The Ice Age sequence in the Quaternary

http://www.gerhardriessbeck.de

Subdivisions of the Quaternary Period

System Series Stage Age (Ma)

Quaternary

Holocene 0–0.0117

Pleistocene

Tarantian (Upper) 0.0117–0.126

Ionian (Middle) 0.126–0.781

Calabrian (Lower) 0.781–1.806

Gelasian (Lower) 1.806–2.588

Milankovitch Cycle Ice sheet changes Interglacial oscillations

Record is based on the analysis of ice layers by 18O, 13C, isotope fractionation, long-lived radioactive isotopes, aerosol and dust composition, and other physical and chemical properties of annual, centennial, and millennial ice layers

Vostok ice core in Antarctica 3740 m

Comparison of the 18O, 2D fractionation based temperature data and CO2 abundance data in the Vostok ice core material shows a strong correlation between the temperature and CO2.

Russian, France, US collaboration

Milankovitch cycle or astronomical forcing The Milankovitch cycles are periodic oscillations associated with the eccentricity of the Earth orbit, the precession of Earth around its axis and the obliquity cycle in the axial tilt of Earth that influence the insolation F0 :

Annual orbit causes summer winter effect, eccentricity of ellipsoid orbit changes from near spherical (now) to elliptic orbit with a periodicity of t≈95,000 years.

Earth rotates around its axis (day night period). The direction of the spin vector precesses in a gyroscopic motion with a period of t≈25,000 years due to the tidal forces of the sun that give Earth an oblate shape.

Earth axis has a tilt with respect to the orbital plane, which varies by 2.4o with a period of t ≈41,000 years changing the extend of seasonal insolation.

Three periodic motions impact the distance of earth or earth’s hemispheres from the sun

Flux=Solar Constant=Insolation): F0=1.37·103 W/m2, Rearth = 6371 km

Solar power incident on earth: S0 = ·Rearth

2·F0=1.75·1017 W Increase in eccentricity causes colder winters because of insolation reduction

Obliquity or change of axis tilt modifies summer-winter differences by bringing northern hemisphere closer to sun

Precession introduces variation in distance between northern and southern hemisphere from sun

Overall impact: F ~1/r2

The change in orbital eccentricity changes the flux as a function of sun-earth distance

Orbital eccentricity

Axis Precession

The spin precession around the axis introduces a regular periodicity for the amplitude of temperature for both the northern and southern seasons

Axis Obliquity

The obliquity or change of spin axis tilt modifies the distance and inflow angle for the solar flux, changing the summer winter temperature difference

Model predictions for Earth temperature variations by coupling various oscillation shows sequence of hot and cold periods.

Fourier Analysis

All three distance fluctuations have a different period determining the periodicity of the solar flux constant or insolation F(t)!

Milankovitch Cycle Demonstrator

Milankovitch theory seems to predict the long-term temperature oscillation pattern reflected in the observational data of the Vostok ice core.

The last four ice-ages

Ice-age 360ka BP 260 ka BP 160 ka BP 40 ka BP

Warm-Period 420ka BP 320 ka BP 240 ka BP 120 ka BP now

Ice sheets and thermohaline circulation The change in insolation changes the extension and volume of the sea ice coverage. Extended ice sheet areas cause higher reflection of incoming flux, reducing insolation and further enhancing the cooling effect. The extension/shrinkage of the ice sheet is a balance between snow accumulation (snow fall) and ablation (melting, slipping, calving) and operates on a delayed timescale compared to Milankovitch forcing. The thermohaline circulation down-wells further south for extended ice sheet conditions, reducing heat transfer north and the associated ablation process. .

Accumulation and ablation of ice shelves Accumulation and ablation of ice depends on the overall temperature conditions the net balance is called ice mass balance. At very cold temperatures the ice mass balance is positive but small, because of the lack of snow fall. Ice sheet can only develop when temperatures are not too low, humidity and wind effects sufficient during winter to generate the necessary precipitation, and the summer are not too warm to reduce melting process. This needs to be included in model!

http://www.gerhardriessbeck.de

Model predictions for ice sheet development in the Pleistocene considering the impact of mid latitude atmospheric circulation (westerlies) on growth rate, thickness, and shape depending on wind direction, precipitation and melting .

growth rate: 105 m2/yr

Roe & Lindzen J. Climate 14 (2000) 1450

Growth rate (height, area, volume) for different feedbacks (SL precipitation, SW wind effects).

The growth and equilibrium conditions depend critically on the insolation as well as the associated local climate factors such as temperature and wind directions !

Initial growth rate of ice sheet slows down with increasing cooling through extended ice sheet, decreasing temperature, humidity and precipitation!

Glacial-Interglacial Oscillations

Appears in ice coverage, sea-level, atmospheric conditions, and ocean currents!

viewed from north

The earth as a box model .

Glacial-interglacial oscillations

Gildor & Tziperman, Paleocanography 15(2000) 605

Included are: Heat exchange between atmosphere and ocean

sisiow

wpwatm

TD

ffTDC

Q

0

Cpw heat capacity, Dw thickness of mixed layer, Dsi thickness of sea ice, potential temperature, time constant for exchange, insulating factor, fow and fsi fractions of open water and sea ice

Salinity exchange by precipitation P (fresh water inflow) and evaporation E

0SEPQatm

S

Temperature exchange and heat flux associated with sea ice melting and freezing processes

TTVC

Q si

si

wpwsi

T

0

Salinity exchange due to sea ice melting and freezing processes with L as latent heat

0SL

QQ

si

si

Tsi

S

100ka glacial oscillation pattern without Milankovitch forcing

Glacial oscillation pattern with Milankovitch forcing

The 100 ka glacial oscillations rely on the feedback coupling of sea ice albedo and precipitation, temperature. Milankovitch forcing, overlaying the oscillator system, yielding temperature, salinity, and THC variations close to observations . .

bo

x fr

acti

on

te

mp

era

ture

oC

Salin

ity

‰

bo

x fr

acti

on

te

mp

erat

ure

oC

TH

C in

po

lar

bo

x

tem

per

atu

re o

C

THC

in p

ola

r b

ox

Salin

ity

‰

bo

x fr

acti

on

From Past to Future

What causes the predictions of rapidly increasing global temperature on such a short time scale? Greenhouse again