THE ASTRONOMICAL THEORY OF CLIMATIC CHANGE
(The Milankovitch Hypothesis)

CHRONOLOGICAL SCALE
109 106 103 102 10 YR
    ORBITAL


GEOGRAPHICAL SCALE
  Local  Regional  Global 
    ORBITAL

Observations on the Earth's orbit:

  1. ORBIT AN ELLIPSE (J. Kepler, 0.0167)
  2. TILT OF AXIS OF ROTATION 23.5o
  3. AXIS OF ROTATION VARIES (120 B.C., Hipparchus compared astronomical observations to Timocharis' 270 B.C.)

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Hypothesis first put forth in 1842 by J.A. Adhemar (Revolutions of the Sea). He explained (glacial) climate in terms of precession alone.
1864, James Croll Philosophical Magazine wrote on orbital change and ice the ice ages

MILUTIN MILANKOVITCH Serbian
Theory fell into disrepute in the 1940's and 1950's
  1. radiocarbon dating shows a lag between the 18K maximum global cooling vs.
    25K predicted by insolation at 65o
  2. scale problem: high frequency glacial advances not explained by curves e.g.,
    Woodfordian, Altonian, Allerod etc.
  3. insolation 18 Ka like now

REVIVAL OF Milankovitch Hypothesis:
FEEDBACKS: Total insolation change small, only the 100,000 yr cycle produces the only change in total global insolation <1%, similar to sunspot cycles.

INSOLATION MINIMUM (decreases)

POSITIVE FEEDBACKS
  1. more snow: increased albedo, cooler atm.
  2. lower sea level: exposed continental shelf, incr. albedo, aerosols
  3. sea ice: fresh water freezes easily, air colder, high albedo
  4. fresh surface water turns off thermohaline circulation
      no production of Atlantic deep water
      no Gulf Current to warm North Atlantic
      slow circulation increases oceanic CO2 (decr. atm. CO2)
NEGATIVE FEEDBACKS
  1. cold ocean - water vapor 20% - 33% less
      lower biol. productivity, + CO2 in atm
      reduced precipitation on glaciers
  2. ice sheet growth



"The Day After Tomorrow"

More Recent model of deep circulation

USE OF ASTRONOMICAL THEORY FOR DATING

Imbrie et al., 1984
SPECMAP sea cores & astronomical theory
  1. Start with six dated benchmarks
  2. Match Peaks
  3. Spectral tuning and adjustment

IMPORTANCE OF SEASONAL INSOLATION

Insolation varies in Space
  1. at equator, precession (22 ka) felt much less than tilt (41 ka)
  2. amplitude of insolation changes greatest at poles
Insolation varies in Time
Davis 1984
  1. relationship of perihelion to equinoxes changes ca. 22,000 yr cycle
  2. each month receives maximum insolation at different times in Holocene
  3. Direct Effects processes or phenomena effected by seasonal insolation
  4. Indirect Effects on atmospheric circulation

EXTENSION OF THE ASTRONOMICAL HYPOTHESIS



Astronomical Theory Readings

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