Millennial-Scale Oscillations

1. Millennial-Scale Oscillations • Many are rapid enough to affect human life spans • Largest and best defined during glaciations • Present in d18O and dust records in Greenland ice core • d18O fluctuations of 5-6‰ • Large compared with overall variations • Negative d18O match increase in dust content • Oscillations referred to as Dansgaard-Oeschger cycles

2. Millennial-Scale Oscillations • 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 • Size of dust large in cold intervals • More evidence for sea salt deposition when cold • Indicates winds were strong

3. Detecting and Dating Oscillations • Detecting millennial-scale oscillation relatively easy • Dating them is not • Dating is necessary for confirming correlations • 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? • Glacial age materials, uncertainly in 14C 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 • Redistribution of fine sediments • Coarse foraminifera and ice rafted-debris settle • 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 d18O in Greenland ice

5. Heinrich Events • 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/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 • 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 • Not just decrease in deposition of foraminifera

7. Cycles or Oscillations? • Some feel represent true cycles of cooling • Followed by ice-rafting • 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 agrees with sense of cooling in Greenland

8. Support for Oscillations • Long cores from ODP • Document millennial-scale oscillations • During 100,000-year and 40,000-year glacial cycles • Benthic foraminifera show changes in d13C during younger oscillations • Suggest that during cooling episodes • NADW slowed particularly during major ice-rafting events • 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 • How did this affect total ice volume? • Oxygen isotope records in Pacific benthic foraminifera • 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 oscillations

10. Millennial-Scale Changes in Europe • 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 • Northern hemisphere away from N. Atlantic • Southern hemisphere

12. A Global Cause? • 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 • Either hemispherical or global scale

13. Testing Global Signal • Evidence in S. hemisphere would strengthen interpretation • Antarctic ice core have short-term d18O signals • Amplitude is much smaller than Greenland • Some hint that signal are opposite • Temperature sea-saw could be related to NADW

14. Ocean Conveyer Belt Circulation • Northward flowing currents in Southern Ocean removes heat • Adds heat to N. Atlantic • 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 • Can be even opposite

15. Millennial-Scale [Greenhouse Gas] • Greenland CH4 show millennial-scale oscillations • However concentrations changes lag temperature changes • CH4 not driver • CO2 not trustworthy because of CaCO3 dissolution in Greenland • No detailed records from Antarctica • Expect changes in CO2 if NADW is a driver

16. Millennial-Scale Oscillation <8K Years Old • 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 • Greenland ice cores

17. N. Atlantic Sediments • 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 today

18. Mountain Glaciers • Oscillation apparent superimposed on gradual cooling • Irregular spacing over last 8,000 years • Poorly dated • 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 • 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 • 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 • 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 • 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

22. Interactions Within 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 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 • 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 • 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 • 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 • Replenish salt and heat to N. Atlantic

26. Salt Oscillator Hypothesis • During times of NADW formation • Ice melting dilutes salinity of N. Atlantic • Eventually slowing or stopping NADW formation • 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 again

27. Hypothesis Testable and Global • Oscillation in NADW should alter atmospheric CO2 • Short-term records not yet available • 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 • Potential to influence Southern Ocean SST • Producing a opposite-phased seesaw (seasaw?) • Unclear if oscillations <4K years linked with NADW

28. Solar Variability • Variations in the strength of Sun • Comparison between 10Be in ice cores and 14C 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 10Be gives indication of production • Residuals affected also by carbon cycle • Oscillations at 420 and perhaps 2,100 years • No production cycle at 1,500 years • Unlikely that strength of Sun • Responsible for variability noted • Why was it greater during glaciations?

30. Where Do We Stand? • Evidence supports reorganization of thermohaline circulation • Accompany Younger Dryas and Heinrich Events • Although reorganization may be a consequence of climate change initiated elsewhere • Probably NADW is primary trigger • Ocean changes likely affected tropical atmosphere dynamics • Drove global atmospheric changes • 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 • Key feature to determine if far-field climate changes predate changes attributable to ocean reorganization • Requires precise dating of events globally • May be doomed by abrupt nature of 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 • Will millennial-scale oscillation warm or cool climate in the future? • 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