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)

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 Δt=t1-t

0=1/λ ln (N

p(t

0)/N

p(t

1))

14C<30ky; orbital tuning (19 ky and 23 ky error 6 ky)

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

Isostatic reboundIsostatic rebound

Process by which the earth’s 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.

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

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

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

Thermohaline circulation

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

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

● 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

● 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 CO

2)? 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 globaleffects

The 1997-1998 El Nino createdtemperatures up to 6oC warmerover Northern America in theregion where the ice sheetsformed during the Ice Ages

Basics and relevant climate feedbacks● 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)

● Milankovitch forcing (changes in incoming solar summer radiation)

● etc... geothermal heating

Milankovitch forcing

● E=(0,0.06); 0.0167● Tilt=(21.9,24.5);

23.439● Precession (19 ky to

23 ky)

Mechanisms of the glacial cycles

● Physical feedbacks: albedo feedback (dT/dt~-albedo~-Vice

) with temperature-precipitation feedback (dV

ice /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~-V

ice gives similar result as before d2V

ice /dt2~-V

ice )

● Various theory based on Milankovitch forcing

Summer insolation● Orbital radiation natural candidate

for theory, but... ● dV

ice/dt=-k(i-i

0) ; i insolation, i

0 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

● 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