Saturday, June 9, 2012


date: Sun, 19 Mar 2006 16:53:00 -0500
from: "Michael E. Mann" <>
subject: Re: Trees
to: Richard Alley <>

Hi Richard,
Thanks for your email, and for your earnest views. There was indeed considerable discussion
of thes issues on friday, the day after your talk. Both Malcolm Hughes and I discussed
these issues in some detail with the committee. Please feel free to take a look at the
presentation I gave to the committee:
There is no doubt that there are issues with the potential non-stationarity of tree
responses to climate, and this introduces caveats. As I pointed out to the committee, these
issues were actually stressed in our '99 article which produced the millennial temperature
reconstruction, the title of which was (emphasis added) "[2]Northern Hemisphere
Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations". The
underlying assumption of our own work has always been that each of the proxies have their
own potential problems, and "multiproxy" approaches are probably the most robust. I don't
have a particular axe to grind about any particular proxy, and recognize that there are
some pretty serious potential problems with all proxies, including ice core delta o18 (as
you're aware, these are not clean paleotemperature proxies at all), and Sr/Ca or o18 from
corals. There is a good discussion of the strengths and weaknesses in all of the proxies in
Jones and Mann (2004): Jones, P.D., Mann, M.E., [3]Climate Over Past Millennia, Reviews of
Geophysics, 42, RG2002, doi: 10.1029/2003RG000143, 2004.
I won't try to defend Rosanne D'Arrigo's analysis, because frankly many in the tree-ring
community feel it was not very good work.You should be aware that her selection criteria
were not as rigorous as those used by other researchers, and the conclusions she comes to
reflect only the data and standardization methods she used--they don't speak for many
other, in my mind, more careful studies. If you want the views of the leading experts in
this community, I would refer you to my colleagues Malcolm Hughes and Keith Briffa, who
have been carefully researching these issues for decades. With your permission, I'd like to
forward your email to them for a more informed response--would that be ok?
>From the questions asked by the community, I really only sensed from one individual the
sort of extreme tree-ring skepticism that you describe. And I frankly think the individual
proved himself to be not especially informed. The committee appeared to be convinced by the
responses I provided to that individual. In short (and please see my presentation for
further information) I made the following points: 1) multiproxy reconstructions that don't
use tree-ring information at all for the long-term variability (Moberg et al, 2005) agree
w/ all other (roughly a dozen now) reconstructions that late 20th century warmth is
anomalous in the context of the past 2000 years at the hemispheric scale. This is one key
point (i.e, the take home conclusion doesn't depend on tree-rings at all!). Another point
I made is that the criticism (by some) that tree-rings underestimate the low-frequency
variability is seriously challenged by the fact that temperature reconstructions based on
data such as northern hemisphere glacier mass balance inversions (i.e. Oerlemans et al,
2005) show less hemispheric LIA cooling than many of the purely tree-ring based
reconstructions. Another point I made in response to this line of criticism is that many
of the long-term tree-ring series used in these reconstructions (see e.g. the recent
Science article by Osborn and Briffa) show late 20th century conditions that are
unprecedented in at least a millennial context. That is to say, if there is some upper
temperature threshold in the past beyond which trees do not record, they do not appear to
have encountered that threshold prior to the late 20th century, because the most positive
anomalies in more than a thousand years are encountered in the last 20th century for most
regions. The Osborn and Briffa science paper (attached) shows that the conclusion of
anomalous 20th century warmth is spatially robust in a pan-hemispheric data set, it does
not just reflect one region (in fact, they show that their conclusion of anomalous late
20th century warmth is robust to the elimination of any three data series used). Of course,
Lonnie Thompson comes to the same conclusion using composites of his tropical ice cores.
i.e., that the late 20th century behavior is anomalous in a greater-than-millennial
context. Of course, tropical ice core delta o18 is difficult to defend as a pure
paleothermometer too, but in this case it is difficult to see where non-climatic impacts
could enter into the anomalous late 20th century behavior.
So, in short, while the issues you mention are real (and have been emphasized by those
actually working in this area for decades, as well as by us in all of our key
publications), the primary conclusions (i.e. that late 20th century warmth is robust in at
least a millennial context) appears robust, and is common to reconstructions whether or not
they use tree-rings to reconstruct the low-frequency variability. . There is yet another
study (embargoed right now in Science) that comes precisely to this same conclusion yet
I'll actually be quite surprised if the committee comes to a *different* conclusion from
that. Nonetheless, I appreciate your comments and your concerns, and your message does
highlight a few issues which would be useful for us to clarify for the committee in case
there is still any misunderstanding of the key points I have raised.
As I said, I'd like to be able to forward your message to Keith and Malcolm, if this is ok
w/ you, so that they can provide a perhaps even better informed response to the criticisms
you raise. So please let me know if that would be ok...
Richard Alley wrote:

Mike--Just a quick follow-up on what I was saying Wednesday. Reconstruction
of temperatures for the last millennium is not my problem, I'm not doing
anything with it, I have no ax to grind, and I'm not tapped into the
deliberations of the NRC committee. But, I think it is highly likely that
the committee will end up casting doubts on the use of tree rings as
paleothermometers. (I actually don't expect the committee to get very
excited about supposed issues with EOF/PCA or similar statistical red
herrings; I think the committee will focus on the indicators rather than on
the methods of aggregating the indicators.) (I also rather expect that the
committee will ask some pointed questions of the ground-temperature
paleothermometers, and especially of groundwater-motion signals.)

The triggering issue was the "divergence" problem as raised by Rosanne
D'Arrigo, that a spatially and temporally complex difference has arisen
between many of the long tree-ring records and the instrumental record more
recently than the calibration period in many cases. This has been in the
literature for a while, as you know much better than I do, and was not
highlighted by Rosanne in her talk, but some committee members jumped on it
in questions, and she was not convincing that trees were thermometers when
it was warm a millennium ago but are not thermometers when it is warm now.
She mentioned the existence of hypotheses (ozone or other pollution damage,
for example), and I believe she tried some of the arguments about spatial
coherence/incoherence of divergence versus nondivergence and of recent
warmth versus medieval warmth, but overall was not convincing to me. (I'm
happy to go into details as to why the arguments were not convincing,
insofar as I captured the arguments, but they were not convincing to me, and
looking around the committee room, I don't think they were convincing to
important members of the committee.)

Under one of my other hats, I raised this issue in a very non-public way
with one individual and a request to keep it quiet, but my questions were
rather quickly circulated well beyond and elicited some fairly warm replies
from some well-respected tree-ring people--I am undoubtedly persona non
grata in some quarters right now. But, having read the arguments, I still
find them non-convincing. I don't believe the tree-ring community knows
with any confidence whether there is a pollution signal, whether there could
have been CO2 fertilization during the calibration period and falling off
since in the way CO2 fertilization seems to behave in natural systems,
whether there is now moisture stress or a snow-cover signal, whether the
most-temperature-sensitive trees simply become less temperature-sensitive
when temperatures become sufficiently high, or whether something else is
going on. Clearly, if some of these are correct, then the "divergence
problem" has no bearing on thermometry of a millennium ago; if others are
correct, then thermometry of a millennium ago is affected. Nor do I believe
that the community has nearly enough data to dismiss the recent signal as
just a northern problem, or as being so anomalous in spatial pattern as to
demonstrate that it must be anthropogenic. (The Cook-Esper 2004 QSR paper
shows the North and South reconstructions as being about 5 normal deviates
apart now but much closer medieval, but the east and west are close now and
were over 3 normal deviates apart in the medieval, so it seems a stretch to
argue that the recent must be anthropogenically anomalous for non-climatic

My suspicion is that the committee will end up noting that the peak
twentieth century warmth in most reconstructions exceeds that of the
tenth/eleventh century interval, and thus that recent warmth being higher is
the leading estimate; however, the confidence in that may not rise much
above 50%. I will be surprised if the committee is much friendlier to the
millennial reconstructions than that, although I obviously could be
completely off base.

I don't want to stir up trouble, I don't want to piss off the tree-ring
people yet again, but I do think that the tree-ring workers (and by
association, all of us who do climate change) have a serious problem, and
have not answered it very well yet. If better answers are out there, I hope
that they come out soon.


Michael E. Mann
Associate Professor
Director, Earth System Science Center (ESSC)

Department of Meteorology Phone: (814) 863-4075
503 Walker Building FAX: (814) 865-3663
The Pennsylvania State University email: [4]
University Park, PA 16802-5013


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