Carbon System Controls on CO2

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Carbon System Controls on CO2
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High Latitude Stratification

• Sigman et al. (2004 Nature 428, 59-63)
• Cold climates promote polar ocean stratification
• Opal MAR decreased at 2.7 mya
• Fewer diatoms
• d15NOM increased or did not change
• Greater NO32- utilization or no change in NO32-
  utilization
• Indicates stratification

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Density at Cold Temperatures

• Variation in seawater density as a function of
  temperature, with salinity at 35
• Density of sea water increases less rapidly as
  the temperature drops towards freezing point
• High latitude SST cold enough so winter cooling
  did not have enough of an effect to overcome
  salinity gradients

From Francois (2004 Nature 428 31-32)
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Salinity Variations Today
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Density ?(cooling)
Density as a function of depth in the modern
wintertime Antarctic and changes in this density
structure for uniform changes in seawater
temperature. Cooling entire water column nearly
doubles the vertical density difference.
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Effect of Stratification on PCO2

• Global cooling an important factor promoting
  high-latitude stratification
• Polar ocean stratification prevents deep ocean
  ventilation
• Traps more carbon in the deep sea
• During Quaternary climatic cycles
• Interglacial periods sufficiently warm to allow
  deep water convection in Antarctic
• Not the North Pacific
• During glacial periods
• Stratification in Antarctica and North Pacific
• Contribute to lower atmospheric CO2

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• Six factors may contribute to glacial atmospheric
  CO2 change
• Four factors tied to carbon cycle through changes
  in
• Nutrient upwelling
• Antarctic surface water chemistry
• Factors contributing most to drop in CO2
• Lower SST
• Stronger biological carbon pump
• Increased CO32- linked to changes in deep ocean
  circulation

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Ice-Driven Climate Responses

• Ice sheets can become drivers of climate within
  the system because
• Large height
• Influences wind direction and temperature
• Bright surface
• Major albedo contrast
• Calve icebergs
• Deliver cold fresh water to oceans

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Glacier Climate Interactions

• Regions in close geographic proximity to ice
  sheet
• The tempo of climate change is set by the change
  in ice sheet
• Ice sheet has highest thermal inertia

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Ice-Driven Responses

• Orbital scale ice sheet rhythms
• Quickly transferred to other parts of the climate
  system
• Atmosphere
• Oceans
• Ice sheets respond to solar insolation
• Other systems respond quickly to change in ice
  sheets

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Ocean Surface Temperature

• N. Atlantic SST should respond to change in N.
  Hemisphere glaciations
• SST track ice volume with no lag
• SST reconstruction from faunal assemblages
• Ice volume and SST follow 41,000 year cycles
• Similar relationships found in younger cores
• Correlate with 100,000 year cycles

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Mechanisms for Ocean Cooling

• Calving icebergs undoubtedly important
• Changes in winds
• GCM sensitivity tests
• Clockwise flow of winds initiated over ice sheets
• Cold winds blown over N. Atlantic
• Replaced warm flow from southwest
• Simulations predict 5-10C drop in SST
• Similar to documented SST change

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Cold Winds Affect Climate

• Pollen changes in France
• N. Europes climate changed
• Warm and moist (trees)
• Cold and dry (herbs)
• Changes correlate with ice volume
• Cold winds from Scandinavian ice sheets
• N. Atlantic ocean colder than today
• Relative warm N. Atlantic moderates Europes
  winter weather

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GCM Test of N. Atlantic SST

• Boundary condition
• N. Atlantic glacial SST
• Output indicated cooling over N. Atlantic,
  European maritime regions and central Eurasia
• Produced lower rainfall
• N. Hemisphere ice sheet growth
• Cooled N. Atlantic
• Cooled Europe and Asia
• Cooling occurred without lag

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Loess Plateaus

• Evidence of ice-driven response found in China
• Plateaus of wind-blown silt
• Deposited by strong winds and dry conditions
• Loess deposition post-date weathered soil at 2.75
  mya
• Onset of dry conditions at glacial inception
• 100,000 year cycle over last 0.5 my

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Onset of Loess Deposition

• Loss of N. Atlantic moisture
• Cold and ice-covered N. Atlantic
• Stop moisture flow to Europe and Asia
• Siberian high-pressure center
• Today source of strong winter winds
• Could have strengthen during glaciers
• Windy and dry conditions deposited loess
• Western N. Pacific
• Greenland ice sheet

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Summary

• Ice volume signal can be transferred far from ice
  sheets
• Altered wind patterns
• Changes in air and sea surface temperature
• Changes in rainfall over land
• Northern Hemisphere ice sheet growth
• Drives the climate signal on orbital time scales
• Ice sheets respond to orbital forcing
• Changes in ice sheets drive other climate
  responses in northern latitudes

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Other Effects of N. Ice Sheets

• Aeolian deposition in western Indian Ocean
• Follows timing of ice sheet growth and melting
• More dust deposited from Arabian desert
• During glacial intervals defined by d18O maxima
• Less dust deposited during interglacial periods

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Effects in South America

• Long cores from eastern Columbian lakes
• Pollen records that alternate between grass and
  trees
• 100,000 year cycles
• Trees grew during rapid warming
• Grassland dominated during slow cooling intervals

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Effects in New Zealand

• Marine sediment core east of New Zealand
• Cyclic variations in tree and grass pollen
• 100,000 year dominant cycle

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Effects in Southern Ocean

• Marine sediment cores in Southern Ocean
• Assemblages of radiolarians
• Indicative of ice cover and cold SST
• Match well colder temperatures in glacial
• Appears that only summer monsoon signal not
  affected by N. Hemisphere ice volume
• Not expected due to summer forcing of monsoon

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Southern Hemisphere Surprises

• Large Antarctic ice sheets do not appear to force
  S. Hemisphere glacial/interglacial changes
• Most land mass of Antarctica now covered
• Not much room to grow so no major expansion of
  ice volume
• In contrast, N. Hemisphere ice sheets fluctuated
  enormously
• If climate signals forced by ice sheets
• Probably forced by changes in N. Hemisphere ice
  sheets

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Seasonal Changes in Precession

• Although insolation changes due to axial tilt in
  N and S Hemispheres in phase
• Precession changes are out of phase
• When one hemisphere is warm (close to Sun)
• Other Hemisphere is cold
• Patterns of insolation changes look different
• Phase of the precession cycle is reversed between
  hemispheres

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Phasing of Insolation and Ice Volume

• 41,000 and 23,000 year components of ice volume
• Lag behind N. Hemisphere insolation by physically
  reasonable amount
• 41,000 year S. Hemisphere cycles has same lag
• 23,000 year ice volume leads S. Hemisphere summer
  insolation forcing
• Unreasonable relationship

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Global Transfer of Signals

• Changes in sea level
• Creating more temperate maritime or harsh
  continental climate regions
• Change in deep water circulation
• Relatively warm and salty water directed away
  from Southern Hemisphere in Atlantic
• Atmospheric CO2 levels
• Lower glacial CO2 levels can cool entire planet
• Reduce the amount of water vapor in atmosphere
• Most reasonable explanation for widespread
  changes
• Which came first CO2 change or glaciations?

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CO2 and Ice Volume

• Records of CO2 and ice volume well correlated
• Both must be related ultimately to orbital
  changes
• Strong correlation also indicates
• Two records are linked

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CO2 Drives Ice Sheets

• Sensible conclusion since CO2 affects temperature
  
• Therefore ice sheet mass balance
• However, CO2 should lead ice volume
• Pattern in records are not consistent with
  premise
• No persistent lag in signals
• Correlation between CO2 and temperature excellent
• CO2 does not lead ice volume

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Ice Sheets Drive CO2

• CO2 signal tracks closely ice volume
• Similar to other ice-driven climate signals
• CO2 record connected to changes in ocean
  circulation and carbon storage
• Ocean circulation responds quickly to changes in
  climate forcing
• Timing of changes in CO2 and ice volume match
  this expectation

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CO2 Feedbacks

• CO2 levels provide positive feedbacks to climate
  system
• Ice sheet growth caused CO2 decrease
• Lower CO2 levels further cool climate
• Increase ice sheet growth
• CO2 levels should also carry ice volume signal
• Other parts of climate system
• Changes in temperature or moisture

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Complete Story Unknown

• Text points to mismatches in CO2 and ice
  volume/temperature records
• Mismatch with temperature is not as great as
  believed only 2-3 years ago
• Mismatch with ice volume real?
• However, records still insufficiently refined
• To answer clearly chicken and egg dilemma
• Part of the uncertainty lies in poorly dated ice
  cores