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Re: Milankovich Cycles



I'm sorry that I have not followed the beginnings of this thread.
I am not a subscriber to the list at which the thread originated 
(apparently the dinosaur list dinosaur@lepomis.psych.upenn.edu)
I was not aware of the discussion until Thomas Lipka cross-posted 
his discussion of Milankovitch Cycles to the vertebrate paleontology 
listserv (vrtpaleo@usc.edu).  I have continued that cross-post (for what 
it is worth).  

While I agree with Thomas Lipka on the general statement that 
Milankovitch cycles play important roles in influencing the 
Earth's climate and through it the biotic system.  I 
disagree with some of of Lipka's statements and conclusions.

Thomas R. Lipka <Tompaleo@aol.com> writes 

>Milankovich cycles are changes in the orbital dynamics of the earth.
>There are three main cycles varying in periodicity of 100,000 ; 40, 00 and
>20,000 year periods.  They DO play a role in forcing the earths climate
>between Ice Ages and Greenhouses.

Although the three main cycles certainly play a role in forcing climate 
during ice ages and greenhouses, other factors, such as continental 
positions and the uplift of major plateaus are probably more important 
in determining larger scale ice age to greenhouse transitions.

>2) 40,000 Year change in the earths precession. Basically, this is a
>variation of the distance between the earth and the sun which can increase or
>decrease the net Solar flux falling on the earth thereby heating or cooling
>the planet.

NO!  Precession is not variation in the the Earth-Sun distance.  
Precession is variation in the direction that the axis of rotation 
points due to the "wobble" of the axis.  It affects the seasonal
timing of the aphelion and perihelion.  Because of this it, it affects
the seasonal balance of radiation.  The radiational effects of precession 
and tilt are closely linked.

>
>3) 20,000 Year variation in the EARTHS TILT (This is the one asked about).
>The earths axial tilt is presently 23.5 degrees or so but  varies between 22
>degrees and almost 24 degrees.

Actually between about 21.5 and 24.5 degrees.  As I noted above the 
tilt and precession signals are closely linked because the variation
in the direction the axis of rotation points is a function of both
factors.

>
>With so many cyclic phenomena having a DIRECT influence on the birth,
>evolution and death  (extinction) of life on earth it should be painfully
>obvious by now that the earth is not an ISOLATED system of detached
>evolutionary events. The concept of the death of the dinosaurs via an
>extraterrestrial mechanism makes alot of sense (an is slowly being proven).
>

I am not sure that you have shown a direct influence of these 
extraterrestrial cycles on biota.  Certainly, some of the cycles 
have influence through agents such as sea-level and climate.

For further discussion of the causes of "ice age-greenhouse" changes
and Milankovitch cycles you can check out the Illinois State Museum's
on-line exhibit "Ice Ages" at 
URL http://www.museum.state.il.us/exhibits/ice_ages/ 

Rick Toomey


------------------------------------------------------------------------
Rickard S. Toomey, III    toomey@museum.state.il.us
Research Associate, Illinois State Museum
phone (217) 524-7908 fax (217) 785-2857
Illinois State Museum Homepage URL http://www.museum.state.il.us/