Re: Milankovich Cycles

["Roy E. Plotnick" <plotnick@xxxxxxxxxxxxxxxxxxxxx>]

I used a modification of Berger's 1978 Fortran program for calculating
insolation.  This program gives the top-of-atmosphere insolation for any
date/year and latitude.  I then ran the program to give the insolation at 60N
at
the summer solstice, at 100K intervals, for the last 300 my.  The resulting
plot
showed clear cycles on the order of 25my.  Needless to say , this was exciting!
But when when I reran it starting 50k ago, the pattern was much more obscure.
I then tried it at a 2k interval (lots of data points!),  at different
latitudes, at different seasons, looked at summer-winter contrast, rates of
change, etc. etc.  You could kind of see a pattern, but it was unclear.  The
clearest pattern I get is a ca. 50 my cycle in the variance.  A big caveat: the
current cycles are not stationary!  I was mostly interested in the idea of
interference patterns, rather thatn in precise dates.
The big surprise to me was the scale of insolation variations during the cycle,
especially at eccentricity and obliquity maximums.  On the order of 15-20% at
higher latitudes!  This has to have a effect on ecosystems, even ignoring the
climatic effects. - Roy

-- 
Roy E. Plotnick
Geological Sciences
University of Illinois at Chicago
845 W. Taylor St.
Chicago, IL 60607

plotnick@plotnick.geol.uic.edu

phone: 312-996-2111
fax: 312-413-2279