cc: tom@ocean.tamu.edu, p.jones@uea.ac.uk, rbradley@geo.umass.edu, k.briffa@uea.ac.uk, mann@virginia.edu
date: Fri, 27 Apr 2001 14:45:07 -0700
from: Chick Keller <ckeller@igpp.ucsd.edu>
subject: Low Frequency signals in Proxy temperatures:
to: "Malcolm K. Hughes" <mhughes@ltrr.arizona.edu>, ckeller@igpp.ucsd.edu

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Dear Malcolm,

Well said indeed!  This helps me to slowly understand what's being 
done and why.

My nagging problem remains however, and that's that there seem to be 
too many paleo records published that show much larger amplitude 
variations.  Now many can be explained, but some look more robust. 
For example I think most people are wondering about the total 
disagreement between isotope temperatures from GISP II and borehole 
temperatures from GRIP and Dye 3.  Here the usual land use caution 
doesn't apply since I don't think the ice above the boreholes has 
changed much?

And if I understand Tom Crowley's note to me, his reconstruction 
averaged normalized records, thus missing large amplitude variations 
such as the Keigwin Sargasso one, which he used, but which shows a 
large amplitude signal tantalizingly similar to the GRIP/Dye 3 
records. (Tom used GISP II which essentially has no low frequency 
amplitude)

So I read all the papers, and am impressed by the painstakingly 
careful work, but still wonder about a world in which the 
hemispherical low frequency temperature amplitude could be (see Jones 
et al Science this week) only about 0.4C between 1000 and about 
1950, while parts of the world could have seen amplitudes of up to 
2C in the same period.  I suppose you could say that, given natural 
forcing only, there can be much larger variance from the mean 
(spatially and temporally) than in the past hundred and fifty years 
when GHG forcing is forcing more uniformity, but does this make sense?

This is why I keep asking questions about the ability of various 
proxies to return low frequency information.

Anything you could say about this would be greatly appreciated.

Finally consider this.  I read recently (don't know the pedigree of 
this number but it WAS published!!) that Milankovitch cooling at this 
point in the Holocene should be about 0.4C/millenium (other plots 
I've seen would suggest about 2.3 to 1/2 of that).  If that's true, 
then all the cooling since the year 1000 is Milankovitch and there's 
no room for variations in solar activity and multiple volcanic 
eruptions.  Now I'm not saying this is the best way to think about 
such things, but it does remind us that much of the cooling seems to 
have been due to Milankovitch, and, given the small amplitude of the 
proxy records, that is a bit worrisome.  What do people think about 
this?

Regards,


Well said Malcolm...

mike

p.s. Chick: You might want to check out the review article by Jones 
et al in the latest Science...

At 01:16 PM 4/26/01 -0700, Malcolm K. Hughes wrote:
>Dear Chick - some thoughts on a couple of the points you raised, 
>Cheers, Malcolm
>1.      There is no reference to the ABD in MBH 98 and 99 because 
>the technique
>was not available at that time - see the dates on Keith's publications that
>describe it.
>2.      There are significant regions where the ABD method is not needed,
>because the trees live much longer than those in the Schweingruber 
>network that
>Keith has been using, and grow under conditions that make only very 
>conservative
>standardization necessary. There is a growing body of evidence that these
>tree-ring records can capture century-to-millennial change accurately (Hughes
>and Graumlich, 1996 and Hughes and Funkhouser 1998, for example). In fact, the
>MBH reconstruction before AD 1400 was largely based on these.
>3.      Keith has pooled information from extremely large regions 
>(presumably to
>get large enough samples), whereas we (MBH) have been particularly 
>interested in
>spatial variability, ruling out the use of ABD.
>4.      The ABD method is new, needs testing, and, I predict, will be modified
>as it is tested.
>5.      The benefit of annual resolution is that direct calibration and
>cross-validation against instrumental records is possible with a 
>high degree of
>rigor. We are relaxing this condition somewhat in our ongoing analyses, and it
>will be interesting to see how the uncertainties increase as one includes
>records with poorer temporal resolution. This is an issue that the 
>advocates of
>such records do not address, so far as I can see.
>
>
>
>Professor Malcolm K. Hughes
>Laboratory of Tree-Ring Research
>W.Stadium 105
>University of Arizona
>Tucson, AZ 85721
>phone 520-621-6470
>fax 520-621-8229

_______________________________________________________________________
                      Professor Michael E. Mann
           Department of Environmental Sciences, Clark Hall
                       University of Virginia
                      Charlottesville, VA 22903
_______________________________________________________________________
e-mail: mann@virginia.edu   Phone: (804) 924-7770   FAX: (804) 982-2137
        http://www.evsc.virginia.edu/faculty/people/mann.shtml

Charles. "Chick" F. Keller,
IGPP.SIO.UCSD - Attn: Chick Keller
9500 Gilman Drive
La Jolla, CA 92093-0225
(858) 822-1510  office
(858) 534-2902  FAX
(858) 456-9002  home
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