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 ● 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 ● Snowfall at lower latitudes Lower global temperature Increase of surface albedo, less incoming solar radiation absorbed by surface 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?