Paleoclimate: Observations and dynamics

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Paleoclimate Observations and dynamics

• Goran Georgievski
• - climate conditions in the long term earth
  history - geology (observation, data analysis and
  interpretation)
• - development of computers and coupling
  ocean-atmosphere models - physics (models and
  theory)

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Proxies and dating

• Instrumental era 200-300 years measurment of
  climatic parameters
• Enviromental parameters for the past,
  reconstructed from proxy variables and empiricaly
  calibrated to the climatic parameters (T, P,...)
  of interest
• Chemical, pysical or biological processes leaves
  record in the sediment fractionate isotopic
  ratio (15N/14N -metabolism of marine bacteria in
  anoxic condition), alkenon (di- and tri-
  unsaturated chains), growth of algae
• 2 sources of data (2x106 years BP) sediment
  cored from the oceans bottom (reconstruction of
  SST, SSS, ventilation, global ice volume) and ice
  core records from Antarctica and Grenland
  (reconstruction of air temperature from isotopic
  ratio 18O/16O)
• Dating natural decay of radioactive isotopes
  ?tt1-t01/? ln (Np(t0)/Np(t1)) 14Clt30ky
  orbital tuning (19 ky and 23 ky error 6 ky)

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Orbital signal in the sediment

• Linking sedimentary cycles to earth orbit
  perturbation
• Changes in incoming solar radiation changes
  sediment properties, fossil communities and
  chemical properties
• Southeren Sicily, Italy sections of carbonate
  cycles or sapropel (brownish colored layers
  enriched in organic C), grey-white, beige-white
  reflects precession cycles, and bold (white)
  reflects eccentricity cycle

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Isostatic rebound

• Process by which the earths crusts is adjusting
  from the pressure of a large ice sheet.
• Using this process certain aspects of the ice
  sheet can be calculated.
• The main pieces of evidence for this rebound are
  the raised shorelines.
• The picture is from the north west coast of
  Norway. The terraces and strand lines can be seen
  to be exposed at a considerable height.

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Observed phenomena

• Ice Sheets and Sea level rise (120-130 m since
  LGM or 50 million km3)
• Lower temperature, in average
• Dansgaard-Oeschger events (rapid warming of
  Grenland)
• Heinrich events (sudden cooling of northeren
  North Atlantic)
• Variations in large scale ocean circulation ( THC
  hysteretic behavior)
• Variations of CO2 distribution

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Sea level changes

• Connected with ice-sheet volume
• causes for relative sea changes water volume,
  volume of the ocean basin, distribution of water
  due to earth rotation changes and various dynamic
  factors
• Reconstruction sea level position (Acropora
  palmata, Fairbanks, 1989), O isotope variations
  (during glaciation oceans depleted in 16O,
  Shackelton, 1987), volumetric ice estimate
  (Flint, 1969)
• Lambeck Chappell, 2001

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Climatic and oceanographic variations in and
around NA fromice cores and marine sediment

• Isotope thermometry O (Dansgaard et al)
• Biomolecular thermometry - alkenon
• Magnetic susceptibility
• Carbon ratio (lower, weak ventilation), 3 states
  high for present day, high but lower than
  present, and low

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Thermohaline circulation
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Hysteresis loop of THC

• Conceptualized climate system representing the
  temperature of the northeren NA as a function of
  fresh water input to the north NA
• Present day upper branch
• Lower regime colder with the large freshwater
  influx

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Models of intermediate complexity (CLIMBER-2)
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Changes in surface air temperature caused by
shutdown of NADW formation (HadCM3)
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Variations in atmospheric CO2 and relative
changes of air temperature (Vostok core)
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Summary of the observation

• Before 3.2 M small oscilations almost stable
  warm condition and no ice sheets
• 2.7 My BP ice-sheet start to wax and wane in a
  periodic cycle, first with the 41 ky periodicity
  which turn into 100 ky about 800 ky ago
• Saw-tooth structure long glaciation (90 ky)
  short deglaciation (10 ky)
• Variations of atmospheric CO2
• Some phase locking to Milankovitch forcing
• Global extent of the glacial signal

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Ice ages theory- open issues

• Besides the need for theory to explain these
  observations, we need to address the following
  question regarding the cycles dynamics
• Are the cycles externally forced? By what? Or
  perhaps internally produced (self sustained)
  within the climate system?
• Are the cycles produced by physical climate
  components (i.e. excluding CO2)? By the
  biogeochemical components? Both? Only amplified
  by CO2 variations that are, in turn, induced by
  the physical system? Which components of the
  physical climate system participate in the
  glacial dynamics and on what time scales?
• Are the cycles driven from northeren hemisphere
  where most of the land ice volume changes occur,
  or from some other region? What phase lags should
  we expect between northeren and southeren
  hemispheres?

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Basics and relevant climate feedbacks

• Energy balance, and the ice albedo feedback
  dT/dt(1-a)SW-LW higher albedo (a) results
  cooling (more ice -gt higher albedo)
• Ice sheets dynamics and geometry (exotic and
  complex) - nonnewtonian
  fluid (stress is related to strain with Glenn's
  law) - parabolic profile (based on
  balance of hydrostatic pressure)
  - accumulation/ablation (complex function of
  height and latitude) - ice streams
  (flow from acc. to abl. zone m/year, transient 4
  km/y) - calving (floating and breaking
  the ice sheet)
  - dust loading (reduces albedo 0.7-gt
  0.1-0.4, 2-3 times more radiation)
• Temperature precipitation feedback (higher T,
  more moisture, stronger hydrological cycle,
  larger accumulation but after some threshold
  higher temperature results in net higer ablation
  )
• Isostatic adjustment (ice sinks into the earth
  crust due to lower density 1/3, and earth rises
  on the border of the ice on the time scale 1000ts
  y)
• Milankovitch forcing (changes in incoming solar
  summer radiation)
• etc... geothermal heating

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Milankovitch forcing

• E(0,0.06) 0.0167
• Tilt(21.9,24.5) 23.439
• Precession (19 ky to 23 ky)

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Mechanisms of the glacial cycles

• Physical feedbacks albedo feedback
  (dT/dt-albedo-Vice) with temperature-precipitati
  on feedback (dVice /dtprecipT) combining gives
  d2T/dt2-T which has oscilatory solutions but
  with to short periodicity, no saw-tooth and no
  nonlinearity
• Isostatic adjustment Load acumulation feedback
  (higher ice sheet elevation -gt colder ice-sheet
  surface -gt less abl. -gt more acc. -gt volume
  increase -gt sinking and moves into area of less
  accumulation more ablation dprecip/dt-Vice
  gives similar result as before d2Vice /dt2-Vice
  )
• Various theory based on Milankovitch forcing

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Summer insolation

• Orbital radiation natural candidate for theory,
  but...
• dVice/dt-k(i-i0) i insolation, i0 mean
  insolation, simplest equation but pure fit to
  observation
• Proxy records shows correlation with precession
  and obliquity but not to the 100 ky cycle of
  eccentricity

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Climate puzzle

• Changes in insolation (Milankovitch cycle)
  initiate glacial cycles
• The rise in atmospheric CO2 levels providing
  strong global warming effect (a better
  understanding of carbon cycle one of the main
  challanges)
• Changes in the ocean circulation