1. An Orbital Theory For Glacial Periods
Pete Bogenschutz
WHOA! Far out!!!!!

2. Outline A brief history Milankovitch cycles
Effects of orbital mechanics on climate
Theory comparison with observations
Recent studies

3. Ice Ages for Dummies Ice Age: Glacial Period:
Extended period of time (several millions of years) with glacial cycles
Most likely function of CO2 concentration, land mass distribution, and possibly astronomical forcing
We're in one now!!!
Glacial Period:
Periods within ice ages of advancing, receeding ice sheets of the poles
Periodicity: 41 – 100 kyr

4. Pioneers
James Croll ( )
Milutin Milankovitch ( )

5. Milankovitch Theory * Eccentricity *
Describes the the shape of the Earth's Orbit around the sun
Eccentricity varies from 0 to 6%
Current eccentricity is 1%
100 kyr cycle with a major variation component of 400 kyr
Aphelion
Perihelion

6. Milankovitch Theory * Precession *
Earth's slow wobble as it spins on axis (similar to a top running down)
North Pole varies from pointing towards Polaris to Vega
Results in greater/less seasonal contrasts depending on position with aphelion and perihelion
Northern Hemisphere Seasons!

7. Milankovitch Theory * Obliquity *
Inclination of the Earth's axis in relation to it's plane of orbit around the Sun
Greater tilt: larger severity in seasons
Less tilt: Solar radiation more evenly distributed

8. Milankovitch Ingredients for Glacial Periods
Minimal tilt!
Summers furthest from the sun
Greatest orbital eccentricity
Minimal Tilt !
Max eccentricity and axis tilted towards Polaris (NH)

9. How To Jump Start Glacial Periods
Slightly warmer winters at the poles, more moisture: more snowfall
Cooler summers globally, less melting of snow
Additional snowfall preceeding winter
Glacial period finally ends once incoming solar radiation becomes maximum at mid latitudes
Increase of surface albedo, less incoming solar radiation absorbed by surface
Snowfall at lower latitudes
Lower global temperature

10. Verification of Milankovitch Theory
Measurements of the per mil enrichment of oxygen 18 from ocean floors
Oxygen isotope ratio reflects the amount of Earth's water frozen in ice
Theory verified except for last one million years

11. Problems With Milankovitch Theory
100 ky problem: Strongest climate signals the past million years associated with eccentricity
400 ky (stage 11) problem: Eccentricity variations do have a strong 400 ky cycle that isn't being detected in climate data
Stage 5 problem: Timing of penultimate interglacial
Effect exceeds climate problem

12. Current Studies
Examination of another possible orbital theory (orbital inclination)
The effects of astronomical forcing (accretion of space dust)
Human Activity influence on glacial cycles
Determine a better understanding on the causes of Ice Ages

13. Summary Glaciation cycles most likely driven by orbital mechanics
Obliquity of the Earth has most profound effect
Other factors are clearly contributing towards severity and timing of glaciation processes
100 kyr problem remains most puzzling
More verification needed, still a young science

14. QUESTIONS?