Lecture 34: Orbital (Milankovitch) Theory of the Ice Ages

warmer, less ice

colder, more ice

Glacial Interglacial

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

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.

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

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.

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

JulyEccentricity grossly exaggerated

152.5 x 106 km

147.5 x 106 km

Obliquity is responsible for seasons

Obliquity

• Current value: 23.5o

• Range: 22o-24.5o

• Period: 41,000 yrs

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).

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

• 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

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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Precession(axial)

Precession of the axis of the earthYear:

AxialPrecession

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

Today

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

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

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)

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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are needed to see this picture.

Eccentricity modulates precession

today

Difference in Northern Hemisphere insolation at summer solstice relative to today

Insolation anomaly at 10 ka relative to present

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)

Ice Decay (Deglaciation)

1. High obliquity (high seasonal contrast)

2. High eccentricity and NH summers during perihelion (hot summers in the north)

Classic Milankovitch Forcing: Insolation at 65°N, June 21

Icedecay

Icegrowth

Milankovitch Theory RevivedMilankovitch Theory Revived

Shackelton & Shackelton & Opdyke, 1972Opdyke, 1972

Pacific deep core V28-238Pacific deep core V28-238

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.

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

Icedecay

Icegrowth

I II III IV V VI

53 ky

41 ky

23 ky

21 ky

19 ky

Power spectrum of June Insolation at 65oN

obliquity

precession

Noteabsenceof 100Kyr power

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

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?

2. The Middle Pleistocene Transition Problem

32

16

Schulz and Zeebe (2006)

Insolation (June 65oN ) Milankovitch Forcing

100

kyr

100

kyr

Schulz and Zeebe (2006)

18O

Insolation

100-kyr world 41-kyr world

?

Icedecay

Icegrowth

I II III IV V VI

3. Stage 11/Termination V Problem

12

11

WeakForcing

StrongResponse

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