An Orbital Theory For Glacial Periods

Pete Bogenschutz

WHOA! Far out!!!!!

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

Ice Ages for Dummies

● Ice Age:– 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

Pioneers

James Croll (1821 - 1890)

Milutin Milankovitch (1879 - 1958)

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

PerihelionAphelion

http://www.homepage.montana.edu/~geol445/hyperglac/time1/milankov.htm

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!

http://www.homepage.montana.edu/~geol445/hyperglac/time1/milankov.htm

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

http://www.homepage.montana.edu/~geol445/hyperglac/time1/milankov.htm

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)

http://www.homepage.montana.edu/~geol445/hyperglac/time1/milankov.htm

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

Increase of surface albedo, less incoming solar radiation absorbed by surface

● Glacial period finally ends once incoming solar radiation becomes maximum at mid latitudes

Lower global temperature

Snowfall at lower latitudes

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

http://en.wikipedia.org/wiki/Milankovitch_cycles

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

Current Studies

● Examination of another possible orbital theory (orbital inclination)

– Human Activity influence on glacial cycles

– Determine a better understanding on the causes of Ice Ages

– The effects of astronomical forcing (accretion of space dust)

http://en.wikipedia.org/wiki/Milankovitch_cycles

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

QUESTIONS?