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 40-50N
• 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 lt8K 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 Earths 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 lt4K 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 dont 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