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“Being Milutin Milankovitch”
Or “From Millennia to
Hours: The Decreasing Scales of the Orbital
Theory of Multin Milankovitch”
Adapted fromDr. Randy Cerveny
Presidential Professor, ASU
Milankovitch Theory
• Milutin Milankovitch, Serbian mathematician and astronomer
• In 1938, hand-computed insolation at 65 N based on orbital variations for two 6-month seasons (summer & winter)
Milankovitch Theory
• He computed variations in three primary orbital parameters:– Obliquity– Eccentricity– Precession of the
Longitude of Perihelion in relation to the Moving Equinox (“Precession”)
Milankovitch Theory
• Obliquity– The variations in the
axial tilt of the Earth. – This orbital
component has a cycle of 42,000 years for the Earth to shift from an axial tilt of 22.1 to 24.5 and back
Milankovitch Theory
• Eccentricity– The variations associated with the shape of the Earth’s orbit
(oblique to circular)– The primary cycle of eccentricity is 93,000 years with a
secondary cycle at 400,000 years.
Milankovitch Theory
• Precession– Most complex of the three mechanisms.– Refers to the season in which perihelion is occurring as a
function of the orientation (or “wobble”) of the Earth’s axis
Milankovitch Theory
• Current Precession– Earth’s perihelion in early
January given the current axial orientation towards the star Polaris
– But 11,000 years from now, perihelion will occur in July
– The primary cycles for precession are 19,000 and 23,000 years.
• Two huge problems in gaining acceptance for his theory– No reliably-dated evidence– Unknown which dates were the most important
• Theory put on back burner until more evidence gathered...
CLIMAP & the ocean corers
• International project in mid-1970s
• Help better understand when the last ice age occurred
• Includes two important research thrusts– First, a chronology of
past climates was begun using deep-sea marine cores.
Hays, Imbrie & Shackleton: Pacemaker to the Ice Ages
• Spectral analyses of cores suggested existence of distinct periodicities
• Closely matched the cycles computed by Milankovitch for the Earth’s three orbital mechanisms
Andre Berger, the modern father of astronomical climate
theory• Second thrust:
Andre Berger (Belgium)– Re-computed
Milankovitch’s astronomical calculations
• Specifically for single month period, specifically June
Berger & SPECMAP
• Berger refined his methodology– Restricted his
calculations to mid-month days
• Specifically focusing on the solstice period
– Led to better correlations with the sea core data
Milankovitch Theory: Paradigm of the 80’s
• By 1980s, deep-sea core data used as the focal point for determining the ice ages
• Concept of “orbital tuning” developed the to make data fit “better”
Then The Devil
Arrives …
• … And, appropriately enough, the place is “Devil’s Hole Nevada”
• Isaac Winograd (Geologist) for USGS, spearheaded a project in the late 80s to reconstruct climate from a vein of calcite
• Laid claim to having the “best dated record of paleoclimate” in existence
• The Problem? Their record doesn’t match the SPECMAP / Milankovitch theory
Meanwhile …The ASU Connection
• While Winograd was working at Devil’s Hole, improvements were being made to Milankovitch theory:– First, Dr. Cerveny
refined Milankovitch calculations from daily insolation totals to hourly
Milankovitch meets Shaffer
• In the 90s, John Shaffer (now at Scottsdale Community College) got interested in orbital theory and the problems raised by Winograd
• He believed there was a way to reconcile the apparent problems:
• He developed “differential insolation” indices!
Two Insolation Curves
• To match the Devil’s Hole proxy record, Shaffer suggested:– Winter tropical (20° N) noontime insolation:
Associated with source region for wintertime precipitation for Devil’s Hole
– Summer extra-tropical (33° N) noontime insolation: Associated with summer heating & evaporation
• Coined the term “Radiation Window”– Coincident periods
of low winter and high summer insolation
• Warmer/drier conditions at Devil’s Hole
• (Usually) Glacial terminations are preceded by– (a) a pre-existing warming and– (b) a radiation window– If atmospheric cooling coincides with a radiation
window, then “failed termination” or an “instadial window”