General Information
Chapman Conference on Mechanisms of Millennial-Scale Global Climate Change
Snowbird Conference Center at the Cliff Lodge
Snowbird, Utah
June 14-18, 1998
(Sunday through Thursday)
Richard B. Alley, Department of Geosciences and Earth System Science Center, Pennsylvania State University, University Park, PA 16802
Program Committee
A.J. Broccoli, NOAA Geophysical Fluid Dynamics Laboratory, Princeton University, Princeton, NJ 08542
Wallace S. Broecker, Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964
William Curry, Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543
Laurent Labeyrie, Centre des Faibles Radioactivities, Laboratoire Mixte CNRS-CEA, Domaine du CNRS 91198, Gif/Yvette Cedex, France
Alan Mix, College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331
This conference will provide an international forum for discussion of possible mechanisms that may account for millennial-scale climate change. Unlike orbital-scale climate change, which has a well-defined forcing mechanism in changes in solar insolation, the cause(s) of millennial-scale climate variation remain(s) unknown. The growing global paleoclimate database relevant to this issue is now of significant breadth to provide the critical information required to address these issues.
The central themes of this conference will address two key questions that arise from evidence of millennial-scale climate change:
(1) What is the sensitivity of various components of the Earth's climate system to millennial-scale climate change?
(2) What are the mechanisms, linkages, and feedbacks which produce millennial-scale climate change?
A subset of these general themes includes:
(i) Is millennial-scale climate change a characteristic only of ice-age boundary conditions, or do those conditions simply amplify an ongoing process?
(ii) Is millennial-scale climate change globally synchronous?
(iii) If so, is the change symmetrical (of equal sign) across both polar hemispheres?
(iv) Are the linkages made through the atmosphere, the deep and surface ocean, or both?
(v) If globally synchronous, does millennial-scale climate change represent an internal climate oscillator originating in the North Atlantic region and transmitted globally through the atmosphere and oceans?
(vi) Does millennial-scale climate change represent a common global forcing mechanism transmitted from the tropics?
(vii) Do changes in the southern hemisphere drive climate change elsewhere?
(viii) What are the feedbacks between various components of the Earth's climate system at these timescales?
(ix) Does all millennial-scale climate change share a common forcing (internal or external), or do several possible forcing mechanisms exist which independently produce millennial-scale climate change?
The primary format of the conference will allow ample time by participants for open discussion of issues within the framework of the schedule. The conference will thus be composed of short (15-20 minute) talks by invited speakers followed by 40-45 minutes of open discussion, during which participants may show a relevant overhead. There will be a half-day poster session during the afternoon of the second day, and an evening open discussion to highlight the main aspects of what we know of the earth system behavior at millennial timescales. An optional one-day field trip to examine the history of Lake Bonneville will follow the conference.
Scientific Program
Mechanisms of Millennial-Scale Global Climate Change
Sunday, June 14
9:00am - 9:50am COFFEE
9:50am - 10:00am Welcome and Introduction
10:00am - 12:00pm Session One: Ice Core Records of Millennial-Scale
Climate Variability Moderator: Peter Clark
10:00am - 10:20am M. L. Bender - Interhemispheric Phasing of
Millennial-Duration Climate Events During the Last 100 ka
10:20am - 11:00am Discussion Moderated by E.W. Wolff
11:00am - 11:20am R.B. Alley, A.-M. Agustsdottir, J.P. Fawcett -
Ice-Core Evidence of Millennial and Slower Variability in North
Atlantic Ocean Heat Transport
11:20am - 12:00pm Discussion Moderated by E. Brook
12:00pm - 1:30pm LUNCH
1:30pm - 6:00pm Session Two: North Atlantic Records of Millennial-Scale
Climate Variability Moderator: Robert Webb
1:30pm - 1:50pm G.C. Bond - A Pervasive 1470-Year Climate Cycle in
North Atlantic Glacials and Interglacials: A Product of Internal or
External Forcing?
1:50pm - 2:30pm Discussion Moderated by C.D. Keeling
2:30pm - 2:50pm L. Labeyrie, H. Leclaire, C. Cortijo, G. Auffret -
The Northwestern Atlantic Surface and Deep Water Variability at 400 Yrs
Resolution Over the Last 150 Kyrs and the Evolution of the Laurentide
Ice Sheet
2:50pm - 3:30pm Discussion Moderated by M. Sarnthein
3:30pm - 4:00pm BREAK
4:00pm - 4:20pm L.D. Keigwin, E.A. Boyle - Rapid Climate
Oscillations of the Last Glacial Cycle in the Western North Atlantic
4:20pm - 5:00pm Discussion Moderated by F. Grousset
5:00pm - 5:20pm T.M. Marchitto, W.B. Curry, J.F. McManus, D.W. Oppo
- Cd and 13C Evidence for Millennial-scale Changes in Ventilation of
the Thermocline and Intermediate Water of the Glacial North Atlantic
5:20pm - 6:00pm Discussion Moderated by T.F. Stocker
8:00pm - 9:30pm Opening Reception
Monday, June 15
9:00am - 12:30pm Session Three: Southern Ocean and Pacific Records of
Millennial-Scale Climate Variability Moderator: Richard
9:00am - 9:20am C.D. Charles, U.S. Ninnemann, P.G. Mortyn -
Thermohaline Circulation and Climate Change in the Southern Hemisphere:
Characterizing the Spectrum of Variability Over Glacial Cycles
9:20am - 10:00am Discussion Moderated by P.J. Bartlein
10:00am - 10:20am A.C. Mix, N.G. Pisias, D.C. Lund, P. Boden, S.W.
Hostetler, P.U. Clark, M. Lyle - Heinrich-Scale Climate Oscillations
in the Northeast Pacific: Atmospheric Transmission or Pacific
Thermohaline Circulation and Heat Transport?
10:20am - 11:00am Discussion Moderated by J.P. Kennett
11:00am - 11:30am BREAK
11:30am - 11:50pm E. Bard, F. Rostek - Abrupt Climatic Changes
During the Last Deglaciation in the North-West Pacific
11:50am - 12:30pm Discussion Moderated by S.C. Brassell
12:30pm - 2:00pm LUNCH
2:00pm - 5:30pm Session Four: Extra-Tropical Records of Millennial-Scale
Climate Variability Moderator: Tony Broccoli
2:00pm - 2:20pm F. Sirocko, M. Staubwasser, D. Leuschner -
Decadal-Scale Evolution of the Subtropical Climate and Hydrography
During High-Frequency Climate Oscillations of the Last 70.000 Years
2:20pm - 3:00pm Discussion Moderated by R.R. Schneider
3:00pm - 3:20pm L.V. Benson, S.P. Lund - The Nonuniform Nature of
Climate Change: Some Examples From the Great Basin of the Western
United States
3:20pm - 4:00pm Discussion Moderated by S.W. Hostetler
4:00pm - 4:30pm BREAK
4:30pm - 4:50pm C. Whitlock - Paleoecological Evidence of
Short-Term Climate Variations in the Northwestern U.S.
4:50pm - 5:30pm Discussion Moderated by T.V. Lowell
Tuesday, June 16
10:00am - 5:30pm Session Five: Longer Timescale Perspectives of
Millennial-Scale Climate Variability Moderator: Robert Webb
9:00am - 9:20am P.J. Bartlein - Signature Patterns of
Large-Scale Climatic Controls on Orbital Timescales
9:20am - 10:00am Discussion Moderated by A. McIntyre
10:00am - 10:20am J.F. McManus, D.W. Oppo, J. Cullen, D. Hodell, M. Raymo
- Beyond the Last Ice Age: Long-Term Records of Millennial-Scale
Variability in the North Atlantic
10:20am - 11:00am Discussion Moderated by N. Koc
11:00am - 5:30pm POSTER SESSION
POSTERS:
1 M. Bar-Matthews, A. Ayalon, A. Kaufman, G.J. Wasserburg -
Mediterranean Paleoclimate During the Last 60 Ky as Derived From
Speleothems, Soreq Cave, Israel
2 D.C. Barber, A.E. Jennings, J.T. Andrews - Did Meltwater Trigger
the Cold Event 8,200 cal. yrs ago? Revised 14C Ages for Drainage of
Glacial Lake Ojibway
3 P.A. Berkman, D.H. Bromwich, L.G. Thompson - Circum-Antarctic
Coastal Environmental Changes and Climate Feedbacks During the
Mid-Holocene
4 G.G. Bianchi, I.N. McCave - Holocene Variability in North
Atlantic
Climate and Deep Flow South of Iceland
5 S.C. Brassell - Alkenone Records of Ocean Warming Since the Last
Glacial Maximum and Prospects for Their Use in Lacustrine Settings
6 C. Buehring, M. Sarnthein - Reconstruction of East-Asian
Monsoon-Climate Variability With Ultra-High-Resolution Sediment-Color
Records From the South China Sea
7 G.K.C. Clarke, Shawn J. Marshall - A Glaciological Perspective
on
Heinrich Events
8 S. Dannenmann, B.K. Linsley, L. Beaufort - The Origin of
Millennial-Scale Variations in the Sulu Sea
9 T. Dokken, E. Jansen, J. Adkins, J.C. Duplessy - Rapid Climatic
Changes Documented From the Nordic Seas During the Last Glacial
Cycle:
Timing and Possible Mechanisms Involved
10 M. Elliot, L. Labeyrie, N. Tinsorat, E. Cortijo, J.C. Duplessy,
G. Bond, J.-L. Turon - Millennium Timescale Iceberg Discharges in
the Irminger Basin During the Last Glacial Period: : Relationship
With the Heinrich Events and Environmental Settings
11 M.B. Elrick, L.A. Hinnov - Millennial-Scale Paleoclimatic
Cyclicity Recorded in Paleozoic Marine Deposits
12 F.X. Gingele, M. Moros - High-Frequency Melting Pulses of the
Fennoscandian Ice Sheet Documented in Late Glacial/Early Holocene
Sediments of the Baltic Sea
13 F.E. Grousset, H. Snoeckx, and M. Revel Did the European Ice
Sheet Surges Trigger the North Atlantic Heinrich Events?
14 B.L. Hall, G.H. Denton - Millennial-Scale Surface-Level Changes
of
Closed-Basin Antarctic Lakes
15 C.J. Heusser, L.E. Heusser, T.V. Lowell - Terrestrial
Paleotemperature Estimates for 10,000 to >50,000 14C yr BP from the
Southern Andes, Chile
16 C.S. Jackson - Sensitivity of Stationary Wave Amplitude to
Laurentide Ice Sheet Topography and the Interpretation of the
Heinrich Event Climate Record
17 C.D. Keeling, T.P. Whorf - An 1800-Year Periodicity in Oceanic
Tidal Dissipation as a Possible Contributor to Millennial-Scale
Global Climate Change
18 J. P. Kennett, I. Hendy, K. Cannariato, R.J. Behl - Climate,
Ventilation, and Biotic Change in the Late Quaternary: Southern
California Margin
19 H.S. Kheshgi, A.G. Lapenis - Apparent Climate Sensitivity
From the Vostok Ice-Core
20 N. Koc, E. Jansen - Climate Variability During a Period With
Different Orbital Forcings Than the Last Climatic Cycle: Evidence
From MIS 10-11
21 S. van Kreveld, U. Pflaumann, M. Sarnthein, P. Grootes,
M.J. Nadeau, H. Erlenkeuser - Correlation Between Temperature
Records From Reykjanes Ridge Sediment and GISP2 Ice-core
22 A.G. Lapenis, H.S. Kheshgi - Climate Sensitivity to Changes
in Global Radiative Forcing: Patterns from the Last Deglaciation
23 A. McIntyre, B. Molfino - Equatorial Phytoplankton and
Millennial Forcing; Who Responds, Who Does Not, and Why With Respect
to Tropical Climate Dynamics
24 K. Mc Intyre, A.C. Ravelo, M.L. Delaney - Millennial-Scale
Variations in North Atlantic Sediments in the Late Pliocene and Early
Pleistocene
25 M. Montoya, H. von Storch, T.J. Crowley - Climate Simulation
for 125 kBP with a Coupled Ocean-Atmosphere General Circulation Model
26 N.G. Pisias, A.C. Mix, L. Heusser - Northeast Pacific Surface
Ocean Response to Millennial Scale Climate Variability
27 R.Z. Poore, L.E. Osterman, W.B. Curry, R.L. Phillips - Marine
Isotope Stages 1 and 3 in the Western Arctic Ocean
28 M.A. Reasoner, M.A. Jodry - Response of the Alpine Timberline
Ecotone to the Younger Dryas Climate Oscillation in the Colorado
Rocky Mountains: Evidence From Two Well-Dated, High-Resolution
Pollen Records
29 M. Sarnthein, L. Wang, H. Erlenkeuser, P. Grootes -
Centennial-to-Millennial-Scale Periodicities in East Asian Monsoon
Climate During the Holocene
30 R.R. Schneider, H.W. Arz - Sub-Milankovitch Variability in the
Transequatorial Atlantic Surface Circulation: A Response to
Subtropical Trade Wind Forcing?
31 J.P. Severinghaus - Annual Layering of Gas Ratios in Ice
Cores
32 Y.Y. Shopov, D.A. Stoykova, D. Ford, L.N. Georgiev, L.
Tsankov - Powerful Millennial-Scale Solar Luminosity Cycles in an
Experimental Solar Insolation Record and Their Significance to the
Termination-II
33 M. Stein, S.L. Goldstein, A. Schramm - High Resolution Record
of the Last Glacial History in Lake Lisan Sediments (Paleo-Dead Sea)
34 J.S. Stoner, J.E.T. Channel - Relative Geomagnetic
Paleointensity as a Millennial Scale Chronostratigraphic Method:
Intrahemispheric Correlation
35 D.A. Stoykova, Y.Y. Shopov, D. Ford, L.N. Georgiev, L.
Tsankov - Powerful Millennial-Scale Solar Luminosity Cycles and
Their Influence Over Past Climates and Geomagnetic Field
36 J.T. Teller, J. Licciardi, P.U. Clark - North American
Meltwater Routing to the North Atlantic During the Last Deglaciation
37 A.H.L. Voelker, M. Sarnthein, H. Erlenkeuser, P.M. Grootes,
F. Niessen, O. Swientek - Millennial Cycles in Climate Records From
the Western Iceland Sea During Isotope Stage 3
38 F. Wagner, W.M. Kuerschner, S.J.P. Bohncke, B. van
Geel - Stomatal Index Analysis Reveals Sensitivity of Atmospheric
(CO2) to Rapid Climate Changes During the Early Holocene
39 R.S. Webb, D.H. Rind, R. Healy, C. Charles - NASA-GISS AGCM
Simulations of Glacial Climate Sensitivity to Ocean Heat Transport
Forced Changed in Atmospheric Water Vapor Content
40 D.E. Wilkins, D.R. Currey - Hemiarid Basin Sensitivity to
Abrupt Climate Change: The Transpecos Closed Basin
41 E.W. Wolff, K.C. Taylor, R. Alley, K. Fuhrer - Timescales and
Phasing of Dansgaard-Oeschger Changes in the Summit Greenland Ice
Evening Discussion: Wrap-Up of Paleoclimate Records: P.U. Clark and
R.S. Webb, Moderators
Wednesday, June 17
9:00am - 12:30pm Session Six: Modern Dynamics of Decadal-Scale and Longer
Climate Variability Moderator: Alan Mix
9:00am - 9:20am L.D. Talley - Contributions of Intermediate Waters,
Deep Waters and Shallow Wind-Driven Gyres to the Overturning
Circulation and Heat Transport
9:20am - 10:00am Discussion Moderated by J.R. Toggweiler
10:00am - 10:20am L.A. Mysak, S.A. Venegas - Decadal Climate
Fluctuations in the Arctic
10:20am - 11:00am Discussion Moderated by S. Rahmstorf
11:00am - 11:30am BREAK
11:30am - 11:50pm R.T. Pierrehumbert - Subtropical Water Vapor as
a Mediator of Rapid Climate Change
11:50am - 12:30pm Discussion Moderated by J. Bates
12:30pm - 2:00pm LUNCH
2:00pm - 5:30pm Session Seven: Modern Dynamics of Decadal-Scale and
Longer Climate Variability, Continued Moderator: Wally Broecker
2:00pm - 2:20pm S.G. Philander - The Role of Wind-Driven Oceanic
Circulation in Climate Changes
2:20pm - 3:00pm Discussion Moderated by A.J. Broccoli
3:00pm - 3:30pm BREAK
3:30pm - 4:10pm A.C. Clement, M.A. Cane - Part I. Variability of
the Tropical Pacific Coupled Ocean-Atmosphere System: Precissional
Response, Abrupt Change and Sub-Milankovich Frequencies. Part II.
Global Connections
4:10pm - 5:30pm Discussion Moderated by D.R. MacAyeal
7:00pm Conference Banquet
8:30pm W.H. Calvin - Climate Instability and Hominid Brain Evolution
Thursday, June 18
8:30am - 12:00pm Session Eight: Modeling Mechanisms of Millennial-Scale
Climate Variability Moderator: Laurent Labeyrie
8:30am - 8:50am T.F. Stocker - Is There a Unique Mechanism for
Abrupt Changes in the Climate System?
8:50am - 9:30am Discussion Moderated by L. Talley
9:30am - 9:50am J.R. Toggweiler, B. Samuel - Energizing the Ocean's
Large-Scale Circulation for Climate Change
9:50am - 10:30am Discussion Moderated by R.T. Pierrehumbert
10:30am - 11:00am BREAK
11:00am - 11:20pm S. Rahmstorf, A. Ganopolski, V. Petoukhov, M.
Claussen - Coupled Ocean-Atmosphere Simulation of the Last Glacial
Maximum
11:20am - 12:00pm Discussion Moderated by S.G. Philander
12:00pm - 1:30pm LUNCH
8:30am - 12:00pm Session Nine: Modeling Mechanisms of Millennial-Scale
Climate Variability, Continued Moderator: Bill Curry
1:30pm - 1:50pm D.R. MacAyeal - Ice Sheet Model Simulations of North
Atlantic Ice Rafting Events: Rogue Ice Streams or Submissive Calving
Margins?
1:50pm - 2:30pm Discussion Moderated by G.K.C. Clarke
2:30pm - 2:50pm S.W. Hostetler, P.U. Clark, P.J. Bartlein, A.C. Mix,
N.J. Pisias - Response of Western North America Surface Processes to
a Canonical Heinrich Event
2:30pm - 3:30pm Discussion Moderated by R.S. Webb
3:30pm - 4:00pm BREAK
4:00pm - 4:20pm A.J. Broccoli - Extratropical Influences on Tropical
Paleoclimates
4:20pm - 5:00pm Discussion Moderated by M.A. Cane
5:00pm - 5:20pm W.S. Broecker - Paleocean Circulation During the Last
Deglaciation: A Bipolar Seesaw?
Please note: Attendance will be limited to the first 100 people who register for the conference.
Funding may be available to provide partial support for students attending the meeting. Application forms for travel support can be obtained from the AGU Meetings Department at the address listed . The deadline for receipt of travel applications is March 11, 1998. Awardees will be selected by the conveners.
Travel Support
Future information pertaining to this conference (i.e., scientific program, housing, registration) will be sent to those who have either submitted an abstract or have asked to be placed on the mailing list.
Those not submitting abstracts who wish to be placed on the mailing list, please contact:
Background Information on Conference Topic
Introduction
The geologic record of climate change clearly supports the basic premise of Mitchell (1976) that climate varies across many timescales. Variations in different frequency bands have been associated with distinct and interrelated driving processes. Within the Quaternary, orbital changes have played a central role in driving the pace of climate changes in concert with associated feedbacks of global-scale processes at periods of 104 to 105 years (Imbrie et al., 1992, 1993).
Superimposed upon orbital-scale changes are abrupt climate variations or events that have occurred at higher, millennial-scale frequencies (103-104 yr). Nearly unrecognized as significant events 10 years ago (cf. Dansgaard et al., 1984), millennial-scale climate variability is now recognized as a characteristic feature of the global climate during the last glaciation. Records from the Greenland Ice Sheet and the North Atlantic clearly demonstrate significant climatic variability on these timescales (Johnson et al., 1992; Lehman and Keigwin, 1992; Bond et al., 1993; Grootes et al., 1993; Oppo and Lehman, 1995; Brook et al., 1996). Evidence from both land and sea now indicate that the climate of regions outside of the North Atlantic also varied at millennial timescales, suggesting the potential for a global response to conditions in the North Atlantic sector (e.g., Broecker, 1994; Clark and Bartlein, 1995; Porter and An, 1995; Behl and Kennett, 1996; Mortyn et al., 1996) or a common global forcing mechanism (e.g., Broecker, 1995; Lowell et al., 1995; Curry and Oppo, 1997). The challenge now is to assess whether events well known in the North Atlantic record are related to events being identified elsewhere, and if so, to identify what the forcing mechanisms are and which processes provide the linkage.
Interhemispheric Synchroneity and Symmetry
Chronology. The two leading issues in understanding any possible mechanism(s) of millennial-scale climate variability are (1) whether these events were synchronous, time transgressive, or unrelated on a global scale, and (2) if synchronous or time transgressive, whether climate change was symmetrical (of equal sign) across both polar hemispheres. Being able to answer these questions rests first with the accuracy of the dating methods used to constrain the chronologies of the records. Because each of the dating methods has its own inherent uncertainties, establishing the phasing relation of rapid climate change requires a degree of accuracy that may fall outside the uncertainties of many of the methods. This holds true even when comparing some chronologies established using the same dating method, such as radiocarbon or various approaches used to date ice cores. Some have argued that the structure of millennial-scale climate change may be a characteristic in itself that supports matching records where the differing chronologies otherwise do not allow direct correlation (e.g., Lehman, 1993; Keigwin and Jones, 1994). The best solution to this problem is to measure two or more different proxies of the climate system in the same core so that phasing relations are not dependent on separate chronologies.
Structure of the Last Glaciation. Records spanning the last glaciation clearly demonstrate that millennial-scale climate change is superimposed on orbital-scale climate change. Variations in the amplitude of the higher-frequency climate change, however, correspond to variations in the amount of global ice volume (primarily northern hemisphere), suggesting that the climatology associated with the presence or absence of large ice sheets modulated the higher frequency climate variability. On this basis, periods when the climate system was most stable correspond to periods of either greatest (marine isotope stages 2 and 4) or least (stage 1) ice volume. In contrast, periods of greatest climatic instability correspond to transitions between ice maxima and minima (stage 5/4 and 2/1 boundaries) and to periods of intermediate ice volume (stage 3).
Last Glacial Maximum. It has long been known that climate change in the Southern Hemisphere paralleled that in the Northern Hemisphere at orbital timescales, despite opposite forcing from solar insolation. This is true for the last glacial maximum (LGM 21 ka), which is simply one end-member state of orbital-scale climate change. The key question has been to identify the linkages between the hemispheres at these timescales (Broecker and Denton, 1989; Imbrie et al., 1992, 1993; Labeyrie et al., 1996).
A variety of new paleoclimate data suggests that the LGM climate of the tropics, at least regionally, was considerably cooler than reconstructed by CLIMAP Project Members (1981) (Rind and Peteet, 1985; Broecker and Denton, 1989; Guilderson et al., 1996; Stute et al., 1996; Schrag et al., 1996; Thompson et al., 1996; Colinvaux et al., 1996; Bard et al., 1997). A new interpretation of the temperature depression at the Summit Greenland site suggests that LGM latitudinal gradients were still significantly steeper than those of today (Cuffey et al., 1995). Nevertheless, the identification of significant cooling of the tropics at a time of known cooling at high latitudes strongly implicates tropics and subtropics as important components in understanding synchronous, symmetrical global climate change (Webb et al., 1997).
Evidence of Synchronous Millennial-Scale Climate Change. As discussed above, the question of interhemispheric synchroneity and symmetry at millennial timescales can only be addressed from records for which the phasing relations are securely known. Establishing sychroneity is most straightforward in records for which more than one climate proxy is present, and thus phase relations can be directly identified. Because of their high accumulation rates, the Greenland ice cores (GRIP and GISP2) provide the highest resolution information on millennial-scale climate change. These cores demonstrate that during stadial (interstadial) events, local climate was colder (warmer) and drier (wetter), wind strengths were greater (lesser), the atmosphere had high (low) dust concentrations, and methane levels were low (high), the latter suggesting significant changes in the area of wetlands.
Cores from the northern North Atlantic (50o-55oN) show that periods of high IRD flux correspond to cold SSTs (stadials), and that SSTs warmed rapidly immediately after IRD events (interstadials) (Bond et al., 1993; Bond and Lotti, 1995; Oppo and Lehman, 1995). Stadial and interstadial events in these marine records are correlated with similar events in Greenland records based on the argument that warm SSTs are expected to result in warm air temperatures over Greenland (Kehman and Keigwin, 1992; Bond et al., 1993; Keigwin and Jones, 1994; Bond and Lotti, 1995). This argument is supported by the Younger Dryas event, where the chronology of both ocean sediments and ice cores is well enough established to demonstrate synchroneity of this cold event (Alley et al., 1993). Cores from the northern (~60oN) and tropical (~5oN) North Atlantic with two or more measured proxies demonstrate that a decrease (increase) in NADW was associated with a decrease (increase) in SSTs, suggesting a linkage between surface and deepwater variability (Oppo and Lehman, 1995; Curry and Oppo, 1997).
Less certain evidence for synchronous climate change is based on correlating records having independent chronologies. Cores from the Cariaco Basin (10oN) suggest increases (decreases) in trade-wind strength across tropical Atlantic at the same time as stadial (interstadial) events in the North Atlantic (Hughen et al., 1996, 1997). In the equatorial Atlantic, weaker (stronger) tropical easterlies correspond to periods of high (low) IRD fluxes (Heinrich events) (McIntyre and Molfino, 1996). The zonal intensity of the easterlies is itself modulated by the strength of the North African monsoon (McIntyre and Molfino, 1996). Fluctuations of closed-lake basins in the tropics (North Africa, Central America) suggest drying (wetting) intervals at the same time as stadials (interstadials) in the North Atlantic (Street-Perrott and Perrott, 1990; Alley at al., 1997). In monsoonal areas of Africa, India, and Asia, lower southwest-monsoon-driven upwelling (Sirocko et al., 1996) and stronger East Asian winter-monsoon winds occurred during Heinrich events (Porter and An, 1995). In Florida, pine-dominated assemblages are correlated with Heinrich events whereas an oak/ragweed/grass assemblage predominates for most of the glacial (Grimm et al., 1993).
In the western U.S., alpine glaciers and ice caps advanced up to and retreated following times of coldest SSTs and maximum IRD flux in the North Atlantic (Heinrich events) (Clark and Bartlein, 1995; Phillips et al., 1996), and possibly experienced smaller fluctuations at approximately the same time as the higher frequency stadial-interstadial (Dansgaard-Oeschger) events (Benson et al., 1996). A similar pattern is suggested for glaciers in Chile which advanced at the same time as stadial and IRD events in the North Atlantic (Lowell et al., 1995). Pluvial lakes in the western U.S. fell during times of high IRD flux to the North Atlantic (Allen and Anderson, 1993; Benson et al., 1996; Oviatt, 1997).
In the northeastern Pacific, oxygenated (oxygen-deficient) intermediate and cold (warm) surface waters occurred along the California margin at the same time as stadials (interstadials) in the North Atlantic region suggesting a change in the sites and/or intensity of global ocean convection (Keigwin and Jones, 1990; Kennett and Ingram, 1995; Behl and Kennett, 1996). Similarly, benthic foraminiferal isotopic records suggest periods of increased (decreased) production of North Pacific Deep Water during stadial (interstadial) conditions in the North Atlantic (Lund and Mix, submitted).
A common chronology based on the 
18O of trapped O2 in Antarctic (Vostok) and Greenland (GISP2) ice cores suggests that longer interstadials in Greenland (>2000 yr) correlate with interstadials in Antarctica (Bender at al., 1994). Because of uncertainties in the ice age-gas age relationship, particularly for Antarctica, however, the phasing relation between events is uncertain. The abrupt reversal of warming during the Younger Dryas may be globally synchronous (Denton and Hendy, 1994; Thompson et al., 1995; Bard et al., 1997), including perhaps in Antarctica (Mayewski et al., 1996). Keigwin and Jones (1994) suggest that similar timing and structure of millennial-scale variability in CaCO3 from the North Atlantic and in the Antarctic ice core records indicates a correlation between the interstadial events. As discussed below, however, there is compelling evidence for asynchroneity between at least some of the Antarctic and northern hemisphere events.
Evidence for Asynchronous Millennial-Scale Climate Change. Several lines of evidence suggest asynchronous climate change between the polar hemispheres. The warm interstadial events recorded by the Greenland ice cores between 20 and 40 ka are either absent or small in the Antarctic record (Sowers and Bender, 1995). During the last glacial-interglacial transition, the northern hemisphere began to gradually warm ~22 ka until 19 ka, followed by cooling until 16 ka, and then an abrupt transition at ~14.7 ka (Johnsen et al., 1992). In contrast, the transition to warmer temperatures began over Antarctica ~20 ka and continued until a cooling (the Antarctic Cold Reversal or ACR) between 12.5 and 14 ka, or nearly 1000 yr earlier than the Younger Dryas event (Jouzel et al., 1995; Sowers and Bender, 1995). Bard et al. (1997) demonstrate a temperature history from 20oS for the last 35 ka that is synchronous with the northern hemisphere, suggesting a break in climate synchroneity at the Antarctic convergence instead of the equator (Broecker, 1996).
Further evidence for asynchronous climate change comes from the Southern Ocean (41oS), where changes in 13C (NADW), which appear to parallel stadial-interstadial events in Greenland, lag changes in 18O of planktonic foraminifera (SSTs), which may parallel the Vostok D record, by 1-2 ka yr (Charles et al., 1996). These results support the chronology of Bender et al. (1994) showing the offset in ages of interstadial events between Antarctica and Greenland. Furthermore, these results suggest that Southern Ocean SSTs cooled (warmed) during times of increased (decreased) rates of NADW formation.
Potential Mechanisms
A number of mechanisms have been proposed to explain millennial-scale climate change. However, many parts of the globe remain unsampled, and thus the response of large areas of the Earth (e.g., the Pacific and Southern Oceans) that may be critical to identifying any particular mechanism remains unconstrained. Furthermore, coupled earth-system numerical models (ice sheet-atmosphere-ocean-biosphere) are still under development which limits the use of these models in the identification of realistic physically-based linkages. Finally, not only may more than one particular forcing mechanism be involved at these timescales, but the nature of the stadial/interstadial forcing and response may vary from one event to the next (Bender et al., 1994). In general, several possible conceptual models can be summarized as follows.
(1) An initial forcing at the high latitudes is transmitted and amplified elsewhere on Earth by way of the oceans and atmosphere (Broecker and Denton, 1989; Imbrie et al., 1992, 1993). The initial forcing may be linked to the effect of the large continental ice sheets on the surface-water hydrography and sites of convection of deep water. The transmission through the ocean may be by way of changes in strength of NADW and its associated heat transport triggered by changes in the surface salinity and temperature in the area of convection (Sigman and Lehman, 1995; Broecker, 1997). Alternatively, Toggweiler and Samuels (1995) argue that as the strength of NADW formation decreases, corresponding increases in deep water formation must occur elsewhere through the effects of wind-driven northward flows of near-surface waters in the Southern Hemisphere that maintain deep-sea ventilation (e.g., Lund and Mix, submitted). Because this process is wind-driven, then a possible feedback exists wherein an initial forcing at northern latitudes is transmitted across the equator, influencing the strength of the Southern Hemisphere westerlies and thus the rate of deep-sea ventilation in the North Atlantic. Finally, changes in sites or intensity of deep water formation (Pacific, Southern Oceans) may occur by changing the temperature and salinity structure of surface waters, either independently of events in the North Atlantic (Labeyrie et al., 1996), or as a direct result of changes in NADW formation and its effect on the global ocean salinity structure (Lund and Mix, submitted) or on the density structure of Southern Ocean waters allowing deep-water formation to occur around Antarctica (Stocker and Wright, 1996; Broecker, 1997).
(2) An initial forcing in tropical regions is propagated elsewhere on Earth by way of the oceans and atmosphere. Cooler tropical SSTs during the LGM implicate the low latitudes as an important component in the global climate system by decreasing water vapor, which then tranmits the signal across both hemispheres through the atmosphere and synchronizes the climate (Broecker, 1995; Webb et al., 1997). If the tropics experience millennial-scale temperature change (Curry and Oppo, 1997), then this can be similarly transmitted globally to synchronize the hemispheres at these timescales. The critical question then becomes identifying the process responsible for forcing the tropics at these timescales. This may involve a change in solar isolation (e.g., Hagelberg et al., 1994; McIntyre and Molfino, 1996) or be driven by changes in the strength of NADW formation, thus identifying an amplifying effect of the tropics on events initiated in the high latitudes.
Allen, B.D., and Anderson, R.Y., Evidence from western North America for rapid shifts in climate during the last glacial maximum, Science, 260, 1920-1923, 1993.
References Cited
Alley, R.B., Meese, D.A., Shuman, C.A., Gow, A.J., Taylor, K.C., Grootes, P.M., White, J.W.C., Ram, M.,Waddington, E.D., Mayewski, P.A., and Zielinski, G.A., Abrupt increase in snow accumulation at the end of the Younger Dryas event, Nature, 362, 527-529,1993.
Bard, E., Arnold, M., Fairbanks, R.G., and Hamelin, B., 230Th-234U and 14C ages obtained by mass spectrometry on corals, Radiocarbon, 35, 191-199, 1993.
Bard, E., Rostek, F., and Sonzongni, C., Interhemispheric synchrony of the last deglaciation inferred from alkenone paleothermometry, Nature, 385, 707-710, 1997.
Behl, R.J., and Kennett, J.P., Brief interstadial events in the Santa Barbara Basin, NE Pacific, during the last 60 kyr, Nature, 379, 243-246, 1996.
Bender, M., Sowers, T., Dickson, M.-L., Orchardo, J., Grootes, P., Mayewski, P.A., and Messe, D.A.,Climate correlations between Greenland and Antarctica during the past 100,000 years, Nature, 372, 663-666, 1994.
Benson, L.V., Burdett, J.W., Kashgarian, M., Lund, S.P., Phillips, F.M., and Rye, R.O., Climate and hydrologic oscillations in the Owens Lake basin and adjacent Sierra Nevada, California, Science, 274, 746-749, 1996.
Bond, G., Broecker, W., Johnsen, S., McManus, J., Labeyrie, L., Jouzel, J., and Bonani, G., Correlations between climate records from North Atlantic sediments and Greenland ice, Nature, 365, 143-147, 1993.
Bond, G., and Lotti, R., Iceberg discharges into the North Atlantic on millennial timescales during the last deglaciation, Science, 267, 1005-1010, 1995.
Broecker, W.S., Massive iceberg discharges as triggers for global climate change, Nature, 372, 421-424, 1994.
Broecker, W.S., Chaotic Climate, Scientific American, November, 62-68, 1995.
Broecker, W.S., Paleoclimatology, Geotimes, 41, 40-41, 1996.
Broecker, W.S., Paleocean circulation during the last deglaciation: A bipolar seesaw?, Paleooceanography, submitted.
Broecker, W.S., and Denton, G.H., The role of ocean-atmosphere reorganizations in glacial cycles, Geochimica et Cosmochimica Acta, 53, 2465-2501, 1989.
Brook, E.J., Sowers, T., and Orchardo, J., Rapid variations in atmospheric methane concentration during the past 110,000 years, Science, 273, 1087-1091, 1996.
Chappell, J., Omura, A., Esat, T., McCulloch, M., Pandolfi, J., Ota, Y., and Pillans, B., Reconciliation of late Quaternary sea levels derived from coral terraces at Huon Peninsula with deep sea oxygen isotope records, Earth and Planetary Science Letters, 141, 227-236, 1996.
Charles, C.D., Lynch-Stieglitz, J., Ninnemann, U.S., and Fairbanks, R.G., Climate connections between the hemisphere revealed by deep sea sediment core/ice core correlations, Earth and Planetary Science Letters, 142, 19-27, 1996.
Clark, P.U., and Bartlein, P.J., Correlation of the late-Pleistocene glaciation in the western United States with North Atlantic Heinrich events, Geology, 23, 483-486, 1995.
CLIMAP Project Members, Seasonal reconstructiions of the Earth's surface at the last glacial maximum, Geological Society of America Map and Chart Series MC-36, 1981.
Colinvaux, P.A., De Oliveira, P.E., Moreno, J.E., Miller, M.C., and Bush, M.B., A long pollen record from lowland Amazonia: Forest and cooling in glacial times, Science, 274, 85-88, 1996.
Cuffey, K.M., Clow, G.D., Alley, R.B., Stuiver, M., Waddington, E.D., and Saltus, R.W., Large Arctic temperature change at the Wisconsin-Holocene glacial transition, Science, 270, 455-458, 1995.
Curry, W.B., and Oppo, D.W., Synchronous, high-frequency oscillations in tropical sea surface temperatures and North Atlantic Deep Water production during the last glacial cycle, Paleoceanography, 12, 1-14, 1997.
Dansgaard, W., Johnsen, S.J., Clausen, H.G., Dahl-Jensen, D., Gundestrup, N., Hamer, C.U., and Oeschger, H., North Atlantic climate oscillations revealed by deep Greenland ice cores. In: Climate processes and climate sensitivity (eds. J.E. hansen and T. Takahashi), AGU Geophysical Monograph, 29, 288-298, 1984.
Denton, G.H., and Hendy, C.H., Younger Dryas age advance of Franz Josef Glacier in the Southern Alps of New Zealand, Science, 264, 1434-1437, 1994.
Edwards, R.L., Beck, J.W., Burr, G.S., Donahue, D.J., Chappell, J., Bloom, A.L., Druffel, E.R.M., and Taylor, F.W., A large drop in atmospheric 14C/12C and reduced melting in the Younger Dryas, documented with 230Th ages in corals, Science, 260, 962-968, 1993.
Grimm, E.C., Jaconson, G.L., Watts, W.A., Hansen, B.C.S., and Maasch, K.A., A 50,000-year record of climate oscillations from Florida and its temporal correlation with the Heinrich events, Science, 261, 198-200, 1993.
Grootes, P.M., Stuiver, M., White, J.W.C., Johnsen, S.J., and Jouzel, J., Comparison of oxygen isotope records from the GISP2 and GRIP Greenland ice cores, Nature, 366, 552-554, 1993.
Guilderson, T.P., Fairbanks, R.G., and Rubenstone, J.L., Tropical temperature variations since 20,000 years ago: Modulating interhemispheric climate change, Science, 263, 663-665, 1994.
Hagelberg, T.K., Bond, G., and deMenocal, P., Milankovitch band forcing of sub-Milankovitch climate variability during the Pleistocene, Paleoceanography, 9, 545-558, 1994.
Hughen, K.A., Overpeck, J.T., Peterson, L.C., and Trumbore, S., Rapid climate changes in the tropical Atlantic region during the last deglaciation, Nature, 380, 51-54, 1996.
Hughen, K.A., Overpeck, J.T., Lehman, S.J., Kashgarian, M., Peterson, L.C., and Alley, R.B., Deglacial 14C calibration, activity, and climate from a marine varve record, Nature, in press, 1997.
Imbrie, J., Boyle, E.A., Clemens, S.C., Duffy, A., Howard, W.R., Kukla, G., Kutzbach, J., Martinson, D.G., McIntyre, A., Mix, A.C., Molfino, B., Morley, J.J., Peterson, L.C., Pisias, N.G., Prell, W.L., Raymo, M.E., Shackleton, N.J., and Toggweiler, J.R., On the structure and origin of major glaciation cycles, 1. Linear responses to Milankovitch forcing, Paleoceanography, 7, 701-738, 1992.
Imbrie, J., Berger, A., Boyle, E.A., Clemens, S.C., Duffy, A., Howard, W.R., Kukla, G., Kutzbach, J., Martinson, D.G., McIntyre, A., Mix, A.C., Molfino, B., Morley, J.J., Peterson, L.C., Pisias, N.G., Prell, W.L., Raymo, M.E., Shackleton, N.J., and Toggweiler, J.R., On the structure and origin of major glaciation cycles, 2. The 100,000-year cycle, Paleoceanography, 8, 699-735, 1993.
Johnsen, S.J., Clausen, H.B., Dansgaard, W., Fuhrer, K., Gundestrup, N., Hammer, C.U., Iversen, P., Jouzel, J., Stauffer, B., and Steffensen, J.P., Irregular glacial interstadials recorded in a new Greenland ice core, Nature, 359, 311-313, 1992.
Jouzel, J., et al., Vostok ice core: A continuous isotope temperature record over the last climate cycle (160,000 years), Nature, 329, 403-408, 1987.
Jouzel, J., Vaikmae, R., Petit, J.R., Martin, M., Duclos, Y., Stievenard, M., Lorius, C., Toots, M., Melieres, M.A., Burckle, L.H., Barkov, N.I., and Kotlyakov, V.M., The two-step shape and timing of the last deglaciation in Antarctica, Climate Dynamics, 11, 151-161, 1995.
Keigwin, L.D., and Jones, G.A., Deglacial climatic oscillations in the Gulf of California, Paleoceanography, 5, 1009-1023, 1990.
Keigwin, L.D., and Jones, G.A., Western North Atlantic evidence for millennial-scale changes in ocean circulation and climate, Journal of Geophysical Research, 99, 12,397-12,410, 1994.
Kennett, J.P., and Ingram, B.L., A 20,000-year record of ocean circulation and climate change from the Santa Barbara basin, Nature, 377, 510-514, 1995.
Labeyrie, L., Labracherie, M., Gorfti, N., Pichon, J.J., Vautravers, M., Arnold, M., Duplessy, J.-C., Paterne, M., Michel, E., Duprat, J., Caralp, M., and Turon, J.L., Hydrographic changes in the Southern Ocean (southeast Insian sector) over the last 230 kyr, Paleoceanography, 11, 57-76, 1996.
Lehman, S., Ice sheets, wayward winds and sea change, Nature, 365, 108-109, 1993.
Lehman, S.J., and Keigwin, L.D., Sudden changes in North Atlantic circulation during the last deglaciation, Nature, 356, 757-762, 1992.
Lowell, T.V., Heusser, C.J., Anderson, B.G., Moreno, P.I., Hauser, A., Heusser, L.E., Schluchter, C., Marchant, D.R., and Denton, G.H., Interhemisphere correlation of late Pleistocene glacial events, Science, 269, 1541-1549, 1995.
Lund, D.C., and Mix, A.C., Millennial-scale deep water oscillations: Reflections of the North Atlantic in the deep Pacific from 10 to 60 kyr BP, submitted to Paleoceanography.
Mayewski, P.A., Twickler, M.S., Whitlow, S.I., Meeker, L.D., Yang, Q., Thomas, J., Kreutz, K., Grootes, P.M., Morse, D.L., Steig, E.J., Waddington, E.D., Saltzman, E.S., Whung, P.-Y., and Taylor, K.C., Climate change during the last deglaciation in Antarctica, Science, 272, 1636-1638, 1996.
McIntyre, A., and Molfino, B., Forcing of Atlantic equatorial and subpolar millennial cycles by precession, Science, 274, 1867-1869, 1996.
Mitchell, J.M., An overview of climatic variability and its causal mechanisms, Quarternary Research, 6, 481-493, 1976.
Mortyn, P.G., Thunnell, R.C., Anderson, D.M., Stott, L.D., and Le, J., Sea surface temperature changes in the Southern California Borderlands during the last glacial-interglacial cycle, Paleoceanography, 11, 415-430, 1996.
Oppo, D., and Lehman, S.J., Suborbital timescale variability of North Atlantic Deep Water during the past 200,000 years, Paleoceanography, 10, 901-910, 1995.
Oviatt, C.G., Lake Bonneville fluctuations and global climate change, Geology, 25, 155-158, 1997.
Phillips, F.M., Zreda, M.G., Benson, L.V., Plummer, M.A., Elmore, D., and Sharma, P., Chronology for fluctuations in late Pleistocene Sierra Nevada glaciers and lakes, Science, 274, 749-751, 1996.
Porter, S.C., and An, Z., Correlation between climate events in the North Atlantic and China during the last glaciation, Nature, 375, 305-308, 1995.
Rind, D., and Peteet, D., Terrestrial conditions at the Last Glacial Maximum and CLIMAP sea-surface temperature estimates: Are they consistent?, Quartenary Research, 24, 1-22, 1985.
Schrag, D.P., Hampt, G., and Murray, D.W., Pore fluid constraints on the temperature and oxygen isotopic composition of the glacial ocean, Science, 272, 1930-1931, 1996.
Sigman, D.M., and Lehman, S.J., Can oceanic nutrient distributions be used to constrain ocean heat transport during the last glacial maximum?, Eos, 76, F284, 1995.
Sirocko, F., Garbe-Schonberg, D., McIntyre, A., and Molfino, B., Teleconnections between the subtropical monsoons and high-latitude climates during the last deglaciation, Science,272, 526-529, 1996.
Sowers, T., and Bender, M., Climate records covering the last deglaciation, Science, 269, 210-214, 1995.
Stocker, T.F., and Wright, D.G., Rapid changes in ocean circulation and atmospheric radiocarbon, Paleoceanography, 11, 773-795, 1996.
Street-Perrot, F.A., and Perrott, R.A., Abrupt climate fluctuations in the tropics: The influence of Atlantic Ocean circulation, Nature, 343, 607-612, 1990.
Stute, M., Forster, M., Frischkorn, H., Serejo, A., Clark, J.F., Schlosser, P., Broecker, W.S., and Bonani, G., Cooling of tropical Brazil (5oC) during the last glacial maximum, Science, 269, 379-383, 1995.
Thompson, L.G., Mosley-Thompson, E., Davis, M.E., Lin, P.-N., Henderson, K.A., Cole-Dai, J., Bolzan, J.F., and Liu, K.-b., Late glacial stage and Holocene tropical ice core records from Huascaran, Peru, Science, 269, 46-50, 1995.
Toggweiler, J.R., and Samuels, B., Effect of Drake Passage on the global thermohaline circulation, Deep-Sea Research, 42, 477-500, 1995.
Webb, R.S., Rind, D.H., Lehman, S.J., Healy, R.J., and Sigman, D., Influence of ocean heat transport on the climate of the Last Glacial Maximum, Nature, 385, 695-699, 1997.
Return to AGU Meetings
