The controversy of the MBH98/99 methods lies in that the
proxies are incorrectly centered on the mean of the period
1902-1995, rather than on the whole time period. The proxy
data exhibiting the hockey stick shape are actually
decentered low. The updated MBH99 reconstruction is given in
Figure 3. This fact that the proxies are centered low is
apparent in Figure 3 because for most of the 1000 years, the
reconstruction is below zero. Because the 'hockey stick'
proxies are centered too low, they will exhibit a larger
effective 'variance', allowing the method to exhibit a
preference for selecting them as the first principal
component.
Figure 8 is a graphic that depicts a number of papers in the
paleoclimate reconstruction area together with some of the
proxies used. We note that many of the proxies are shared.
Using the same data also suggests a lack of independence.
The MBH98/99 work has been sufficiently politicized that
this community can hardly reassess their public positions
without losing credibility. Overall, our committee believes
that the MBH99 assessment that the decade of the 1990s was
the likely the hottest decade of the millennium and that
1998 was likely the hottest year of the millennium cannot be
supported by their analysis.
--------------------------------------------------------
Original source URL:
http://www.urban-renaissance.org/urbanren/publications/Wegman[2].pdf
Ad Hoc Committee Report on the 'Hockey Stick' Global Climate Reconstruction
Testimony of Edward J. Wegman
I would like to begin by circumscribing the substance of our report.
We were asked to provide an independent verification by statisticians
of the critiques of the statistical methodology found in the papers
of Drs. Michael Mann, Raymond Bradley and Malcolm Hughes published
respectively in Nature in 1998 and in Geophysical Research Letters in
1999. These two papers have commonly been referred to as MBH98 and
MBH99. The critiques have been made by Stephen McIntyre and Ross
McKitrick published in Energy and Environment in 2003 and in Energy
and Environment and in Geophysical Research Letters in 2005. We refer
to these as MM03, MM05a, and MM05b respectively. We were also asked
about the implications of our assessment. We were not asked to assess
the reality of global warming and indeed this is not an area of our
expertise. We do not assume any position with respect to global
warming except to note in our report that the instrumented record of
global average temperature has risen since 1850 according to the MBH
99 chart by about 1.2o centigrade. In the NAS panel Report chaired by
Dr. North, .6o centigrade is mentioned in several places.
Our panel is composed of Edward J. Wegman (George Mason University),
David W. Scott (Rice University), and Yasmin H. Said (The Johns
Hopkins University). This Ad Hoc Panel has worked pro bono, has
received no compensation, and has no financial interest in the
outcome of the report.
[Go to Figure 1]
MBH98, MBH99 use several proxy indicators to measure global climate
change. Primarily, these include historical records, tree rings, ice
cores, and coral reefs. More details of proxies are given in the
report and mentioned in the written testimony. [The width and density
of tree rings vary with climatic conditions (sunlight, precipitation,
temperature, humidity, and carbon dioxide and nitrogen oxides
availability), soil conditions, tree species, tree age, and stored
carbohydrates in the trees. The width and density of tree rings are
dependent on many confounding factors, making isolation of the
climatic temperature signal uncertain. It is usually the case that
width and density of tree rings are monitored in conjunction in order
to more accurately use them as climate proxies. Ice cores are the
accumulation of snow and ice over many years that have recrystallized
and have trapped air bubbles from previous time periods. The
composition of these ice cores, especially the presence of hydrogen
and oxygen isotopes, provides a picture of the climate at the time.
The relative concentrations of the heavier isotopes in the condensate
indicate the temperature of condensation, allowing for ice cores to
be used in global temperature reconstruction. In addition to the
isotope concentration, the air bubbles trapped in the ice cores allow
for measurement of the atmospheric concentrations of trace gases,
including greenhouse gases carbon dioxide, methane, and nitrous
oxide.]
[Go to Figure 2]
Some examples of tree ring proxy series are given in Figure 2. Most
of the proxy series show little structure, but the last two show the
characteristic 'hockey stick' shape. The principal component-like
methodology in MBH 98/99 preferentially emphasizes these shapes as we
shall see.
Principal component analysis methodology is at the core of the
MBH98/99 analysis methodology. Principal component analysis is a
statistical methodology often used for reducing datasets with many
variables into datasets with fewer, but composite variables. The time
series proxy data involved are transformed into their principal
components, where the first principal component is intended to
explain most of the variation present in the data variables. Each
subsequent principal component explains less and less of the
variation. In the methodology of MBH98/99, the first principal
component is used in the temperature reconstruction.
[Go to Figure 3]
Two principal methods for temperature reconstructions have been used;
CFR (climate field construction used in MBH98/99) and CPS
(climate-plus-scale). The CFR is essentially the principal component
based analysis and the CPS is a simple averaging of climate proxies.
The controversy of the MBH98/99 methods lies in that the proxies are
incorrectly centered on the mean of the period 1902-1995, rather than
on the whole time period. The proxy data exhibiting the hockey stick
shape are actually decentered low. The updated MBH99 reconstruction
is given in Figure 3. This fact that the proxies are centered low is
apparent in Figure 3 because for most of the 1000 years, the
reconstruction is below zero. Because the 'hockey stick' proxies are
centered too low, they will exhibit a larger effective 'variance',
allowing the method to exhibit a preference for selecting them as the
first principal component. The net effect of this decentering using
the proxy data in MBH98 and MBH99 is to produce a 'hockey stick'
shape. Centering on the overall mean is a critical factor in using
the principal component methodology properly.
[Go to Figure 4]
To illustrate this, we consider the North America Tree series and
apply the MBH98 methodology. The top panel shows the result from the
de-centering. The bottom panel shows the result when the principal
components are properly centered. Thus the centering does make a
significant difference to the reconstruction.
[Go to Figure 5]
To further illustrate this, we digitized the temperature profile
published in the IPCC 1990 report and applied both the CFR and the
CPS methods to them. The data used here are 69 unstructured noise
pseudo-proxy series and only one copy of the 1990 profile. The upper
left panel illustrates the PC1 with proper centering. In other words,
no structure is shown. The other 3 panels indicate what happens using
principal components with an increasing amount of de-centering.
Again, the single series begins to overwhelm the other 69 pure noise
series. Clearly, these have a big effect.
It is not clear that Mann and associates realized the error in their
methodology at the time of publication. Our re-creation supports the
critique of the MBH98 methods.
In general, we found the writing in MBH98 and MBH99 to be somewhat
obscure and incomplete and the criticisms by MM03/05a/05b to be
valid. The reasons for setting 1902-1995 as the calibration period
presented in the narrative of MBH98 sounds plausible, and the error
may be easily overlooked by someone not trained in statistical
methodology. We note that there is no evidence that Dr. Mann or any
of the other authors in paleoclimate studies have had significant
interactions with mainstream statisticians.
Because of this apparent isolation, we decided to attempt to
understand the paleoclimate community by exploring the social network
of authorships in temperature reconstruction.
[Go to Figure 6]
We found that at least 43 authors have direct ties to Dr. Mann by
virtue of coauthored papers with him. Our findings from this analysis
suggest that authors in the area of this relatively narrow field of
paleoclimate studies are closely connected. Dr. Mann has an unusually
large reach in terms of influence and in particular Drs. Jones,
Bradley, Hughes, Briffa, Rutherford and Osborn.
[Go to Figure 7]
Because of these close connections, independent studies may not be as
independent as they might appear on the surface. Although we have no
direct data on the functioning of peer review within the paleoclimate
community, but with 35 years of experience with peer review in both
journals as well as evaluation of research proposals, peer review may
not have been as independent as would generally be desirable.
[Go to Figure 8]
Figure 8 is a graphic that depicts a number of papers in the
paleoclimate reconstruction area together with some of the proxies
used. We note that many of the proxies are shared. Using the same
data also suggests a lack of independence.
The MBH98/99 work has been sufficiently politicized that this
community can hardly reassess their public positions without losing
credibility. Overall, our committee believes that the MBH99
assessment that the decade of the 1990s was the likely the hottest
decade of the millennium and that 1998 was likely the hottest year of
the millennium cannot be supported by their analysis. Recommendations
Recommendation 1. Especially when massive amounts of public monies
and human lives are at stake, academic work should have a more
intense level of scrutiny and review. It is especially the case that
authors of policy-related documents like the IPCC report, Climate
Change 2001: The Scientific Basis, should not be the same people as
those that constructed the academic papers.
Recommendation 2. We believe that federally funded research agencies
should develop a more comprehensive and concise policy on disclosure.
All of us writing this report have been federally funded. Our
experience with funding agencies has been that they do not in general
articulate clear guidelines to the investigators as to what must be
disclosed. Federally funded work including code should be made
available to other researchers upon reasonable request, especially if
the intellectual property has no commercial value. Some consideration
should be granted to data collectors to have exclusive use of their
data for one or two years, prior to publication. But data collected
under federal support should be made publicly available.
Recommendation 3. With clinical trials for drugs and devices to be
approved for human use by the FDA, review and consultation with
statisticians is expected. Indeed, it is standard practice to include
statisticians in the application-for-approval process. We judge this
to be a good policy when public health and also when substantial
amounts of monies are involved, for example, when there are major
policy decisions to be made based on statistical assessments. In such
cases, evaluation by statisticians should be standard practice. This
evaluation phase should be a mandatory part of all grant applications
and funded accordingly.
Recommendation 4. Emphasis should be placed on the Federal funding of
research related to fundamental understanding of the mechanisms of
climate change. Funding should focus on interdisciplinary teams and
avoid narrowly focused discipline research.
[figures and graphs in PDF original]
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