lecture 34: orbital (milankovitch) theory of the ice ages
TRANSCRIPT
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Lecture 34: Orbital (Milankovitch) Theory of the Ice Ages
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warmer, less ice
colder, more ice
Glacial Interglacial
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Orbital forcing of Earth’s climate
Changes in Earth’s orbital geometry(eccentricity, tilt, precession)
Changes in the seasonal distribution ofInsolation (heat) as a function of latitude
Glacial-interglacial climate change
Amplified by other processes
CO2, albedofeedback
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James Croll• Scottish (1821-1890)
• Millwright, carpenter, tea shopkeeper, electrical sales, hotelkeeper, insurance salesman, janitor, Geologic Survey, Fellow of the
Royal Societydecreases in winter radiation would favor snow accumulation, coupled this to the idea
of a positive ice-albedo feedback to amplify the solar variations.
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Milutin Milankovitch, Serbian mathematician(1879-1958) •1941, “Canon of Insolation of the Earth and Its Application to the Problem of the Ice Ages,” 626 pages
•improved upon Croll's work partly by making more precise calculations (all done by hand!)
•Emphasized the importance of decreases in summer radiation which favors snow accumulation and glacial advance
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The equilibrium line of a glacier is
the location whereaccumulation
equals ablation If accum>ablation,
glacier advancesIf ablation>accum
glacier retreats
Decreased summer insolation lowers the equilibrium lineand glacier advances.
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50 0100150200250300
Latit
ude
of e
quiv
alen
t ins
olat
ion
65
60
70
75
600 550 500 450 400 350 300
Cool summersin northern hemisphere
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JulyEccentricity grossly exaggerated
152.5 x 106 km
147.5 x 106 km
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Obliquity is responsible for seasons
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Obliquity
• Current value: 23.5o
• Range: 22o-24.5o
• Period: 41,000 yrs
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Effect of Obliquity on Insolationdifference in obliquity from 22 to 24.5o with other parameters held at present values
Effect on insolation is greatest at high latitudesSame sign for respective summer season (hemispheric response is in phase).
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varies at a period of41 kyrs.Obliquity
frequency [1/ky] Period [ky] Amplitude 0.02439 40.996 0.011168 0.02522 39.657 0.004401 0.02483 40.270 0.003010 0.01862 53.714 0.002912 0.03462 28.889 0.001452
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• Current value: 0.017• Range: 0-0.06• Period(s): ~100,000 yrs ~400,000 yrs
Eccentricity
Eccentricity =a - p
a + p
a = apehlion distancep = perihelion distance
a a p
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Eccentricity changesthe total insolationreceived by theEarth but the difference is small!
0.5 W m-2/342.7 W m-2 = 0.15%
Dominant periods are at ~400 and near 100 kyrs
Eccentricity frequency [1/ky] Period [ky] Amplitude 0.00246 406.182 0.010851 0.01055 94.830 0.009208 0.00807 123.882 0.007078 0.01014 98.607 0.005925 0.00769 130.019 0.005295
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QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.
Precession(axial)
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Precession of the axis of the earthYear:
AxialPrecession
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Precession of the Ellipse
• Elliptical shape of Earth’s orbit rotates
– Precession of ellipse is slower than axial precession
– Both motions shift position of the solstices and equinoxes
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Today
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Precession of the Equinoxes
• Earth’s wobble and rotation of its elliptical orbit produce precession of the solstices and equinoxes– One cycles takes
23,000 years• Simplification of complex
angular motions in three-dimensional space
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Extreme Solstice Positions• Today June 21 solstice is near aphelion (July 4)
– Solar radiation a bit lower making summers a bit cooler
• Configuration reversed ~11,500 years ago– Precession moves June solstice to perihelion– Solar radiation a bit higher, summers are warmer– Assumes no change in eccentricity
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Effect of precession from its minimum value (boreal winter at perihelion) to its maximum value (boreal summer)
Precession affects insolation at both high and low latitudes.Opposite sign in northern and southern hemispheres for respective season(out of phase)
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Dominant periodsat ~19, 22 and 24 kyrs
Precession frequency [1/ky] Period [ky] Amplitude 0.04221 23.690 0.018839 0.04467 22.385 0.016981 0.05275 18.956 0.014792 0.05236 19.097 0.010121 0.04326 23.114 0.004252
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QuickTime™ and a decompressor
are needed to see this picture.
Eccentricity modulates precession
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today
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Difference in Northern Hemisphere insolation at summer solstice relative to today
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Insolation anomaly at 10 ka relative to present
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Ice Growth (glacial) Configuration(cool summers)
1. Low obliquity (low seasonal contrast)
2. High eccentricity and NH summers during aphelion (cold summers in the north)
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Ice Decay (Deglaciation)
1. High obliquity (high seasonal contrast)
2. High eccentricity and NH summers during perihelion (hot summers in the north)
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Classic Milankovitch Forcing: Insolation at 65°N, June 21
Icedecay
Icegrowth
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Milankovitch Theory RevivedMilankovitch Theory Revived
Shackelton & Shackelton & Opdyke, 1972Opdyke, 1972
Pacific deep core V28-238Pacific deep core V28-238
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Variations in the Earth's Orbit: Pacemaker of the Ice Ages J. D. Hays, John Imbrie, N. J. ShackletonScience, 194, No. 4270, (Dec. 10, 1976), pp. 1121-1132
(eccentricity)
(obliquity)
(precession)
Marine oxygen isotope record shows
the same periodicities predicted by
orbital forcing.
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Problems for the Milankovitch Theory
Milutin Milankovitch
1879-1958
1. 100,000-year problem
4. Causality Problem:Timing of Term II
2. The Mid Pleistocene transition problem
3. Stage 11 (Termination V) problem
So many problems
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Icedecay
Icegrowth
I II III IV V VI
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53 ky
41 ky
23 ky
21 ky
19 ky
Power spectrum of June Insolation at 65oN
obliquity
precession
Noteabsenceof 100Kyr power
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2
2.5
3
3.5
4
4.50 50 100 150 200 250 300 350 400
( )Time ka
0
5
10
15
0 0.02 0.04 0.06 0.08 0.1
frequency
100 ky
41 ky
23 ky53 ky
41 ky
23 ky
21 ky
19 ky
Insolation65oN
Benthic18O
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Forcing
Response
1. The 100-kyr Problem
small
big
?
Classic Milankovitch forcing(might consider alternatives)
Why does the climate system have so much 100-kyr powerwhen the forcing is so small?
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2. The Middle Pleistocene Transition Problem
32
16
Schulz and Zeebe (2006)
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Insolation (June 65oN ) Milankovitch Forcing
100
kyr
100
kyr
Schulz and Zeebe (2006)
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18O
Insolation
100-kyr world 41-kyr world
?
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Icedecay
Icegrowth
I II III IV V VI
3. Stage 11/Termination V Problem
12
11
WeakForcing
StrongResponse
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A. L. Thomas, G. M. Henderson, et al., 2009. Penultimate Deglacial Sea-Level Timing from U/Th Dating of Tahitian Corals. Science doi:10.1126/science.1168754 (23 April) 2009
Termination II/Stage 5 Problem
4. Causality Problem sea level begins to rise before boreal summer insolation