Understanding Milankovitch Cycles and Climate Variability

1. Chapter 14—Part 2 Milankovitch cycles/ Chaotic obliquity variations

2. Marine 18O record in carbonate sediments • Remember: • High 18O  low T • Low 18O  high T • (because polar ice is • depleted in 18O)

3. Ice Age Cycles: 100,000 years between ice ages Smaller cycles also recorded every 41,000 years*, 19,000 - 23,000 years *This cycle dominates prior to 0.9 kA

4. Asymmetric cycles: • Slow cooling • Rapid warming after Bassinot et al. 1994

5. Eccentricity (orbit shape) 100,000 yrs 400,000 yrs Obliquity (tilt) 41,000 yrs Precession (wobble) 19,000 yrs 23,000 yrs 22o http://www.geo.lsa.umich.edu/~crlb/COURSES/205/Lec20/lec20.html

6.   Q: What makes eccentricity vary?A: The gravitational pull of the other planets • The pull of another • planet is strongest • when the planets • are close together • The net result of • all the mutual inter- • actions between • planets is to vary the • eccentricities of their • orbits

7. Eccentricity Variations • Current value: 0.017 • Range: 0-0.06 • Period(s): ~100,000 yrs ~400,000 yrs

8. 800 kA Today Unfiltered Orbital Element Variations 0.06 65o N solar insolation Imbrie et al., Milankovitch and Climate, Part 1, 1984

9. Q: What makes the obliquity and precession vary?A: First, consider a better known example… Example: a top • Gravity exerts a torque • --i.e., a force that acts • perpendicular to the spin • axis of the top • This causes the top to • precess and nutate g

10. Q: What makes the obliquity and precession vary?A: i) The pull of the Sun and the Moon on Earth’s equatorial bulge N g g Equator • The Moon’s torque on • the Earth is about twice • as strong as the Sun’s S

11. Q: What makes the obliquity and precession vary?A: ii) Also, the tilting of Earth’s orbital plane N  N S  • Tilting of the orbital plane is like • a dinner plate rolling on a table • If the Earth was perfectly spherical, • its spin axis would always point in • the same direction but it would make • a different angle with its orbital plane • as the plane moved around S

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

13. N  S Precession Variations • Range: 0-360o • Current value: Perihelion occurs on Jan. 3  North pole is pointed almost directly away from the Sun at perihelion • Periods*: ~19,000 yrs ~23,000 yrs Today *Actual precession period is 26,000 yrs, but the orienta- tion of Earth’s orbit is varying, too (precession of perihelion)

14. N Today 11,000 yrs ago  N S  S Which star is the North Star today?

15. N Today 11,000 yrs ago  N S  S Which star was the North Star at the opposite side of the cycle? Polaris

16. N 11,000 yrs ago* Today  N S  S Vega Polaris *Actually, Vega would have been the North Star more like 13,000 years ago

17. 800 kA Today Unfiltered Orbital Element Variations 0.06 65o N solar insolation Imbrie et al., Milankovitch and Climate, Part 1, 1984

18. Ref: Imbrie et al., 1984 Eccentricity Obliquity Precession Filtered Orbital Element Variations 800 kA Today

19. Interestingly, Earth’s obliquity variations would • be quite different if the Earth didn’t have a Moon • The obliquity would vary chaotically from 0-85o • on a time scale of tens of millions of years • Chaos: Mathematically,this term is used to • describe dynamical systems in which small • changes in initial conditions lead to large • changes in the solution after some period of • time

20. Earth’s obliquity with and without the Moon Chaotic region Daylength (with no moon) Laskar and Robutel (1993)

21. Back to the climate story…

22. Milutin Milankovitch, Serbian mathematician 1924--he suggested solar energy changes and seasonal contrasts varied with small variations in Earth’s orbit He proposed these energy and seasonal changes led to climate variations NOAA

23. Optimal Conditions for Glaciation: • Low obliquity (low seasonal contrast) • High eccentricity and NH summers during aphelion (cold summers in the north) • Milankovitch’s key insight: • Ice and snow are not completely melted during very cold summers. • (Most land is in the Northern Hemisphere.)

24. N  S • Optimal Conditions for Deglaciation: • High obliquity (high seasonal contrast) • High eccentricity and NH summers during perihelion (hot summers in the north) Today 11,000 yrs ago N  S Optimal for glaciation Optimal for deglaciation

25. NH Insolation vs. Time

26. O isotopes—the last 900,000 yrs Peak NH summertime insolation after Bassinot et al. 1994

27. Big Mystery of the ice ages: Why is the eccentricity cycle so prominent? The change in annual average solar insolation is small (~0.5%), but this cycle records by far the largest climate change Two possible explanations: 1) The eccentricity cycle modulates the effects of precession (no change in insolation when e = 0) 2) Some process or processes amplify the temperature change. This could take place by a positive feedback loop

28. What are some possible glacial climate feedbacks?

29. Temperature Planetary albedo What are some possible glacial climate feedbacks? 1) Ice-Albedo 2) CO2 variations Snow and Ice cover