Puzzle: Ice Ages!

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Puzzle Ice Ages!

• Occur with a period of 250 million yr
• Cycles of glaciation within the ice age occur
  with a period of 40,000 yr
• Most recent ice age began 3 million yr ago and
  is still going on!

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Last Glacial Maximum 18,000 yr ago 32 of land
covered with ice Sea level 120 m lower than now
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Cycles of glaciation/deglaciation what cause
them??

• Milankovitch Hypothesis they occur due to cyclic
  changes in the summer insolation of Northern
  Hemisphere. These cyclic climate changes are, in
  turn, due to variations in the Earths orbital
  parameters
• Precession (26,000 yr cycle)
• Eccentricity (varies from 0.00 to 0.06 with
  100,000 and 400,000 yr cycles)
• Axis tilt (varies from 24.5o to 22.1o with 41,000
  yr cycle

Milutin Milankovitch 1920
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axis
Celestial equator
As a result, planes of the ecliptic and celestial
equator make an angle 23.5o
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The axis tilt causes the seasons!!
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Drawn for northern latitudes, these are the paths
the sun takes across the sky on the equinoxes and
solstices. Can you see that the summer path is
longer (and therefore that the summer sun stays
in the sky longer)?
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Seasons - summary

• Seasons are NOT caused by varying distances from
  the Earth to the Sun
• The primary cause of seasons is the 23.5 degree
  tilt of the
• Earth's rotation axis with respect to the plane
  of the ecliptic.

The Seasons in the Northern Hemisphere
Note the Earth is actually closest to the Sun in
Northern Hemisphere Winter, on January 4! This
reduces seasonal variations in present epoch.
Perihelion 147.09 106 km Aphelion 152.10
106 km
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26,000 yr cycle
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• Varies from 0.00 to 0.06 (currently 0.017)
• Periodicity 100,000 and 400,000 yr
• Eccentricity cycle modulates the amplitude of
  the precession cycle

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As a result, the flux of solar radiation received
by the Earth oscillates with different
periodicities and amplitudes This triggers
changes in climate
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Tmax -30 C Tmin -90 C
Ice sheet 3500-4000 m thick
Vostok
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