# Lecture 17 Milankovitch cycles

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### Lecture 17—Milankovitch cycles:

Lecture 17—Milankovitch cycles Abiol 574

### Slide2:

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

### Slide3:

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

### Slide4:

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

### Q: What makes eccentricity vary? A: The gravitational pull of the other planets:

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

### Eccentricity Variations:

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

### Slide7:

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

### Slide8:

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

### Q: What makes the obliquity and precession vary? A: First, consider a better known example…:

Q: What makes the obliquity and precession vary? A: First, consider a better known example… g 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 Example: a top

### Slide10:

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

### Slide11:

Q: What makes the obliquity and precession vary? A: ii) Also, the tilting of Earth’s orbital plane N  S  S N 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

### Obliquity Variations:

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

### Precession Variations:

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 *Actual precession period is 26,000 yrs, but the orienta- tion of Earth’s orbit is varying, too (precession of perihelion) Today

### Slide14:

Which star is the North Star today?

### Slide15:

Polaris Which star was the North Star at the opposite side of the cycle?

### Slide16:

Polaris Vega *Actually, Vega would have been the North Star more like 13,000 years ago

### Slide17:

NOAA 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

### Slide18:

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.)

### Slide19:

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

### NH Insolation vs. Time:

NH Insolation vs. Time

### O isotopes—the last 900 m.y.:

after Bassinot et al. 1994 O isotopes—the last 900 m.y. Peak NH summertime insolation

### Slide22:

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