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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 firstname.lastname@example.org)
I was not aware of the discussion until Thomas Lipka cross-posted
his discussion of Milankovitch Cycles to the vertebrate paleontology
listserv (email@example.com). 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
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
>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
Rickard S. Toomey, III firstname.lastname@example.org
Research Associate, Illinois State Museum
phone (217) 524-7908 fax (217) 785-2857
Illinois State Museum Homepage URL http://www.museum.state.il.us/