the ice age sequence in the quaternarynsl/lectures/phys20054/15lecture 12... · 2015. 4. 14. ·...
TRANSCRIPT
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
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