Wednesday, May 2, 2012


date: Sat, 11 Mar 2006 15:27:19 -0500 (EST)
from: <>
subject: NRC and IPCC millennial temperatures

Friends in the IPCC WG1 AR4--

My impression is that, for good reasons, the US NRC panel looking at
the record of temperatures over the last millennium or two is not going
to strongly endorse the ability of proxies to detect warming above the level
of a millennium ago, and that a careful re-examination of the
Chapter 6 wording and its representation in the TS and SPM would
be wise. Some of you have seen some of the discussion that follows,
in some of the rapid-fire emails over the last day or two, but I'd like
to clarify a little.

Please note that I am NOT on the NRC committee, do not speak for them, and
have no "inside" knowledge of what they are doing. I was asked to
testify to them, and I heard remarks from some other speakers and questions
from the committee in public forum. I did NOT represent the IPCC to the
committee, either; I stated that although I was proud to be participating
with the IPCC, I absolutely was not speaking for, representing, or presaging
anything in the IPCC. (I was, however, favorably quite impressed with the NRC
committee and their efforts.) Someone else may have a different impression of
what went on; this is mine.

Among the presentations, involving borehole temperatures, corals, glaciers and
ice cores, and historical records, that which to me seemed to interest
the committee most was from Rosanne d'Arrigo, who reported (among many other
things) on a just-published study in which northern tree-ring sites were
revisited and updated, and in which many of those sites failed to track the
recent warming documented instrumentally. She did not make a big deal out
of this, but several of the questions afterward from the committee focused on
this "divergence" problem. (And to note, Rosanne did not discover the
divergence problem, which has been around and discussed for a while; her
testimony, including the recent large effort to update some tree-ring records,
stirred interest from some committee members.)

I would also note that one of the committee members was asking each presenter
whether the presenter believed that temperatures could be reconstructed for
1000 years ago within 0.5 C, and that the presenters were answering with some
qualified version of "no".

My guess is that the NRC committee will put these things together, find some
papers on ozone damage and CO2 fertilization, consider Rosanne's statement
that the preferred temperature-sensitive trees are rare and in restricted places
(and thus that a prolonged warming could easily move those trees out of the
sensitive band), and conclude that tree-ring reconstructions include larger
errors than are returned by any of the formal statistics from calibration
or aggregation of records, and thus that there is less confidence than
previously believed in the relative warmth of recent versus Medieval times.
I also consider it possible that they will point out the difficulty of using
a composite temperature history consisting of proxy and instrumental data if
some of the proxy data do not track the more-recent part of the instrumental

The IPCC must be the IPCC, not the NRC. But, if the IPCC and NRC look very
different, there will be much comment, and we will have to be very sure.
More importantly, I believe that real issues are raised here, and that
better discussion of this should be included in chapter 6, and probably
brought forward at least into the TS. I know I'm not in chapter 6, I know
I'm not a tree-ring expert, and I know I'm sticking my nose in where it might
not belong or be welcome. But the flurry of emails in the last couple of days
has not convinced me that this one can be ignored; indeed, I am more convinced
that there exist issues that the IPCC must discuss more thoroughly.

My impression of the status (and my thoughts about what chapter 6 might say)
from a whole lot of quick reading, your emails, and the testimony and
questions I heard, is along the lines of:

--> The TAR highlighted a temperature history composited from multi-proxy
paleoclimatic indicators plus the instrumental record, showing anomalous
recent warmth, with the recent warmth emerging well above the 95% confidence
interval for the last millennium.

--> The multi-proxy paleoclimatic indicators reflect tree-ring results more
than any other source.

--> Tree-ring records are responsive to many factors, and great care and
effort go into isolating the temperature signal from other signals.

--> Tree-ring data, in common with essentially all paleoclimatic data, are not
collected in a continually updated "operational" fashion analogous to that used
for meteorological data, so the data sets end at different times; data used
in the multi-proxy reconstructions cited in the TAR ended between the 1990s
and the 1940s. This difficulty motivated the need to include instrumental
as well as proxy data in the reconstructions.

--> In those data, there was some suggestion of non-temperature influences
on the tree-ring reconstructions; in particular, some of the
most-recent records did not record the full amplitude of the instrumental
warming. This has come to be known as the "divergence" issue.

--> Much research has been conducted since the TAR, and additional evidence
of divergence has emerged in some records, causing some aggregated
reconstructions from proxy records to show less warming than does the
instrumental record.

--> There are many hypotheses for non-temperature influences on tree-ring
records, including: (i) recent damage (as by ozone); (ii) recent fertilization
(as by CO2); and (iii) decreasing sensitivity of tree-ring growth to
temperature with increasing temperature (once it's warm enough, the trees are
primarily responsive to other things). The nature of these and their timing
relative to the interval in which tree-ring data were calibrated to
instrumental records would control the effects on climate reconstructions.
In general: (i) would mean that recent warmth is underestimated but warmth
from a millennium ago is not; (ii) would mean that recent warmth is
overestimated but warmth from a millennium ago is not; and (iii) would mean
that both recent warmth and warmth from a millennium ago are underestimated.

--> Various arguments have been advanced to support (i), (ii), or (iii),
with many workers in the field favoring (i). Nonetheless, further
characterizing recent non-temperature influences on tree-ring growth
remains an open research question, and no broad consensus has emerged on
(i), (ii), (iii), or something else.

--> These considerations do not affect the conclusion that recent warmth
is anomalous over the last few centuries; the strong correlations of the
proxy data with temperature over the instrumental record, and the strong
tree-ring signals, are evident.

--> These considerations do not affect the best estimate that recent warmth
is greater than that of a millennium ago; the central estimate from proxy data
of latter-twentieth-century warmth is still above that of a millennium ago, with
greater spatial conherence recently in the signal.

--> These considerations do somewhat affect the confidence that can be attached
to the best estimate of recent warmth versus that of a millennium ago. If the
paleoclimatic data could be confidently be interpreted as paleotemperatures,
then joining the paleoclimatic and instrumental records would be appropriate,
and the recent warmth would clearly be anomalous over the last millennium and
beyond. By demonstrating that some tree-ring series chosen for temperature
sensitivity are not fully reflecting temperature changes, the divergence issue
widens the error bars and so reduces confidence in the comparison between
recent and earlier warmth.


Richard B. Alley
Evan Pugh Professor
Department of Geosciences, and Earth and Environmental Systems Institute
The Pennsylvania State University
517 Deike Building
University Park, PA 16802, USA
ph. 814-863-1700
fax 814-863-7823

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