Saturday, March 17, 2012


cc: Tim Osborn <>
date: Thu Oct 2 15:58:11 2008
from: Keith Briffa <>
subject: Re: recognize this?!
to: Edward Cook <>

Dear Ed
Thanks for these comments - and while I agree with them , I do not necessarily concur on
the "fashionable" opinion these days that IPCC has made a mistake in stressing the
temperature issue and the rank magnitudes of late Holocene warm periods. It is undeniable
that hydroclimatic variability , past and future, is of enormous scientific and societal
importance. However, the IPCC must follow the published literature and to a large extent
the assessment must maintain a reasonable degree of continuity. Just as it is vital to
understand spatial variability and mechanisms concerning temperature (and precipitation)
changes, the extent of published knowledge has not, as yet, supported a strong emphasis on
these topics. The focus on the MWP was perhaps to some degree a response to the
misinformation peddled by certain climate warming sceptics, but I believe it was justified
to devote the amount of limited space allotted to this section to the area of large-scale
temperature reconstructions, especially considering the extent of the recent literature and
the attacks on the TAR hockey stick. I hope we did a reasonable job in assessing the
evidence honestly. I am in no doubt that future IPCC reports will reflect a growing body
of evidence for the existence of large natural variability in moisture conditions and ,
hopefully, the dynamic mechanisms whereby temperature and moisture have varied over space
in recent millennia. In our defence I would also say that the AR4 clearly pointed to the
importance of the issue of natural drought occurence and cited the best relevant work
demonstrating this - ie your own.
My beef with Esper is not because his conclusion is wrong - merely that his piece wrongly
impugns the IPCC. Through a subtle combination of selective focus, blatant
misrepresentation of the text, and a complete failure to acknowledge the circumspect
language and explicit caveats therein, he builds a straw man and succeeds in publishing a
trivial, unoriginal idea.
At 14:48 01/10/2008, you wrote:

Hi Keith and Tim,
I have quickly read through the Esper paper and have the following comments to make.
First, I hadn't seen it before, so it is all new to me. It is certainly true that Jan
did not do a proper job citing Briffa et al. (1992). That was a clear mistake,
especially given that Douglass (1929) was cited for crossdating. I also note that Jan
did not cite Osborn et al. (1997) on adjusting the variance in series for sample size
changes. That too was an clear oversight given that Frank et al. (2007) was cited.
Hopefully, neither was done intentionally. I tend to give people the benefit of a doubt
on that unless it is a chronic problem in their publications. The latter issue of
variance adjustment is also relevant to the discussion concerning spatial homogeneity or
lack thereof. Am I correct in assuming that some form of variance adjustment was made to
the series used in the AR4 report? I haven't read the report closely enough to recall if
that was done. If it was done, that would tend to force the data towards an appearance
of greater homogeneity, I would guess, hence the relative stability of the bootstrap
intervals, etc.. In any case, I do tend to agree with Jan that nothing very definitive
can be said about the spatial homogeneity of the putative MWP until we get more records
to look at that truly express temperature and not something else.
The whole issue of whether or not the MWP was more spatially heterogeneous or not is a
huge "red herring" in my opinion anyway. A growing body of evidence clearly shows that
hydroclimatic variability during the putative MWP (more appropriately and inclusively
called the "Medieval Climate Anomaly" or MCA period) was more regionally extreme (mainly
in terms of the frequency and duration of megadroughts) than anything we have seen in
the 20th century, except perhaps for the Sahel. So in certain ways the MCA period may
have been more climatically extreme than in modern times. The problem is that we have
been too fixated on temperature, especially hemispheric and global average temperature,
and IPCC is enormously guilty of that. So the fact that evidence for "warming" in
tree-ring records during the putative MWP is not as strong and spatially homogeneous as
one would like might simply be due to the fact that it was bloody dry too in certain
regions, with more spatial variability imposed on growth due to regional drought
variability even if it were truly as warm as today. The Calvin cycle and
evapotranspiration demand surely prevail here: warm-dry means less tree growth and a
reduced expression of what the true warmth was during the MWP.
That is my take on the Esper and Frank paper, with obvious editorial comments included
as well.
Dr. Edward R. Cook
Doherty Senior Scholar and
Director, Tree-Ring Laboratory
Lamont-Doherty Earth Observatory
Palisades, New York 10964 USA
Email: [1]
Phone: 845-365-8618
Fax: 845-365-8152
On Sep 29, 2008, at 11:06 AM, Keith Briffa wrote:

X-Mailer: QUALCOMM Windows Eudora Version
Date: Mon, 29 Sep 2008 09:50:59 +0100
To: Keith Briffa <[2]>
From: Tim Osborn <[3]>
Subject: recognize this?!
Dr Timothy J Osborn, Academic Fellow
Climatic Research Unit
School of Environmental Sciences
University of East Anglia
Norwich NR4 7TJ, UK
e-mail: [4]
phone: +44 1603 592089
fax: +44 1603 507784
web: [5]
sunclock: [6]

Professor Keith Briffa,
Climatic Research Unit
University of East Anglia
Norwich, NR4 7TJ, U.K.
Phone: +44-1603-593909
Fax: +44-1603-507784
[7] <esper frank IPCC on MWP hetero 2008.pdf>

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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