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

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

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

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

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

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

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 ? How does northern ice signal transfer to SH? Arabian dust in Indian Ocean South America pollen New Zealand pollen

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?

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

CO2: global cooling

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

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

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

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

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

Bipolar Seesaw NH 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?

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

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

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

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 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

Why greater ice volume occurs later stage of the glacial cycles? Early stage: Sliding on soil Later stage: building on rock 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 atmosphere Ice 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

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

Forcing and feedback Insolation Ice Climate CO2