Lecture 10: Orbital Control of Ice Sheets

(Chapter 9)

Ice sheet cover at 20,000 years ago

The Last Glacial Maximum (LGM)

Ice-sheet balance

Slow Accumulation vs.

fast Ablation

Annual mean temperature

Implication:

which season is most important for ice sheet?

Winter vs. summer

Milankovitch theory

Small tilt +Aphelion

=>low summer insolation

favor glaciation

Large tilt + Perihelion

=> High summer insolation

favor deglaciation

Milankovitch theory

High summer insolation

=> net melting

=> deglaciation

Low summer insolation

=> net accumulation

=> glaciation

The Last Glacial Maximum (LGM): 21,000 yrs ago

Why the greatest ice sheet not over Siberia?

How does Insolation Control Ice Sheet

Equilibrium line, Climate point and ice-sheet formation

Cold

Warm

High insolation

northern/high equilibrium line

northern climate point

less ice-sheet formation

Low insolation

southern/low equilibrium line

southern climate point

more ice-sheet formation

Two positive feedbacks

Ice-albedo feedback

Ice-elevation feedback

Thermal inertial and Phase lag

Ice volume lags insolation

mdT/dt = SeiΩt-bT

Ice volume

lags Insolation

by about ¼ cycle

(ocean effect on seasonal cycle!)

Phase lag

Bedrock sinking

and

Bedrock rebound

Bedrock sinking

Bedrock feedbacks:

Initial elastic response is negative feedback;

Later viscous response

is positive feedback relative to the elastic sinking response by producing a delayed sinking response

Bedrock feedbacks

The cycle of ice-sheet growth and decay

Conceptual model for ice-sheet cycle:

•Repeat constant insolation

•Threshold temperature: 0oC summer, -10oC annual mean

Deep sea 18O and ice volume history

18O ~ O18/O16

O16 light, easy to evaporate

Deep sea higher 18O

more ice volume over land (evaporated to be accumulated)

Quantify δ18O changes

Δ (18Oland*land ice volume)+ Δ( 18Oocean*ocean volume)=0

18Oland* Δ land ice volume + Δ 18Oocean* ocean volume=0

1/(-40) = 18Oocean/ 18Oland = -Δ land ice volume / ocean volume = -100m sl/4000m sl = -1/40

Deep sea 18O and ice volume history~1o/oo

Sm

all glaciation phaseL

arge glaciation phase

but, relative magnitude ?

Revival of Milankovith Theory

1976

The remaining question:

100 kyr cycle?

Insolation Forcing and Ice Volume Response

Sm

all glaciation phase

Large glaciation

phase

Ice sheet 18O in the last 150,000 years Dominant 100 kyr cycle, why?

125 ka:

Last interglacial

21 ka:

Last Glacial Maximum

0 ka: Interglacial

Ice Core 18O in the last 100,000 years

is it ice volume effect?

Coral reefs lives in coastal water

– a proxy of sea level

Confirming sea level change

Uplifted coral reef terraces:

New Guinea (western Pacific)

Uplift of coral reefs:

Quantifying the ice volume from the sea level change -110m (115ka) to +6m (125ka)

LGM

IPCC/AR4 2007

Sea Level Reconstruction of the last 30 kyr

References for Further reading and analysis

• Hays et al., 1976, Variations in the Earth’s Orbit: pacemaker of the ice ages.” Science, 194, 1121-

• P. Huybers, C. Wunsch, Nature 434, 491 (2005).

• P. Huybers, 2006, Early Pleistocene Glacial Cycles

• and the Integrated Summer Insolation Forcing. Science, 313,508-511

End of Lecture 10