“Being Milutin Milankovitch” Or “From Millennia to Hours: The Decreasing Scales of the Orbital Theory of Multin Milankovitch” Adapted from Dr. 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”

The Future? The latest Milankovitch research suggests that next ice age will be one of the mildest of the last half million years