1. Lecture 13 Orbital-Scale Interactions, Feedbacks and Unresolved Problems The Cause of Glacial Cycles?
(Chapter 11)

2. Why glacial cycle started about 2.75 ma?
The Question!
~1o/oo
The questions:
Why 41 kyr cycle from 2.75 to 0.9 ma?
Why 100 kyr after 0.9 ma?
Why glacial cycle started about 2.75 ma?
Small glaciation phase
Large glaciation phase

3. Given a small eccentricity forcing and a modest tilt forcing !
The Question!
Why 41 kyr cycle from 2.75 to 0.9 ma?
Why 100 kyr after 0.9 ma?
Given a small eccentricity forcing and a modest tilt forcing !
Small glaciation phase
Large glaciation phase

4. with 23kyr summer insolation domiant!
Small glaciation phase
Large glaciation phase
Why 41 kyr Cycle?
with 23kyr summer insolation domiant!

5. Proposal 1:
High-than-normal insolation at perihelion is offset by the shorter length of summer month so no net 23 kyr anomaly
Problem:
forget about calendar month, caloric month or celestial month, still more summer insolation in summer for ablation

6. Proposal 2:
High-than-normal insolation at perihelion in the NH is offset by the reduced insolation in the SH,
Also SH d18O ~ -60 vs. NH ~ -40 so only 60% Antarctic melting is sufficient to compensate NH melting).
Problem 1:
NH is dominated by 41kyr not 23 kyr response
Problem 2:
Antarctic sufficiently cold little melting

7. Summer insolation
North vs. South
Out-of-phase (23kyr)
In phase (41kyr)

8. Plus CO2 positive feedback
Hard to quantify CO2 feedback

9. with 23kyr summer insolation dominant!
Why 100 kyr Cycle?
with 23kyr summer insolation dominant!
41 kyr as strong as before!
Small glaciation phase
Large glaciation phase

10. NH response in phase with ice
European Climate Response

11. NH response in phase with ice
East Asia windblown dust

12. Tropical, SH response in phase with ice, so ?
How does northern ice signal transfer to SH?
Arabian dust
in Indian Ocean
South America
pollen
New Zealand pollen

13. Ice-Driven Response NH response
Secondary Response to North Atlantic Sea Surface Temperature

14. Ice-Driven Responses North America Wind
(and its feedback on the ice-sheet growth)

15. Regions of Ice-Driven Responses Mid-high latitude Northern Hemisphere
Why 100 kyr signal strong in both hemispheres?

16. CO2, Ice Sheet Forcing ΔSST ΔTair LGM CO2 CO2 is a global forcing
Why CO2 changes at 100 kyr?
Liu et al., 2005

17. CO2: global cooling

18. Antarctic, Greenland and CO2, Ice-Volume
In phase with ice volume

19. Evidence of ice as a response to insolation
Ice lag Insolation:
Evidence of ice as a response to insolation

20. The Role of Southern Ocean, CO2, Tropics?
Ice volume
Tropical SST
Antarctic air temp
Visser et al., 2003, Nature

21. Why Southern Ocean changes first? Proposal I: SH Spring Insolation
Stout et al., 2007,

22. Why Southern Ocean changes first?
Proposal II: Melting from the NH Ice sheet
SO/deep Pacific
leads
tropical SST, CO2, NH
(e.g. Stott et al., 2007)
Obs
Model
He et al., 2010, in prep

23. Bipolar Seesaw NH to SH, but opposite sign

24. 100 kyr cycle: SH response to NH ice volume, which in turn seems to follow the summer insolation in the NH via ice melting global synchrony (in phase)  implication?

25. Back to the last question:
How is 100 kyr cycle generated (or amplified) by the orbital forcing?
with 23kyr summer insolation dominant!
41 kyr as strong as before!
Small glaciation phase
Large glaciation phase

26. Deglaciation Pacemaker
Strong summer insolation peaks
4-5 precession cycles
92 kyr or 115 kyr

27. Orbital interaction
115 30 72 95 45
2-3 tilt cycles
82 kyr or 123 kyr

28. Lesson from the last 150 kyrs
4-5 precession cycles, 92 kyr or 115 kyr
2-3 tilt cycles, 82 kyr or 123 kyr
So the 100 kyr cycle occurs either kyr or kyr!

29. Ice-sheet modeling
Insolation (dominant 23,000, 41,000 year cycles) can generate some 100,000 year response

30. Why greater ice volume occurs later stage of the glacial cycles?
Early stage:
Sliding on soil
Later stage:
building on rock
Clark P.,

31. Simple response to insolation
After ice-sheet exceeds a critical value: strong (CO2) feedbacks kick in
Imbrie hypothesis, SPECMAP

32. Ice-Driven Climate Response: fast response through the atmosphere
Ice forcing:
Height=> affect winds
Albedo=> affect temperature
Calve iceberges =>melting water=>THC

33. Reference for Reading
Huybers P. and C. Wunsch, 2005: Obliquity pacing of the late Pleistocene glacial terminations. Nature, 434,
Huybers P., 2006: Early Pleistoncene glaicial cycle and the integrated summer insolation forcing. 313,
Huybers P. and G. Denton, 2008: Antarctic temperature at orbital time scales controled by local summer duration. Nature Geo.,
Stout et al., 2007: Southern Hemisphere and Deep-Sea warming led Deglacial Atmospheric CO2 Rise and Tropical Warming. Science. 318,
Raymo M. and P. Huybers, 2008: Unlocking the mysteries of the ice ages. Nature, 451,
Cheng et al., 2009, Ice Age Terminations, Science, 326, 248-

34. End of Lecture 13

35. How to identify cause/effect in the observation?
This is a typical chicken and egg problem.
Hints may be found in the lead and lag because of the fast and slow responses of various components of the system
Direct insolation driven climate changes, follows the phase of insolation (e.g. monsoon)
Direct ice-dirven climate changes follows the phase of insolation (e.g. most 100 kyr global changes)

36. The Role of CO2: forcing and/or feedback?
Ice Volume and CO2 almost in phase,
with CO2 leading ~2kyr,
is it long enough as forcing or part of feedback

37. Global ice/climate: Northern or Southern summer insolation, which to follow?
Consistent with northern insolation forcing at both 41kyr and 23 kyr
Consistent with southern insolation forcing at 41kyr
Against Antarctic ice sheet control of global ice volume at 23 kry

38. Forcing and feedback
Insolation
Ice
Climate
CO2