Millennial-Scale Oscillations
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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 Oscillations
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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
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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
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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
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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
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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?
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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
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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 icerafting 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
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
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Millennial-Scale Changes in Europe
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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 Oscillations
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Similar scale oscillations have been found
 Northern hemisphere away from N. Atlantic
 Southern hemisphere
A Global Cause?
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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
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
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Ocean Conveyer Belt Circulation
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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]
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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
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
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N. Atlantic Sediments
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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
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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
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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
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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
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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
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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
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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 Camps
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Changes in dynamics of the tropical atmosphereocean 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
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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
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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
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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
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Variations in the strength of Sun
10Be in ice cores and 14C in
 Comparison between
tree rings
 Link production rates to sun strength
 Variability don’t show millennial-scale oscillations
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?
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Where Do We Stand?
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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
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?
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Future Oscillations
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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