Transcript document

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 oceanatmosphere models - physics (models and
theory)
Proxies and dating
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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 Δt=t1-t0=1/λ ln (Np(t0)/Np(t1))
14C<30ky; orbital tuning (19 ky and 23 ky error 6 ky)
Orbital signal in the sediment
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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
Isostatic rebound
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Process by which the earth’s
crusts is adjusting from the
pressure of a large ice sheet.
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Using this process certain
aspects of the ice sheet can be
calculated.
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The main pieces of evidence for
this rebound are the raised
shorelines.
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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.
Observed phenomena
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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
Sea level changes
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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
Climatic and oceanographic
variations in and around NA fromice
cores and marine sediment
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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
Thermohaline circulation
Hysteresis loop of THC
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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
Models of intermediate complexity
(CLIMBER-2)
Changes in surface air temperature
caused by shutdown of NADW
formation (HadCM3)
Variations in atmospheric CO2 and
relative changes of air temperature
(Vostok core)
Summary of the observation
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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
Ice ages theory- open issues
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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?
El Nino has global
effects
The 1997-1998 El Nino created
temperatures up to 6 oC warmer
over Northern America in the
region where the ice sheets
formed during the Ice Ages
Basics and relevant climate feedbacks
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Energy balance, and the ice albedo feedback: dT/dt=(1-α)SW-LW;
higher albedo (α) results cooling (more ice -> 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-> 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)
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Milankovitch forcing (changes in incoming solar summer radiation)
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etc... geothermal heating
Milankovitch forcing
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E=(0,0.06); 0.0167
Tilt=(21.9,24.5);
23.439
Precession (19 ky to
23 ky)
Mechanisms of the glacial cycles
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Physical feedbacks: albedo feedback (dT/dt~-albedo~-Vice) with
temperature-precipitation feedback (dVice /dt~precip~T) 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 -> colder ice-sheet surface -> less abl. -> more acc. -> volume
increase -> sinking and moves into area of less accumulation more
ablation: dprecip/dt~-Vice gives similar result as before d2Vice /dt2~-Vice
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Various theory based on Milankovitch forcing
Summer insolation
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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
Climate puzzle
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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