Upload
scarlett-bradley
View
217
Download
3
Tags:
Embed Size (px)
Citation preview
Lecture 13 Orbital-Scale Interactions, Feedbacks and
Unresolved ProblemsThe Cause of Glacial Cycles?
(Chapter 11)
The Question!~1o/oo
Sm
all glaciation phaseL
arge glaciation phase
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!
Sm
all glaciation phase
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!
Sm
all glaciation phaseL
arge glaciation phase
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!
Sm
all glaciation phaseL
arge 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 ?
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!
Sm
all glaciation phaseL
arge glaciation phase
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