLIVAR (1994).

109. Etheridge et al. (1996).

110. Atkinson et al. (1987).

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How can we understand and rise to the environmental challenges of global change? One clear answer is to understand the science of global change, not solely in terms of the processes that control changes in climate and the composition of the atmosphere, but in how ecosystems and human society interact with these changes. In the last two decades of the twentieth century, a number of such research efforts--supported by computer and satellite technology--have been launched. Yet many opportunities for integration remain unexploited, and many fundamental questions remain about the earth's capacity to support a growing human population.

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