1. Millennial-Scale Oscillations Many are rapid enough to affect human life spans Many are rapid enough to affect human life spans Largest and best defined during glaciations Largest and best defined during glaciations  Present in  18 O and dust records in Greenland ice core   18 O fluctuations of 5-6‰  Large compared with overall variations  Negative  18 O match increase in dust content  Oscillations referred to as Dansgaard-Oeschger cycles

2. Millennial-Scale Oscillations Apparent in the GRIP/GISP Greenland cores Apparent in the GRIP/GISP Greenland cores  Oscillations 2,000-3,000, some 5,000 years  Average is about 4,000 years Dust apparently sourced from northern Asia Dust apparently sourced from northern Asia  Size of dust large in cold intervals More evidence for sea salt deposition when cold More evidence for sea salt deposition when cold  Indicates winds were strong

3. Detecting and Dating Oscillations Detecting millennial-scale oscillation relatively easy Detecting millennial-scale oscillation relatively easy  Dating them is not  Dating is necessary for confirming correlations Problems involved are twofold Problems involved are twofold  Can the archive record millennial-scale oscillations?  Deep sea sediments deposited cm 1000 y -1  Typically easy with high sedimentary rates to show that oscillations exist How accurately can the oscillations be dated? How accurately can the oscillations be dated?  Glacial age materials, uncertainly in 14 C date about the length of the cycle  May be dated, cannot determine lead/lag relationship

4. Oscillations in N. Atlantic Sediments High sedimentation rate drift deposits High sedimentation rate drift deposits  Redistribution of fine sediments  Coarse foraminifera and ice rafted-debris settle Revealed millennial-scale oscillations and ice rafting events Revealed millennial-scale oscillations and ice rafting events  Called Heinrich events  Polar species and ice rafted debris indicated  Cold waters  Icebergs present Match changes in  18 O in Greenland ice Match changes in  18 O in Greenland ice

5. Heinrich Events When Greenland became cold, dry and windy When Greenland became cold, dry and windy  North Atlantic ocean temperature decreased and icebergs were present  Dating sufficient over last 30K years to confirm correlation Not sufficient to determine lead/lagsNot sufficient to determine lead/lags  Pattern was slow cooling  Followed (typically) by ice-rafting event  Rapid warming after ice-rafting event

6. Source of Icebergs Most ice rafted debris found 40-50°N Most ice rafted debris found 40-50°N  Icebergs from northeastern margin of Laurentide ice sheet  Iceland  Northern Scotland  Earliest events not from Laurentide Detailed study showed large increases in rate of deposition of ice-rafted debris Detailed study showed large increases in rate of deposition of ice-rafted debris  Not just decrease in deposition of foraminifera

7. Cycles or Oscillations? Some feel represent true cycles of cooling Some feel represent true cycles of cooling  Followed by ice-rafting Icebergs were dumped into N. Atlantic from Iceland every 1,500 years Icebergs were dumped into N. Atlantic from Iceland every 1,500 years  Despite climatic conditions  At some point a threshold was reached  Triggered large influx of icebergs Not all evidence has this regular pattern Not all evidence has this regular pattern Not all agrees with sense of cooling in Greenland Not all agrees with sense of cooling in Greenland

8. Support for Oscillations Long cores from ODP Long cores from ODP  Document millennial-scale oscillations  During 100,000-year and 40,000-year glacial cycles Benthic foraminifera show changes in  13 C during younger oscillations Benthic foraminifera show changes in  13 C during younger oscillations  Suggest that during cooling episodes  NADW slowed particularly during major ice- rafting events Oscillations occur in Greenland and N. Atlantic Oscillations occur in Greenland and N. Atlantic  Changes in air and surface-ocean temperatures  Ice sheet margins and ice rafting  In deep water formation

9. Changes in Ice Volume If icebergs formed and melted If icebergs formed and melted  How did this affect total ice volume? Oxygen isotope records in Pacific benthic foraminifera Oxygen isotope records in Pacific benthic foraminifera Deposits sense global ice volume but not local ice melting Deposits sense global ice volume but not local ice melting  Show generally small variations (0.1‰)  Less than 10 m change in sea level Gross changes in the size of ice sheets unlikely cause of oscillationsGross changes in the size of ice sheets unlikely cause of oscillations

10. Millennial-Scale Changes in Europe Greenland ice sheet temperatures correlate Greenland ice sheet temperatures correlate  European soil type  Warm intervals rich in clay and organic carbon  European pollen  Similar change to larger scale climate changes

11. Millennial-Scale Oscillations Similar scale oscillations have been found Similar scale oscillations have been found  Northern hemisphere away from N. Atlantic  Southern hemisphere

12. A Global Cause? Millennial-scale oscillation in Santa Barbara Basin Millennial-scale oscillation in Santa Barbara Basin  Match fluctuations in Greenland ice core  Warm intervals in Greenland match warm and productive intervals in California margin sediments May indicate separate regional responses to more pervasive cause of climate change May indicate separate regional responses to more pervasive cause of climate change  Either hemispherical or global scale

13. Testing Global Signal Evidence in S. hemisphere would strengthen interpretation Evidence in S. hemisphere would strengthen interpretation  Antarctic ice core have short-term  18 O signals  Amplitude is much smaller than Greenland  Some hint that signal are opposite Temperature sea-saw could be related to NADW Temperature sea-saw could be related to NADW

14. Ocean Conveyer Belt Circulation Northward flowing currents in Southern Ocean removes heat Northward flowing currents in Southern Ocean removes heat  Adds heat to N. Atlantic Suggests that even distant millennial-scale oscillation Suggests that even distant millennial-scale oscillation  Can be driven by N. Atlantic  As a response to changes in NADW formation Response to this forcing can be different in different environments Response to this forcing can be different in different environments  Can be even opposite

15. Millennial-Scale [Greenhouse Gas] Greenland CH 4 show millennial-scale oscillations Greenland CH 4 show millennial-scale oscillations  However concentrations changes lag temperature changes  CH 4 not driver CO 2 not trustworthy because of CaCO 3 dissolution in Greenland CO 2 not trustworthy because of CaCO 3 dissolution in Greenland  No detailed records from Antarctica  Expect changes in CO 2 if NADW is a driver

16. Millennial-Scale Oscillation <8K Years Old Although lower in amplitude, oscillation exist Although lower in amplitude, oscillation exist  Fluctuations weak and show variations of 2,600 year cycle Changes in sea salt have ~2,600 year cycle Changes in sea salt have ~2,600 year cycle  Greenland ice cores

17. N. Atlantic Sediments Slight increases in very small sand sized grains Slight increases in very small sand sized grains  Depositional intervals of 1,500-2,000 years  Probably transported by large icebergs That are common in N. Atlantic todayThat are common in N. Atlantic today

18. Mountain Glaciers Oscillation apparent superimposed on gradual cooling Oscillation apparent superimposed on gradual cooling  Irregular spacing over last 8,000 years  Poorly dated Oscillations present Oscillations present  Cyclic nature of the oscillation  Not well known (1,500 versus 2,500 years)

19. Causes of Oscillations Hypotheses must explain key questions Hypotheses must explain key questions  What initiates the oscillations?  How are they transmitted to other parts of the climate system where they have been documented?  Why are they stronger during glaciations than during interglaciations? Hypotheses include Hypotheses include  Natural oscillations in the internal behavior of N. hemisphere ice sheets  The result of internal interactions among several parts of the climate system  A response to solar variations external to the climate system

20. Physics of Change Poorly Understood Explanation must address Explanation must address  States among which the climate system has jumped  Mechanism by which the climate system can be triggered to jump from one climate state to another  Invoke a telecommunication system by which the message can be transmitted globally  Must have a “flywheel” capable of holding the system in a given state for centuries

21. Processes Within Ice Sheets Ice sheets obvious choice since strong glacial signal Ice sheets obvious choice since strong glacial signal  Margins of ice sheets can change rapidly  Perhaps movement of marine ice sheets from one “pinning point” to another  Ice sheets break of forming flotilla of icebergs Hard to argue that ice sheets can recover from such losses in just 1,500 years Hard to argue that ice sheets can recover from such losses in just 1,500 years

22. Interactions Within Climate System Such interactions require several components of the climate system Such interactions require several components of the climate system  Function as nearly equal partners  Continuously interact Must have similar response times and the right response time Must have similar response times and the right response time  Must not take over and drive the entire climate system  A natural for this response in NADW

23. Current Thinking – Two Camps Multiple state of thermohaline circulation Multiple state of thermohaline circulation  Trigger – catastrophic input of fresh water to N. Atlantic  Flywheel – sluggish dynamics of internal ocean  Missing – change of interactions capable of producing immediate large and widespread atmospheric impacts

24. Current Thinking – Two Camps Changes in dynamics of the tropical atmosphere- ocean system Changes in dynamics of the tropical atmosphere- ocean system  Since tropical convective systems constitute the dominant element in Earth’s climate system  Trigger most like resides in the region that house the El Nino-La Nina cycle  Telecommunication not a problem!  No evidence for multiple states of of tropical atmosphere-ocean system  Unless it affects deep ocean, no flywheel capable of locking the atmosphere into one of its alternate states

25. Another Broecker Hypothesis Salt oscillator hypothesis Salt oscillator hypothesis NADW removes heat and salt from N. Atlantic NADW removes heat and salt from N. Atlantic  Heat melts ice and delivers fresh water to N. Atlantic reducing salinity Gulf Stream and N. Atlantic Drift transport heat and salt to subpolar Atlantic Gulf Stream and N. Atlantic Drift transport heat and salt to subpolar Atlantic  Replenish salt and heat to N. Atlantic

26. Salt Oscillator Hypothesis During times of NADW formation During times of NADW formation  Ice melting dilutes salinity of N. Atlantic  Eventually slowing or stopping NADW formation When NADW does not form When NADW does not form  Less salt removed and little heat transported north  Ice sheets stop melting N. Atlantic gets salty and NADW starts to form againN. Atlantic gets salty and NADW starts to form again

27. Hypothesis Testable and Global Oscillation in NADW should alter atmospheric CO 2 Oscillation in NADW should alter atmospheric CO 2  Short-term records not yet available Change in N. Atlantic SST would affect atmospheric temperatures – possible telecommunication Change in N. Atlantic SST would affect atmospheric temperatures – possible telecommunication  Atmospheric circulation patterns  Could alter jet stream and affect other regions (e.g., Santa Barbara Basin) NADW eventually interacts with ACC NADW eventually interacts with ACC  Potential to influence Southern Ocean SST  Producing a opposite-phased seesaw (seasaw?) Unclear if oscillations <4K years linked with NADW Unclear if oscillations <4K years linked with NADW

28. Solar Variability Variations in the strength of Sun Variations in the strength of Sun  Comparison between 10 Be in ice cores and 14 C in tree rings  Link production rates to sun strength  Variability don’t show millennial-scale oscillations

29. Solar Variability: Problems Age of tree rings exact and 10 Be gives indication of production Age of tree rings exact and 10 Be gives indication of production  Residuals affected also by carbon cycle Oscillations at 420 and perhaps 2,100 years Oscillations at 420 and perhaps 2,100 years  No production cycle at 1,500 years  Unlikely that strength of Sun Responsible for variability notedResponsible for variability noted  Why was it greater during glaciations?

30. Where Do We Stand? Evidence supports reorganization of thermohaline circulation Evidence supports reorganization of thermohaline circulation  Accompany Younger Dryas and Heinrich Events Although reorganization may be a consequence of climate change initiated elsewhere Although reorganization may be a consequence of climate change initiated elsewhere  Probably NADW is primary trigger Ocean changes likely affected tropical atmosphere dynamics Ocean changes likely affected tropical atmosphere dynamics  Drove global atmospheric changes Missing – mechanism for transmitting the signal from deep ocean to tropical atmosphere Missing – mechanism for transmitting the signal from deep ocean to tropical atmosphere  Time scales of only a few decades

31. Status of Millennial-Scale Oscillation Proof of underlying mechanism must come from climate records Proof of underlying mechanism must come from climate records Key feature to determine if far-field climate changes predate changes attributable to ocean reorganization Key feature to determine if far-field climate changes predate changes attributable to ocean reorganization Requires precise dating of events globally Requires precise dating of events globally  May be doomed by abrupt nature of events Current search for precursor events Current search for precursor events  What is happening just prior to Heinrich event? Cooling? Warming?

32. Future Oscillations Changes rapid enough to affect human populations Changes rapid enough to affect human populations  Will millennial-scale oscillation warm or cool climate in the future? Ignoring anthropogenic greenhouse gases Ignoring anthropogenic greenhouse gases  Slow natural cooling of N. hemisphere  Likely interrupted by rapid millennial-scale cooling events  Nature of the oscillations during the last 8K years  Makes future changes difficult to predict