[NatureNS] The Hockey stick and astronomical cycles

Date: Mon, 27 Nov 2006 12:07:29 -0400
From: Stephen Shaw <srshaw@dal.ca>
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&
Quoting David & Alison Webster <dwebster@glinx.com>:>
> Hi Patrick & All, Nov 26, 2006
> Thanks for these URLs. The top one led me to
> http://www.aip.org/history/climate/cycles.htm#M_52_
> which contains a good account (or so it seems to me) of climatology 
> over the years.
>
> One passage in this account has me baffled and perhaps someone can 
> explain. The passage being "The changes in the atmosphere could also 
> answer the old persuasive objection to Milankovitch's theory -- if 
> the timing of ice ages was set by variations in the sunlight falling 
> on a given hemisphere, why didn't the Southern Hemisphere get warmer 
> as the Northern Hemisphere cooled, and vice-versa? The answer was 
> that changes in atmospheric CO2 and methane physically linked the two 
> hemispheres, warming or cooling the planet as a whole.(52*) "
>
> What configuration of tilt and orbital distortion could lead to one 
> hemisphere receiving more insolation in e.g. summer than the other 
> hemisphere would receive 6 months later ? Tilt would have to be 
> symmetrical and even if the sun was at times not exactly at the 
> intersection of the major and minor axis of the orbital ellipse, 
> surely this assymetry would not flip in the space of 6 months.
> Yours truly, Dave Webster, Kentville
>
Dave:  Puzzling and interesting. While it's fresh in the mind I think 
the answer
may be that you are focussing on the primary effect, the change in insolation
with earth's obliquity. As you say, this must be symmetrical, just phase
shifted 6 months in the two hemispheres. However, the more important secondary
effect, the amplification of this, is not symmetrical.
According to the AIP sources, part of the reason Milankovitch's (M) ideas were
discounted for several decades was that the primary effects predicted are so
feeble -- only a ~1 percent change for the extremes of obliquity --  while the
global cooling during periods of glaciation was much more severe than this
would predict.  (Another reason was that the M cycles didn't match the
simplicity of the 4 periods of glaciation for which there was evidence back
then: this evidence was simply incomplete -- there turned out to be many more
than 4, which do match M cycles better).  So you need secondary amplification
mechanism(s), found for example in changes in the earth's albedo.  The N
hemisphere cools a bit in summer as the obliquity of earth's axis increases a
little (axial tilt relative to the plane of its orbit, one of the M cycles).
It then snows in winter a bit more and longer and the snow comes a bit earlier
in the fall in the N. hemisphere, so summer snow cover increases on 
land.  This
increases the reflectivity of part of the N. surface, so it cools a bit more
than it otherwise would do.  This slight extra cooling persists through the
next year, so the the snow cover increases some more and so does the cooling
until finally all of the N. land surface is covered by snow, the albedo 
cooling
effect is now massive and then secondly, the N. seas finally freeze over too,
locking
up water as snow fails to melt and turns to glacial ice.  The same weak 
primary
driving force, lowered insolation, acts 6 months out of phase in the S.
hemisphere, but now has little effect because there is not much land 
down there
to
capture the extra snow, only a thin bit down in Patagonia (in contrast to
Canada + N. Europe + Russia-Siberis in the north). So the snow falls and melts
on the sea which cools a bit but doesn't change its reflectivity, so 
there's no
amplification in the south.
This leads to the apparent paradox mentioned in the clip, that the 
Antarctic and
Greenland ice core evidence points to symmetric world-wide cooling during the
glacial periods, not to an effect just concentrated in the N. 
hemisphere.  This
requires an explanation linking climates in the two hemispheres.  The
explanation offered is that the N and S hemispheres' climates are linked via
the "greenhouse gases", water vapour, methane and CO2. I don't know how this
works since I don't have the supercomputer model, but guessing simplistically,
much of the vegetation dies in the north and the swamps freeze over, so 
the CO2
and Me levels emitted into the atmosphere drop, and their greenhouse
contributions fall, amplifying the northern albedo cooling some more.  Because
atmospheric mixing is relatively fast (relative to ocean mixing), this effect
proceeds with little delay in the S and well as the N, linking the two 
climates
via the changes in greenhouse gas concentrations (not sure if this also 
applies
to water vapour though).  The huge difference in land masses in the N and S
critical to this explanation is mentioned briefly in reference 16 of 
your clip,
or of one of the AIP links that I pursued -- I forgot to note which.
Steve

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