cc: "Michael E. Mann" <mann@virginia.edu>, esper@ldeo.columbia.edu, Jonathan Overpeck <jto@u.arizona.edu>, Keith Briffa <k.briffa@uea.ac.uk>
date: Wed, 2 May 2001 10:59:37 -0400
from: Edward Cook <drdendro@ldeo.columbia.edu>
subject: Re: hockey stick
to: tom crowley <tom@ocean.tamu.edu>

>Ed,
>
>heard some rumor that you are involved in a non-hockey stick reconstruction
>of northern hemisphere temperatures.  I am very intrigued to learn about
>this - are these results suggesting the so called Medieval Warm Period may
>be warmer than the early/mid 20th century?
>
>any enlightenment on this would be most appreciated, Tom
>
>
>
>Thomas J.  Crowley
>Dept. of Oceanography
>Texas A&M University
>College Station, TX  77843-3146
>979-845-0795
>979-847-8879 (fax)
>979-845-6331 (alternate fax)

Hi Tom,

As rumors often are, the one you heard is not entirely accurate. So, I will
take some time here to explain for you, Mike, and others exactly what was
done and what the motivation was, in an effort to hopefully avoid any
misunderstanding. I especially want to avoid any suggestion that this work
was being done to specifically counter or refute the "hockey stick".
However, it does suggest (as do other results from your EBM, Peck's work,
the borehole data, and Briffa and Jones large-scale proxy estimates) that
there are unresolved (I think) inconsistencies in the low-frequency aspects
of the hockey stick series compared to other results. So, any comparisons
with the hockey stick were made with that spirit in mind.

What Jan Esper and I are working on (mostly Jan with me as second author)
is a paper that was in response to Broecker's Science Perspectives piece on
the Medieval Warm Period. Specifically, we took strong exception to his
claim that tree rings are incapable of preserving century time scale
temperature variability. Of course, if Broecker had read the literature, he
would have known that what he claimed was inaccurate. Be that as it may,
Jan had been working on a project, as part of his post-doc here, to look at
large-scale, low-frequency patterns of tree growth and climate in long
tree-ring records provided to him by Fritz Schweingruber. With the addition
of a couple of sites from foxtail pine in California, Jan amassed a
collection of 14 tree-ring sites scattered somewhat uniformly over the
30-70 degree NH latitude band, with most extending back 1000-1200 years.
All of the sites are from temperature-sensitive locations (i.e. high
elevation or high northern latitude. It is, as far as I know, the largest,
longest, and most spatially representative set of such
temperature-sensitive tree-ring data yet put together for the NH
extra-tropics.

In order to preserve maximum low-frequency variance, Jan used the Regional
Curve Standardization (RCS) method, used previously by Briffa and myself
with great success. Only here, Jan chose to do things in a somewhat radical
fashion. Since the replication at each site was generally insufficient to
produce a robust RCS chronology back to, say, AD 1000, Jan pooled all of
the original measurement series into 2 classes of growth trends: non-linear
(~700 ring-width series) and linear (~500 ring-width series). He than
performed independent RCS on the each of the pooled sets and produced 2 RCS
chronologies with remarkably similar multi-decadal and centennial
low-frequency characteristics. These chronologies are not good at
preserving high-frquency climate information because of the scattering of
sites and the mix of different species, but the low-frequency patterns are
probably reflecting the same long-term changes in temperature. Jan than
averaged the 2 RCS chronologies together to produce a single chronology
extending back to AD 800. It has a very well defined Medieval Warm Period -
Little Ice Age - 20th Century Warming pattern, punctuated by strong decadal
fluctuations of inferred cold that correspond well with known histories of
neo-glacial advance in some parts of the NH. The punctuations also appear,
in some cases, to be related to known major volcanic eruptions.

Jan originally only wanted to show this NH extra-tropical RCS chronology in
a form scaled to millimeters of growth to show how forest productivity and
carbon sequestration may be modified by climate variability and change over
relatively long time scales. However, I encouraged him to compare his
series with NH instrumental temperature data and the proxy estimates
produced by Jones, Briffa, and Mann in order bolster the claim that his
unorthodox method of pooling the tree-ring data was producing a record that
was indeed related to temperatures in some sense. This he did by linearly
rescaling his RCS chronology from mm of growth to temperature anomalies. In
so doing, Jan demonstrated that his series, on inter-decadal time scales
only, was well correlated to the annual NH instrumental record. This result
agreed extremely well with those of Jones and Briffa. Of course, some of
the same data were used by them, but probably not more than 40 percent
(Briffa in particular), so the comparison is based on mostly, but not
fully, independent data. The similarity indicated that Jan's approach was
valid for producing a useful reconstruction of multi-decadal temperature
variability (probably weighted towards the warm-season months, but it is
impossible to know by how much) over a larger region of the NH
extra-tropics than that produced before by Jones and Briffa. It also
revealed somewhat more intense cooling in the Little Ice Age that is more
consistent with what the borehole temperatures indicate back to AD 1600.
This result also bolsters the argument for a reasonably large-scale
Medieval Warm Period that may not be as warm as the late 20th century, but
is of much(?) greater significance than that produced previously.

Of course, Jan also had to compare his record with the hockey stick since
that is the most prominent and oft-cited record of NH temperatures covering
the past 1000 years. The results were consistent with the differences shown
by others, mainly in the century-scale of variability. Again, the Esper
series shows a very strong, even canonical, Medieval Warm Period - Little
Ice Age - 20th Century Warming pattern, which is largely missing from the
hockey stick. Yet the two series agree reasonably well on inter-decadal
timescales, even though they may not be 1:1 expressions of the same
temperature window (i.e. annual vs. warm-season weighted). However, the
tree-ring series used in the hockey stick are warm-season weighted as well,
so the difference between "annual" and "warm-season weighted" is probably
not as large as it might seem, especially before the period of instrumental
data (e.g. pre-1700) in the hockey stick. So, they both share a significant
degree of common interdecal temperature information (and some, but not
much, data), but do not co-vary well on century timescales. Again, this has
all been shown before by others using different temperature
reconstructions, but Jan's result is probably the most comprehensive
expression (I believe) of extra-tropical NH temperatures back to AD 800 on
multi-decadal and century time scales.

Now back to the Broecker perspectives piece. I felt compelled to refute
Broecker's erroneous claim that tree rings could not preserve long-term
temperature information. So, I organized a "Special Wally Seminar" in which
I introduced the topic to him and the packed audience using Samuel
Johnson's famous "I refute it thus" statement in the form of "Jan Esper and
I refute Broecker thus". Jan than presented, in a very detailed and well
espressed fashion, his story and Broecker became an instant convert. In
other words, Wally now believes that long tree-ring records, when properly
selected and processed, can preserve low-frequency temperature variability
on centennial time scales. Others in the audience came away with the same
understanding, one that we dendrochronologists always knew to be the case.
This was the entire purpose of Jan's work and the presentation of it to
Wally and others. Wally had expressed some doubts about the hockey stick
previously to me and did so again in his perspectives article. So, Jan's
presentation strongly re-enforced Wally's opinion about the hockey stick,
which he has expressed to others including several who attended a
subsequent NOAA meeting at Lamont. I have no control over what Wally says
and only hope that we can work together to reconcile, in a professional,
friendly manner, the differences between the hockey stick and other proxy
temperature records covering the past 1000 years. This I would like to do.

I do think that the Medieval Warm Period was a far more significant event
than has been recognized previously, as much because the high-resolution
data to evaluate it had not been available before. That is much less so the
case now. It is even showing up strongly now in long SH tree-ring series.
However, there is still the question of how strong this event was in the
tropics. I maintain that we do not have the proxies to tell us that now.
The tropical ice core data are very difficult to interpret as temperature
proxies (far worse than tree rings for sure and maybe even unrelated to
temperatures in any simple linear sense as is often assumed), so I do not
believe that they can be used alone as records to test for the existence of
a Medieval Warm Period in the tropics. That being the case, there are
really no other high-resolution records from the tropics to use, and the
teleconnections between long extra-tropical proxies and the tropics are, I
believe, far too tenuous and probably unstable to use to sort out this
issue.

So, at this stage I would argue that the Medieval Warm Period was probably
a global extra-tropical event, at the very least, with warmth that was
persistent and probably comparable to much of what we have experienced in
the 20th century. However, I would not claim (and nor would Jan) that it
exceeded the warmth of the late 20th century. We simply do not have the
precision or the proxy replication to say that yet. This being said, I do
find the dismissal of the Medieval Warm Period as a meaningful global event
to be grossly premature and probably wrong. Kind of like Mark Twain's
commment that accounts of his death were greatly exaggerated. If, as some
people believe, a degree of symmetry in climate exists between the
hemispheres, which would appear to arise from the tropics, then the
existence of a Medieval Warm Period in the extra-tropics of the NH and SH
argues for its existence in the tropics as well. Only time and an enlarged
suite of proxies that extend into the tropics will tell if this is true.

I hope that what I have written clarifies the rumor and expresses my views
more completely and accurately.

Cheers,

Ed

==================================
Dr. Edward R. Cook
Doherty Senior Scholar
Tree-Ring Laboratory
Lamont-Doherty Earth Observatory
Palisades, New York  10964  USA
Phone:  1-845-365-8618
Fax:    1-845-365-8152
Email:  drdendro@ldeo.columbia.edu
==================================


