North American Fruit Explorers mailing list at ibiblio

[Bob Melvin <hasnah8@yahoo.com>]

Hi,

     With regard to the question about global warming several thousand
years ago, the most logical answer seems to be the Milankovitch cycles.

     These have to do with the fact that the orbital dynamics of the
Earth cause the amount of sunlight in each hemisphere (northern or
southern) to vary slowly over thousands of years.

     Here is a simple example:  Currently, the Sun is closest to the
Earth in early January.  That gives us (here in the northern
hemisphere) milder winters than we would expect otherwise.  In several
thousand years, the Sun will be farthest from the Earth in January. 
That should make our winter and summer temperatures more extreme than
they are now.  This is but one example of orbital effects.  There are
also others.  Please see this article:
http://en.wikipedia.org/wiki/Milankovitch_cycles

     I think the explanation under the heading: "Earth's movements" and
the accompanying graphs are particularly good.

     Here is one thing that may seem confusing:  The precession of the
equinoxes is usually referred to as taking about 25,800 years or
(rounded off) 26,000 years.  Yet, one basic cycle mentioned is 22,000
years.  The trick here is that not only are the equinoxes shifting, but
the point on the Earth's orbit where we are closest to the Sun is also
shifting.  So the winter solstice and the perihelion point will match
up every 22,000 years or so.  As I said, there are other effects, too.

--"Bob Melvin" <hasnah8@yahoo.com>, <rfmelvin@ncsu.edu> 
Ph. 919-215-2814  (Anyone may call at any time.) 


>
----------------------------------------------------------------------
> 
> Message: 1
> Date: Sat, 23 Sep 2006 07:32:46 EDT
> From: Jwlehman@aol.com
> Subject: Re: [NAFEX] Early Winter?
> To: nafex@lists.ibiblio.org
> Message-ID: <46f.72ea4500.3246755e@aol.com>
> Content-Type: text/plain; charset="us-ascii"
>
> Jerry 
> 
> PS. Perhaps someone could answer this question for me off line. Where
> did the 
> greenhouse gasses come from that started the global warming 25,000
> years ago? 
>  Are greenhouse gasses the main ingredient?   

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