THE ASTRONOMICAL THEORY OF CLIMATIC CHANGE
(The Milankovitch Hypothesis)

Observations on the Earth's orbit:

1. ORBIT AN ELLIPSE (J. Kepler, 0.0167)
   
   • eccentricity varies ±± 98,000 yr period (0.0005 - 0.0607)
     
     radius = 155 x 10⁶ km, so varies 9.3 x 10⁶ km
   • TOTAL INSOLATION (incoming solar radiation) CHANGES VERY LITTLE
     
     currently: Jan 3 perihelion, July 4 aphelion
2. TILT OF AXIS OF ROTATION 23.5^o
   
   • tilt (obliquity) varies ± 41,000 yr period (20^o - 25^o)
     Seasons in N - S Hemispheres Opposite
     
     Equinox Mar 20 - Sept 22
     Solstice June 21 - Dec 21
     
3. AXIS OF ROTATION VARIES (120 B.C., Hipparchus compared astronomical observations to Timocharis' 270 B.C.)
   
   • today Polaris is the pole star
     
   • 6000 B.P. pole star in handle of big dipper
     
   • i.e., changing relationship between perihelion and solstice
     
   • Precession of the equinoxes ± 19,000, 23,000 yr period
     

click on image for larger version

• His hypothesis was based on the mathematics of d'Alembert (1754), who calculated precession due to gravitational pull of moon and sun on earth
  
• Adhemar predicted alternation of glaciations between hemispheres; that is,
  
  • Today: N. hemisphere interglacial, S. hemisphere glacial (Antarctica?)
    
  • 11,000 y.a.: N. hemisphere glacial, S. hemisphere interglacial
    
  • Precession Cycle ~ 20k, North-South see-saw
    1. TBS: Thermal Bipolar Seesaw
    2. MIS 20K precession cycle depressed prior to 700,000 B.P. by N-S cancellation of ice volume (Raymo, Lisiecki Nisancioglu, 2006)
• Alexander von Humboldt quashed Adhemar's theory: pointed out that although seasonal amounts change, total amount same, cold winter balanced by warm summer
  This is still the problem: must postulate:
  
  1. seasonal insolation is critical
     
  2. one hemisphere is dominant
     

• used both eccentricity cycle and precession cycle
  
• applied mathematics of U. Leverrier, effects of other planets on the eccentricity of Earth's orbit.
  
• 100,000 yr cycle. Last 10,000 yr low eccentricity, greater insolation
  
  (discrepancies between calculations, and astronomical observations lead to the discovery of Neptune)
  Croll added two more elements to the Theory:
  
  1. positive feedback mechanism: cold winters lead to permanent snow fields at high latitudes. These produced even greater cooling that resulted in changes in atmospheric circulation.
     
  2. 1875 took into account changes in Tilt (obliquity) of the Axis
     

• 1904 Ph.D. Vienna Inst. Tech.
  
• 1909 Univ. Belgrade - Professor of Applied Mathematics
  Milankovitch produced integrated curves combining all of the orbital elements:
  
  1. Eccentricity of Orbit 100,000 yr
     
  2. Tilt of Axis 41,000 yr
     
  3. Position of Equinoxes 22,000 yr
     
  Published 3 short papers 1912, 13, 14 - generally ignored
  
• 1914 taken prisoner by Austro-Hungarian Army (medic) - released in Dec. Prof. Czuber
  
• 1920 Mathematical Theory of Heat Phenomena Produced by Solar Radiation included modern climate of Mars, Venus
  
  W. Koppen read 1920 paper asked to collaborate with Milankovitch and study past climate. (Koppen was convinced that summer temperature in the Arctic was the key to glaciations)
• 1924 Koppen and Wegner (son-in-law) publ. Climates of the Geologic Past included Milankovitch's curves 55, 60, 65^o N
  
• 1930 Milankovitch published, Mathematical Climatology and the Astronomical Theory of Climatic Changes
  
  • included 8 latitudes 5^o N to 75^o N
    
  • can lead to greater temperature gradients
    
  • seasonal insolation changes more important at low latitudes
    
• 1938 Astronomical methods for investigating Earth's Historical Climate. included feedback mechanism: increased snow, albedo
  
• 1941 Canon of Insolation and the Ice Age Problem.
  

1. radiocarbon dating shows a lag between the 18K maximum global cooling vs.
   25K predicted by insolation at 65^o
   
2. scale problem: high frequency glacial advances not explained by curves e.g.,
   Woodfordian, Altonian, Allerod etc.
   
3. insolation 18 Ka like now
   

• 1955 long records: C. Emiliani ¹⁸O/¹⁶O noted general correspondence to Milankovitch's curves
  
• 1956 accurate chronology: Erickson et al.(1956) Barbados raised beaches dated at 82, 105, and 125 K
  
• 1969 terrestrial conformation: Kukla (1970) published loess chronology dated by Uranium-Thorium the dates of for terminations match 45^o N insolation
  
• 1969 paleomagnetic dating of long ocean cores confirms age of earlier cycles as predicted by the Theory
  
• 1974 J. Imbrie (e.g., Imbrie & Kipp 1971, Hays et al. 1974) publishes climatic model based on insolation for 45^o which match oxygen isotope record for core V28-238 -- 95% variance ¹⁸O/¹⁶O explained by insolation at 45^oN Imbrie's insolation curves
• 1978 A. Berger publishes recalculated, accurate formulae for insolation changes
  

More Recent model of deep circulation

USE OF ASTRONOMICAL THEORY FOR DATING

1. Start with six dated benchmarks
   
   • core top, isotope stages 2.22, 2.24, 6, B/M, Jarmillo
     
2. Match Peaks
   
   • assume linear sedimentation between dated points
     
3. Spectral tuning and adjustment
   
   • adjust sedimentation rates so ¹⁸O variance = 19, 23, 41, 98 yr
     

IMPORTANCE OF SEASONAL INSOLATION

1. at equator, precession (22 ka) felt much less than tilt (41 ka)
   
2. amplitude of insolation changes greatest at poles
   

1. relationship of perihelion to equinoxes changes ca. 22,000 yr cycle
   
2. each month receives maximum insolation at different times in Holocene
   
   (45°N)
   
   • fall - 5,000 ya
     
   • summer - 9,000 ya
     
   • spring - 15,000 ya
     
   
3. Direct Effects processes or phenomena effected by seasonal insolation
   
   • upper tree line highest, July temperature greatest, 9,000 ya. Spruce
     (Ritche et al., 1983) Poplar (Bartlein et al 1995)
     
   • continental interiors: L. Baikal biogenic productivity 100, 41, 23, 19 Kyr cyclicity
     (Colman et al 1995) Devils H (Winograd et al, 1988)
     
4. Indirect Effects on atmospheric circulation
   
   • Asian and African Monsoon (Ping et al, 1995) wettest, +85% in Chinese arid regions, 25% drier LGM (Maher & Thompson, 1995)
     
   • Arizona Monsoon
     

EXTENSION OF THE ASTRONOMICAL HYPOTHESIS

• climatic effects of orbital cycles apparent in pre-Quaternary sediments
  
• regularity of cycles permit prediction of future climatic change