Re: Peer review (was Re: Wisconsin professor says global warming a hoax!)

In article <op.txk1k1lzhv4qyg@murphus>, "Tom Linden" <tom@xxxxxxxxxxxxxx> writes:
On Fri, 24 Aug 2007 09:07:00 -0700, <david20@xxxxxxxxxxxxxxxx> wrote:

In article <op.txjgu10bhv4qyg@xxxxxxxxxxxxxx>, "Tom Linden"
<tom@xxxxxxxxxxxxxx> writes:
On Thu, 23 Aug 2007 08:54:05 -0700, <david20@xxxxxxxxxxxxxxxx> wrote:

That is not accurate, the period of precession of the equinoxes is
25,600
years, IIRC, advancing one degree about every 70 years (perhelion
currently is
January 4)

The predominant astronomical cycle affecting glaciation for the last
800,000 years has been a 100,000 year cycle of ice ages punctuated by
briefer
usually 9000 - 12000 year long interglacials. There are a number of
different
astronomical cycles which acting together may explain this. I believe
the
interglacial can generally be thought of as lasting for about one half
of the
equinox precessional period (the exact length depending upon how all
the
cycles mesh together and probably also modulated by other
non-astronomical
factors). The variation caused by the precession of the equinoxes
obviously
occurs repeatedly during the 100,000 year period but only leads to
interglacial
conditions at the beginning/end of the 100,000 year cycle.
However as indicated in some of the links below more recent findings
from ice
cores point to some interglacials having lasted much longer than half
the
equinox precessional period.



Because there are a number of different repeating factors ( google
Milankovitch)
there is a beat phenomenon that can occur resulting in extremes.
I copied the following article which I think is well put together
http://www.kednos.com/physics/CLIMATOLOGY/ICEAGE.HTML

Note the graph on insolation, the 100K period to which you refer is
clearly significant,
the 400K period is caused by perturbation of the earths orbit by
planetary
alignments
resulting in the eccentricity becoming as high as 0.04 (currently almost
circular, 0.01)
but also not that the last ice age in which the ice was several km thick
on northern Europe
was maybe 12000 years ago and that is outside the 100KA cycle.


I'm not sure then why you objected so strenuously to my statement that
the last
interglacial started about 11500 years ago (there are a number of
different
ways to measure when the interglacial started and hence figures from
10,000
upto 12,000 years ago are often quoted but I'm sure you can't have been
objecting on that basis - The links I provided used all of those
figures).

I don't think it was strenuous;-) In any event my reading of your post
left the
impression that ice ages occurred on 100KA cycle and all I was saying is
that it is
about every 26000 years. But then that may have been my misreading.

Except complete glacial/interglacial cycles do NOT occur every 26000 years or
last for just 26000 years ( or 21000 years see note).
Results from cores show they have been occuring for the last 800,000 years on
a cycle of about 100,000 years. Before that they were occuring on a cycle of
about 41,000 years. (both of these periods are associated with Milankovitch
cycles). The vast majority of the cycle being under glacial conditions with
a comparatively short interglacial period.

The cycle of the precession of the equinoxes appears to be a modifier of those
dominant cycles rather than itself being the dominant cycle (and would be
expected to be providing some contribution to warming the earth for half of
the precession cycle and contributing to cooling for the other half. However
apart from at the beginning/end of the dominant cycle this warming is not
strong enough to cause an interglacial).

(
note. In Milankovitch cycles the precession of the equinoxes cycle is the
period of the precession of the equinoxes with respect to perihelion which
is 21000 years rather than 26000 years
see
http://en.wikipedia.org/wiki/Milankovitch_cycles
and
http://en.wikipedia.org/wiki/Precession_of_the_equinoxes#Anomalistic_precession

)


David Webb
Security team leader
CCSS
Middlesex University










The only other thing you could have been objecting to was the statement
that
in the 1970s it was thought that interglacial's generally lasted about
11,000
years again the links provide adequate support for that statement.

I'm also not sure what you mean by "outside the 100KA cycle" in your last
remark.

See

http://en.wikipedia.org/wiki/image:Ice_Age_Temperature.png

for a diagram showing Antarctic temperature changes during the last
several
glacial/interglacial cycles over the past 450,000 years (unfortunately
the date
scale isn't linear - stretching out the recent past and compressing the
more
distant past).


David Webb
security team leader
CCSS
Middlesex University




--
PL/I for OpenVMS
www.kednos.com



--
PL/I for OpenVMS
www.kednos.com
.

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