1. Lecture 10: Orbital Control of Ice Sheets
(Chapter 9)

2. Ice sheet cover at 20,000 years ago The Last Glacial Maximum (LGM)

3. Ice-sheet balance Slow Accumulation vs. fast Ablation Implication:
Annual mean temperature
Implication:
which season is most important for ice sheet?
Winter vs. summer

4. Milankovitch theory Small tilt +Aphelion =>low summer insolation
favor glaciation
Large tilt + Perihelion
=> High summer insolation
favor deglaciation

5. Milankovitch theory
High summer insolation
=> net melting
=> deglaciation
Low summer insolation
=> net accumulation
=> glaciation

6. The Last Glacial Maximum (LGM): 21,000 yrs ago
Why the greatest ice sheet not over Siberia?

7. How does Insolation Control Ice Sheet
Equilibrium line, Climate point and ice-sheet formation
Cold
Warm

8. 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

9. Two positive feedbacks
Ice-albedo feedback
Ice-elevation feedback

10. Thermal inertial and Phase lag
Ice volume lags insolation
mdT/dt = SeiΩt-bT

11. Phase lag Ice volume lags Insolation by about ¼ cycle
(ocean effect on seasonal cycle!)

12. Bedrock sinking
Bedrock sinking
and
Bedrock rebound

13. Bedrock feedbacks 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

14. The cycle of ice-sheet growth and decay

15. Conceptual model for ice-sheet cycle:
Repeat constant insolation
Threshold temperature: 0oC summer, -10oC annual mean

16. 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)

17. 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

18. Deep sea 18O and ice volume history
~1o/oo
Small glaciation phase
Large glaciation phase

19. Revival of Milankovith Theory
1976
but, relative magnitude ?

20. Insolation Forcing and Ice Volume Response
Small glaciation phase
Large glaciation phase
The remaining question:
100 kyr cycle?

21. Ice sheet 18O in the last 150,000 years Dominant 100 kyr cycle, why?
0 ka: Interglacial
21 ka:
Last Glacial Maximum
125 ka:
Last interglacial

22. Ice Core 18O in the last 100,000 years
is it ice volume effect?

23. Confirming sea level change
Coral reefs lives in coastal water
– a proxy of sea level

24. Uplifted coral reef terraces:
New Guinea (western Pacific)

25. Uplift of coral reefs:
Quantifying the ice volume from the sea level change -110m (115ka) to +6m (125ka)

26. Sea Level Reconstruction of the last 30 kyr
LGM
IPCC/AR

27. 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,

28. End of Lecture 10