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

2. in carbonate sediments
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 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)
Precession
(wobble)
19,000 yrs
23,000 yrs
22o

6. Q: What makes eccentricity vary
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:
Period(s): ~100,000 yrs
~400,000 yrs

8. Unfiltered Orbital Element Variations 65o N solar insolation Today
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
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. the Earth is about twice as strong as the Sun’s
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
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. 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  N S
*Actual precession period
is 26,000 yrs, but the orienta-
tion of Earth’s orbit is varying,
too (precession of perihelion)

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

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

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

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

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

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 Daylength (with no moon)
Chaotic
region
Daylength (with
no moon)
Laskar and Robutel (1993)

21. Back to the climate story…

22. Milutin Milankovitch, Serbian mathematician
                      
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. 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 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. What are some possible glacial climate feedbacks? 1) Ice-Albedo
2) CO2 variations
Temperature
Planetary
albedo
Snow and
Ice cover