Lecture 13 Orbital-Scale Interactions, Feedbacks and

Unresolved ProblemsThe Cause of Glacial Cycles?

(Chapter 11)

The Question!~1o/oo

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The questions:

1)Why 41 kyr cycle from 2.75 to 0.9 ma?

2)Why 100 kyr after 0.9 ma?

3)Why glacial cycle started about 2.75 ma?

The Question!

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Large glaciation

phase

1) Why 41 kyr cycle from 2.75 to 0.9 ma?

2) Why 100 kyr after 0.9 ma?

Given a small eccentricity forcing and a modest tilt forcing !

Why 41 kyr Cycle?

with 23kyr summer insolation domiant!

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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

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

Summer insolation

North vs. South

Out-of-phase (23kyr)

In phase (41kyr)

Plus CO2 positive feedback

Hard to quantify CO2 feedback

Why 100 kyr Cycle?

with 23kyr summer insolation dominant!

41 kyr as strong as before!

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NH response in phase with ice

European Climate Response

NH response in phase with ice

East Asia windblown dust

Tropical, SH response in phase with ice, so ?

Arabian dust in Indian Ocean

South Americapollen

New Zealand pollen

How does northern ice signal transfer to SH?

Ice-Driven Response

NH response

Secondary Response to North Atlantic Sea Surface Temperature

Ice-Driven Responses

North America Wind

(and its feedback on the ice-sheet growth)

Regions of Ice-Driven Responses

Mid-high latitude Northern Hemisphere

Why 100 kyr signal strong in both hemispheres?

LGMLGM

CO2CO2

ΔSST ΔTair

Liu et al., 2005Liu et al., 2005

CO2, Ice Sheet Forcing

CO2 is a global forcing

Why CO2 changes at 100 kyr?

CO2: global cooling

Antarctic, Greenland and CO2, Ice-Volume

In phase with ice volume

Ice lag Insolation:

Evidence of ice as a response to insolation

Visser et al., 2003, Nature

The Role of Southern Ocean, CO2, Tropics? The Role of Southern Ocean, CO2, Tropics?

Ice volume

Tropical SSTAntarctic air temp

Stout et al., 2007,

Why Southern Ocean changes first?Why Southern Ocean changes first?Proposal I: SH Spring Insolation Proposal I: SH Spring Insolation

He et al., 2010, in prep

SO/deep Pacific leads tropical SST, CO2, leads NH (e.g. Stott et al., 2007)

Obs

Model

Why Southern Ocean changes first?Why Southern Ocean changes first?Proposal II: Melting from the NH Ice sheet Proposal II: Melting from the NH Ice sheet

Bipolar SeesawNH to SH, but opposite sign

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?

How is 100 kyr cycle generated (or amplified) by the orbital forcing?

with 23kyr summer insolation dominant!

41 kyr as strong as before!

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Back to the last question:

Deglaciation Pacemaker

Strong summer insolation peaks

4-5 precession cycles92 kyr or 115 kyr

Orbital interaction

115

30

72

95

45

2-3 tilt cycles82 kyr or 123 kyr

Lesson from the last 150 kyrs

4-5 precession cycles, 92 kyr or 115 kyr2-3 tilt cycles, 82 kyr or 123 kyrSo the 100 kyr cycle occurs either 82-92 kyr or 115-123 kyr!

Ice-sheet modeling

Insolation (dominant 23,000, 41,000 year cycles) can generate some 100,000 year response

Early stage:

Sliding on soil

Later stage:

building on rock

Why greater ice volume occurs later stage of the glacial cycles?

Clark P.,

Simple response to insolation

After ice-sheet exceeds a critical value: strong (CO2) feedbacks kick in

Imbrie hypothesis, SPECMAP

Ice-Driven Climate Response:

fast response through the atmosphereIce forcing:

Height=> affect winds

Albedo=> affect temperature

Calve iceberges =>melting water=>THC

Reference for Reading

• Huybers P. and C. Wunsch, 2005: Obliquity pacing of the late Pleistocene glacial terminations. Nature, 434, 491-494

• Huybers P., 2006: Early Pleistoncene glaicial cycle and the integrated summer insolation forcing. 313, 508-511

• 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, 435-438

• Raymo M. and P. Huybers, 2008: Unlocking the mysteries of the ice ages. Nature, 451, 284-285

• Cheng et al., 2009, Ice Age Terminations, Science, 326, 248-

End of Lecture 13

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)

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

Against Antarctic ice sheet control of global ice volume at 23 kry

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

Insolation

Ice

CO2

Forcing and feedback

Climate