An Orbital Theory For Glacial Periods

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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
●
●
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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
●
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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?

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