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: 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

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 Aphelion Perihelion 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 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

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) The effects of astronomical forcing (accretion of space dust) http://en.wikipedia.org/wiki/Milankovitch_cycles Human Activity influence on glacial cycles Determine a better understanding on the causes of Ice Ages

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?