Monday, January 16, 2012


date: Thu Jan 20 10:04:49 2005
from: Keith Briffa <>
subject: Re: Re:
to: Edward Cook <>

will be discussing all this early next week with Gerrard. He is doing the US stuff at least
. We wish to do some longer (based on station records) stuff for some European locations
and try some reconstructions against oak data also.
I am trying to track down the NAO MSc thesis but it might be that the guy only looked at
post 1950 data - will let you know.
I am attaching the short 2000 year section from the ZOD of the IPCC report and the text of
a "box" on the MWP (both confidential for now)
but if we can get more space , it needs expanding to cover SH and more hydro . They also
want an appendix on standardisation - so you will be involved in this also.
Really happy to get critical comment here . There is no doubt that this section will
attract all the
venom from the sceptics. I find myself in the strange position of being very skeptical of
the quality of all present reconstructions, yet sounding like a pro greenhouse zealot here!
Told Peck that you (and Jan) will be CLAS
15:51 19/01/2005, you wrote:

Hi Gerard,
I wouldn't contact Dai until you are certain about the problem in his calculations.
Perhaps simply showing him a comparison of your PDSIs for Europe and his at the same
grid point location with freak values would be enough to stimulate a useful initial
discussion, or do as Keith suggests with maps. I don't know Dai at all to say how he
handles possible criticism. I do know that Trenberth, his co-author, is extremely
defensive and combative when ever criticized about anything because he figures that he
is smarter than everyone else and virtually infallible. So at the start, if you do
contact Dai, do not include Trenberth in the communication. Let Dai sort that issue out
I am glad to hear that the SC-PDSI also appears to improve things for Europe as well. It
appears to be the way to go. With respect to the program, have you written your SC-PDSI
program in Fortran? And if so, is it possible to get a copy of the source code?
I would also appreciate seeing a copy of your European SC-PDSI paper when you feel
comfortable with sending it to me.
On Jan 19, 2005, at 10:09 AM, Gerard van der Schrier wrote:

Dear Ed,
Thanks for sending me the pdfs of those articles. And the recommendation to read the
supplementary material.
I've computed the self-calibrating PDSI for Europe only (but it is trivial to do that
for other regions in the world). The SC-PDSI is a real improvement over the 'ordinary'
PDSI: many thanks for suggesting to look into this! About two weeks ago, I gave a first
draft of a paper on the European SC-PDSI to Keith and Phil (haven't heard anything
since). Initially the idea was to make an update to a publication of Keith on European
PDSI from 1992.
About the Dai et al. PDSI: they used the code available from the NGDC. This code is a
modified version from the 1980s, when internal memory of the computers was very limited.
I'm certainly not a computer-gizmo, but my guess about the mistake in Dai et al. is that
they simply took this code, put some do-loops around it (to compute PDSI for every
gridbox), but then failed to properly declare and initialize the arrays. Fortran might
get internal memory places mixed up when declarations & initializations are not properly
sorted out, especially when you are working with a code that it written to be as
economical as possible with internal memory. At least: this all happened to me when I
had my first try on computing PDSI. I also got freak values of 20 or so. The PDSI
algorith then makes sure that these freak values are quickly damped away, which makes
their impact relatively short.
I haven't told Dai et al. about this. Should I? Keith urged me to explicitly show one of
their most horrible PDSI maps and compare it to ours. They also have freak values in
Thanks for the comments about the preferred format. I'll stick to that.
I'll be in touch.
Cheers, Gerard

Hi Gerard,
I had a nice talk with Keith yesterday about a number of things including asking about
your progress in generating monthly gridded PDSIs over global land areas. Keith
indicated that you had successfully programmed the self-calibrating PDSI algorithm and
had calculated gridded PDSIs for at least Europe (if I understood him correctly). That
is great.
Do you have any timetable for producing gridded PDSIs over global land areas? I am very
interested in comparing what your PDSIs with those of Dai first of all. (Maybe you have
already done that) Keith indicated that you discovered a computational error in Dai's
program that produced unusually extreme PDSI values in some cases. (Has Dai and that
miserable prat Trenberth been told about this? As you might tell, I have genuine dislike
for that arrogant Kiwi) That is not surprising to me because I found PDSIs >20 in some
of Dai's Mexican grid point estimates. I also suspect that the quality of the monthly
precipitation and temperature data used by you is better than that used by Dai. So,
between not having the computational error and having what I suspect to be better
quality input data, your PDSI data will be a significant improvement over Dai's data.
Added to that, the self-calibrating PDSI algorithm would appear to add even more
improvement. All this being the case, the next step would be for me to re-do my North
American PDSI reconstructions with your gridded PDSI data when they are available.
Please let me know when this might be the case. I also have a Chinese graduate student
here from Lanzhou who will work on producing gridded PDSI reconstructions for China and
surrounding land areas (e.g. Mongolia) from long tree-ring records. Your gridded PDSI
data over that region would also be extremely useful for that project.
I also talked with Keith about the eventual format of the global land area PDSI file. As
you know, Paul Krusic has written a very handy Fortran program for extracting
sub-regions of monthly climate data from the Mitchell et al. (2003) 0.5� global data
sets. It would be great if your eventual global PDSI data, derived from the same
Mitchell et al. data (I believe), were organized in the same format as Mitchell's data
so that Paul's program could be used with minimal modification.
Finally, I have attached pdfs of my recent drought paper in Science and its Supporting
Online Materials for you interest (the latter is very important to read in my humble
opinion). Also, my recent QSR paper on the Esper et al. estimates of past temperature is
Dr. Edward R. Cook
Doherty Senior Scholar and
Director, Tree-Ring Laboratory
Lamont-Doherty Earth Observatory
Palisades, New York 10964 USA
Phone: 845-365-8618
Fax: 845-365-8152

Dr. Edward R. Cook
Doherty Senior Scholar and
Director, Tree-Ring Laboratory
Lamont-Doherty Earth Observatory
Palisades, New York 10964 USA
Phone: 845-365-8618
Fax: 845-365-8152

Professor Keith Briffa,
Climatic Research Unit
University of East Anglia
Norwich, NR4 7TJ, U.K.

Phone: +44-1603-593909
Fax: +44-1603-507784

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