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Millennial-Scale Oscillations. Many are rapid enough to affect human life spans Largest and best defined during glaciations Present in d 18 O and dust records in Greenland ice core d 18 O fluctuations of 5-6 ‰ Large compared with overall variations

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millennial scale oscillations
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
millennial scale oscillations2
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
detecting and dating oscillations
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
oscillations in n atlantic sediments
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
heinrich events
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
source of icebergs
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
cycles or oscillations
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
support for oscillations
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
changes in ice volume
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
millennial scale changes in europe
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
millennial scale oscillations11
Millennial-Scale Oscillations
  • Similar scale oscillations have been found
    • Northern hemisphere away from N. Atlantic
    • Southern hemisphere
a global cause
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
testing global signal
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
ocean conveyer belt circulation
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
millennial scale greenhouse gas
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
millennial scale oscillation 8k years old
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
n atlantic sediments
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
mountain glaciers
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)
causes of oscillations
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
physics of change poorly understood
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
processes within ice sheets
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
interactions within climate system
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
current thinking two camps
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
current thinking two camps24
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
another broecker hypothesis
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
salt oscillator hypothesis
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
hypothesis testable and global
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
solar variability
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
solar variability problems
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?
where do we stand
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
status of millennial scale oscillation
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?
future oscillations
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