Transcript Milankovitch

“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, handcomputed insolation at
65 N based on orbital
variations for two 6month 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 refined his
methodology
– Restricted his
calculations to midmonth days
• Specifically focusing
on the solstice period
– Led to better
correlations with the
sea core data
Berger &
SPECMAP
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