California State University Long Beach 

 Milankovitch Theory (1920’s to 1930’s!)

• Earth’s orbital cycles controls solar insolation
• Solar insolation at different latitudes forces climate change
• "Pacemaker of the ice ages"
• Combination of cycles with different periodicities
• Obliquity (tilt)
• Varies from 21.5° to 24.5°
• Presently at 23.5°
• 41,000 yr cycle
• Most important near the poles
• Precession (wobble)
• Determines the season of perihelion
• ~ 22,000 yr cycle
• A combination of 19 kyr and 23 kyr cycles
• Most important near the equator
• Eccentricity (how circular?)
• 100,000 yr cycle
• A combination of 95 kyr and 123 kyr cycles
• 413,000 yr cycle
• Effects all latitudes
• Surprisingly strong influence
• Quaternary glacial-interglacial cycles
• Documented by oxygen isotopes
• Remarkable cyclicity
• Spectral analysis gives important frequencies
• Sawtooth pattern
• Milankovitch cyclicity also in:
• Fossil abundances
• Organic content of marine sediments
• Windblown clay and silt (aridity)

• Problems with Milankovitch forcing
• Strength of the 100 kyr glacial/interglacial cycle
• If not Milankovitch forcing...then what?
• Tilting of orbital plane may intercept space dust
• ³He trapped in micrometeorites
• Accumulates in ocean sediments
• Shows 100 kyr cycles for last 800 kyr
• But apparently out of phase with ice ages
• Dynamic response to crustal loading by ice
• Response of Ice sheets to internal dynamics
• 5,000 - 7,000 year lag time between forcing and climate change
• Synchronous climate and glacial change in both hemispheres
• Similar magnitude temperature change in both hemispheres
• Extreme rapidity of deglaciation (climatic warming)

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