Re: Milankovitch cycles

[Jan Hollan <jhollan@amper....muni.cz>]

Dear Constantin,

at last I managed to read my old notes on glaciation as given in the 
  http://amper.ped.muni.cz/gw/articles/
 
> First, I assume that to favor glaciation, the key thing is to have cool
> summers for 65 degrees north latitude. If this is so, then will glaciation
> be favored by:
> 	1. minimum axial tilt?
> 	2. maximum orbital eccentricity with northern summer occurring at
> the aphelion?

1. Yes, 2. Yes. But the aphelion is to be very far, and it is not the
present case.

> Is it correct that, on a long term basis,  we should now be leaving an
> interglacial and entering a glacial? That is, we should be at the end of a
> roughly 18,000 year interglacial and about to begin a 90,000 year glacial,
> ignoring minor wiggles in the climate curve? 

NOT AT ALL, this false belief is just the reason I wrote my notes using
the BDL model/data. 

> Where are we now in the three M cycles? Is our axial tilt increasing or
> decreasing? How eccentric is our orbit and is it increasing or decreasing in
> ellipticity? And is our winter solstice close to the aphelion but moving
> towards the perihelion? 

The main fact is, that the present eccentricity is very low and will
remain so for long, despite its ~80 ka oscillations.  It will rise to
moderately high values some 250 ka from now, but then just the moments of
northern summer aphelia will have high axial tilt, so no large prospect
for glaciation emerges. The minima of 65degN summer insolation at 130 ka
and 170 ka from now are not very deep. 

The first really deep minimum, comparable with the onsets of the two last
glaciations, comes really just some 620 ka from now -- due to very large
value of excentricity in spite of a diminished amplitude of axial tilt
compared to the past 0.5 Ma. 

The present excentricity is so negligible, that there is very little
difference in aphelion and perihelion, and it further diminishes. Aphelion
is midsummer now (beginning of July), but it matters little. 

I should probably include a graph of these orbital variables in the text
given above (I had just some drafts for me before). Meanwhile I post the
graphs I made today in the above-given directory (700-250 and 300-1000
ones, meaning spans from -250 ka to 700 ka, like the one with insolation
sums in the article, and, for comparison with past orbital parameters, one
going from -1000 ka to 300 ka), in the subdirectory
  http://amper.ped.muni.cz/gw/articles/graphs/

The numeric values are apparent from the graphs, just the axial tilt is a
bit inconveniently expressed as 30 crad + (something between 9 crad and 12
crad). 

I have also included the insolation data in the same scale, to enable
comparing the prints (*l.eps means landscape for merely copying it to a PS
printer, I hope ``letter'' instead of an A4 format will be OK).

with best regards,
Jenik

PS. 
 I don't publish scientific papers a long time already, considering myself
a teacher only. But, as a review paper, my article on future astronomical
impossibility of an ice age, coud be maybe useful, to help to wipe that
absurd myth, what's your opinion? What could be a suitable media to
publish it? 
  
--------------------------------------------------------------------------
                                Jan Hollan
              N. Copernicus Observatory and Planetarium in Brno
Kraví hora 2, CZ - 616 00 Brno                        +420 (5) 41 32 12 87

                                   home:
Lipová 19, 602 00 Brno                                         43 23 90 96

                 member of the Society for Sustainable Living
             and volunteer of the Ecological counselling Veronica
Panská 9, 602 00 Brno, Czechia                  fax:  +420 (5) 42 21 05 61

e-mail: hollan@ped....cz             http://astro.sci.muni.cz/pub/hollan
--------------------------------------------------------------------------