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

Lecture 34: Orbital (Milankovitch) Theory of the Ice Ages. warmer, less ice. Holocene. Last glaciation. colder, more ice. Interglacial. Glacial. Orbital forcing of Earth’s climate. Changes in Earth’s orbital geometry (eccentricity, tilt, precession).

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

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

  2. warmer, less ice Holocene Last glaciation colder, more ice Interglacial Glacial

  3. Orbital forcing of Earth’s climate Changes in Earth’s orbital geometry (eccentricity, tilt, precession) Changes in the seasonal distribution of Insolation (heat) as a function of latitude Amplified by other processes CO2, albedo feedback Glacial-interglacial climate change

  4. James Croll • Scottish (1821-1890) • Millwright, carpenter, tea shopkeeper, electrical sales, hotelkeeper, insurance salesman, janitor, Geologic Survey, Fellow of the Royal Society decreases in winter radiation would favor snow accumulation, coupled this to the idea of a positive ice-albedo feedback to amplify the solar variations.

  5. 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 by making more precise calculations of solar insolation (all done by hand!) • Emphasized the importance of decreases in summer radiation which favors snow accumulation and glacial advance

  6. The equilibrium line of a glacier is the location where accumulation equals ablation If accum>ablation, glacier advances If ablation>accum glacier retreats Decreased summer insolation lowers the equilibrium line and glacier advances.

  7. Cool summers in northern hemisphere 60 65 70 Latitude of equivalent insolation 75 250 200 150 100 0 300 50 550 500 450 400 350 300 600

  8. Eccentricity grossly exaggerated Today’s orbital parameters 152.5 x 106 km July 147.5 x 106 km

  9. Obliquity is responsible for seasons Dec 21 June 21 Mar 21 Sept 21

  10. Obliquity • Current value: 23.5o • Range: 22o-24.5o • Period: 41,000 yrs

  11. Effect of Obliquity on Insolation difference in obliquity from 22 to 24.5o with other parameters held at present values Boreal summer W m-2 summer austral Effect on insolation is greatest at high latitudes Same sign for respective summer season (hemispheric response is in phase).

  12. varies at a period of 41 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

  13. Eccentricity a a p • Current value: 0.017 • Range: 0-0.06 • Period(s): ~100,000 yrs ~400,000 yrs a - p Eccentricity = a + p a = aphelion distance p = perihelion distance

  14. Eccentricity changes the total insolation received by the Earth but the difference is small! 0.5 W m-2/ 1370 W m-2 = 0.04% Dominant periods are at ~400 and near 100 kyrs Eccentricity frequency [1/ky]Period [ky]Amplitude 0.00246406.1820.010851 0.0105594.8300.009208 0.00807123.8820.007078 0.0101498.6070.005925 0.00769130.0190.005295

  15. Precession (axial)

  16. Precession of the axis of the earth Year: Axial Precession

  17. 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 relative to Earth-Sun distance

  18. The effect of axial and elliptical precession is the change the timing during the year of perihelion and aphelion 152.5 x 106 km July 147.5 x 106 km

  19. Precession of the Equinoxes July 4 a • 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 p Jan 4 p a a p a p

  20. 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 Cool summers Warm summers

  21. Effect of precession from its minimum value (boreal winter at perihelion) to its maximum value (boreal summer) Boreal summer summer austral Precession affects insolation at both high and low latitudes. Opposite sign in northern and southern hemispheres for respective season (out of phase)

  22. Dominant periods at ~19, 22 and 24 kyrs Precession frequency [1/ky]Period [ky]Amplitude 0.0422123.6900.018839 0.0446722.3850.016981 0.0527518.9560.014792 0.0523619.0970.010121 0.0432623.1140.004252

  23. Eccentricity modulates the amplitude of precession 100, 400 kyrs a p2 a=p1 19, 23 kyrs

  24. The three orbital parameters combine to change boreal summer insolation today

  25. Last Ice Age Difference in Northern Hemisphere insolation at summer solstice relative to today Holocene interglacial

  26. Ice Growth (glacial) Configuration • (cool summers) • Low obliquity (low seasonal contrast) • NH summers during aphelion (cold summers in the north) • High eccentricity

  27. Ice Decay (Deglaciation) Configuration • High obliquity (high seasonal contrast) • NH summers during perihelion (hot summers in the north) • High eccentricity

  28. Classic Milankovitch Forcing: Insolation at 65°N, June 21 Ice decay Ice growth

  29. Milankovitch Theory Revived The measurement of long, continuous oxygen isotope variations in deep-sea were instrumental in providing evidence for the Milankovitch theory of the ice ages. Pacific deep core V28-238 Shackleton & Opdyke, 1972

  30. Variations in the Earth's Orbit: Pacemaker of the Ice Ages J. D. Hays, John Imbrie, N. J. Shackleton Science, 194, No. 4270, (Dec. 10, 1976), pp. 1121-1132 (eccentricity) (obliquity) (precession) Marine oxygen isotope record shows the same periodicities predicted by orbital forcing.

  31. Marine isotope record provides support for the Milankovitch Theory but there are still some unanswered questions So many problems 1. 100,000-year problem Milutin Milankovitch 1879-1958 2. The Mid Pleistocene transition problem 3. Stage 11 (Termination V) problem 4. Causality Problem: Timing of Term II

  32. I II III IV V VI Ice decay 5 1 11 9 7 13 14 8 4 10 2 6 12 Ice growth

  33. 100 ky 41 ky 23 ky Insolation 65oN Benthic 18O

  34. 1. The 100-kyr Problem Classic Milankovitch forcing (might consider alternatives) Forcing small ? big Response Why does the climate system have so much 100-kyr power when the forcing is so weak? but recall that eccentricity also modulates amplitude of the precession cycle

  35. 2. The Middle Pleistocene Transition 32 16 Schulz and Zeebe (2006)

  36. Insolation (June 65oN ) Milankovitch Forcing Schulz and Zeebe (2006) 100 kyr 100 kyr

  37. 18O 100-kyr world 41-kyr world Insolation (forcing) ?

  38. 3. Stage 11/Termination V Problem I II III IV V VI Ice decay 11 Strong Response 12 Weak Forcing Ice growth

  39. 4. Causality Problem sea level begins to rise before boreal summer insolation Termination II/Stage 5 Problem 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

  40. NATURE|Vol 451|17 January 2008| Milutin Milankovitch (1879-1958)

  41. Non-linear ice volume models invoking threshold response How to explain the 100-kyr power? Ice Sheet Growth Lags Summer Insolation

  42. Model 1: Calder (1974) 18O Ice volume V = ice volumei = summer insolation at 65°N i0 = insolation threshold k = kA (accumulation) if i < i0 k = kM (melting) if i > i0 18O (observed) Ice Volume model Model produces some 100-kyr power

  43. Model 2: Imbrie and Imbrie (1980) • Written in dimensionless form (i.e., variables are divided by a scaling value) 18O Ice volume dV/dt = (i-V)/ Not observed In climate record (time series might be too short) V = ice volume i = summer insolation at 65°N t = tM if V > i (melting) t = tA otherwise Ice Volume model 18O

  44. Hypothesis: Roy et al. (2004) • 41 kyr world • Less ice volume • Smaller, thinner ice sheet • Ice sheets responded linearly to • obliquity forcing • 100-kyr world • New source of low-frequency variability: • Ice volume and thickness increases • Ice sheet mass is capable of surviving • weak summer insolation maxima • Deglaciations begin to skip precession • and/or obliquity cycles • Lenthens duration of glacial cycle • New source of high-frequency variability: • Large, thick ice-sheet begins behaving • dynamically (non-linearly) • 100-kyr cycle driven by internal ice • sheet dynamics (paced by insolation)

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