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AD HOC COMMITTEE REPORT ON THE ‘HOCKEY

STICK’ GLOBAL CLIMATE RECONSTRUCTION2

 

EXECUTIVE SUMMARY

 

The Chairman of the Committee on Energy and Commerce as well as the Chairman of

the Subcommittee on Oversight and Investigations have been interested in an

independent verification of the critiques of Mann et al. (1998, 1999) [MBH98, MBH99]

by McIntyre and McKitrick (2003, 2005a, 2005b) [MM03, MM05a, MM05b] as well as

the related implications in the assessment. The conclusions from MBH98, MBH99 were

featured in the Intergovernmental Panel on Climate Change report entitled Climate

Change 20013: The Scientific Basis. This report concerns the rise in global temperatures,

specifically during the 1990s. The MBH98 and MBH99 papers are focused on

paleoclimate temperature reconstruction and conclusions therein focus on what appear to

be a rapid rise in global temperature during the 1990s when compared with temperatures

of the previous millennium. These conclusions generated a highly polarized debate over

the policy implications of MBH98, MBH99 for the nature of global climate change, and

whether or not anthropogenic actions are the source. This committee, composed of

Edward J. Wegman (George Mason University), David W. Scott (Rice University), and

Yasmin H. Said (The Johns Hopkins University), has reviewed the work of both articles,

as well as a network of journal articles that are related either by authors or subject matter,

and has come to several conclusions and recommendations. This Ad Hoc Committee has

worked pro bono, has received no compensation, and has no financial interest in the

outcome of the report.

 

Global Climate Reconstruction Methodology

 

MBH98, MBH99 use several indicators to measure global climate change. Primarily,

these include historical records, tree rings, ice cores, and coral reefs. 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. However, tree ring density is

useful in paleoclimatic temperature reconstructions because in mature trees, tree rings

vary approximately linearly with age. The width and density of tree rings are dependent

on many confounding factors, making it difficult to isolate the climatic temperature

signal. 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. Because isotopes of water vapor exhibit a lower vapor pressure, when

the temperature falls, the heavier water molecules will condense faster than the normal

water molecules. The relative concentrations of the heavier isotopes in the condensate

indicate the temperature of condensation at the time, 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.

The air bubbles may also contain traces of aerosols, which are produced in great

concentrations during volcanic eruptions.

 

Coral is similar to trees in that the growth and density of the coral is dependent upon

temperature. X-rays of coral cross sections show the relative density and growth over

time. High density layers of coral are produced during years of high ocean surface

temperatures. Hence, corals can be calibrated to estimate sea surface temperatures.

 

Principal Component Analysis and the CFR and CPS Methodology

 

Principal component analysis is a method often used for reducing multidimensional

datasets to lower dimensions for analysis. In this context, dimensions refer to the number

of distinct 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 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, and also has the highest explained variance.

This method is intended for dimension reduction. In most datasets, the first principal

component should be the least smooth (because of the higher variance). However, in

MBH98, MBH99, the proxy data are incorrectly centered, which inflates the variance of

certain proxies and selectively chooses those decentered proxies as the temperature

Reconstruction.

 

There are several time series models that exist for the purpose of modeling series with

dependence, including autoregressive, moving averages, autoregressive moving average

models, and long memory processes. MBH98 and MBH99 focus on simple signal plus

superimposed noise models for paleoclimate temperature reconstruction. Because of

complex feedback mechanisms involved in climate dynamics, it is unlikely that the

temperature records and the data derived from the proxies can be adequately modeled

with a simple temperature signal with superimposed noise. We believe that there has not

been a serious investigation to model the underlying process structures nor to model the

present instrumented temperature record with sophisticated process models.

 

Two principal methods for temperature reconstructions have been used; CFR4 (climate

field construction) and CPS (climate-plus-scale). The CFR is essentially a principal

component analysis and the CPS is a simple averaging of climate proxies, which are then

scaled to actual temperature records. The controversy of Mann’s methods lies in that the

proxies are centered on the mean of the period 1902-1995, rather than on the whole time

period. This mean is, thus, actually decentered low, which will cause it to exhibit a larger

variance, giving it preference for being selected 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 the mean is a critical factor in using the principal

component methodology properly. It is not clear that Mann and associates realized the

error in their methodology at the time of publication. Because of the lack of full

documentation of their data and computer code, we have not been able to reproduce their

research. We did, however, successfully recapture similar results to those of MM. This

recreation supports the critique of the MBH98 methods, as the offset of the mean value

creates an artificially large deviation from the desired mean value of zero.

 

Findings

 

In general, we found MBH98 and MBH99 to be somewhat obscure and incomplete and

the criticisms of MM03/05a/05b to be valid and compelling. We also comment that they

were attempting to draw attention to the discrepancies in MBH98 and MBH99, and not to

do paleoclimatic temperature reconstruction. Normally, one would try to select a

calibration dataset that is representative of the entire dataset. The 1902-1995 data is not

fully appropriate for calibration and leads to a misuse in principal component analysis.

However, the reasons for setting 1902-1995 as the calibration point presented in the

narrative of MBH98 sounds reasonable, 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 paleoclimatology studies have had significant

interactions with mainstream statisticians.

 

In our further exploration of the social network of authorships in temperature

reconstruction, 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 paleoclimate studies are closely connected and thus ‘independent studies’ may not

be as independent as they might appear on the surface. This committee does not believe

that web logs are an appropriate forum for the scientific debate on this issue.

It is important to note the isolation of the paleoclimate community; even though they rely

heavily on statistical methods they do not seem to be interacting with the statistical

community. Additionally, we judge that the sharing of research materials, data and results

was haphazardly and grudgingly done. In this case we judge that there was too much

reliance on peer review, which was not necessarily independent. Moreover, the work has

been sufficiently politicized that this community can hardly reassess their public

positions without losing credibility. Overall, our committee believes that Mann’s

assessments that the decade of the 1990s was the hottest decade of the millennium and

that 1998 was the hottest year of the millennium cannot be supported by his analysis.

 

Read the whole Report   or:  http://www.uoguelph.ca/~rmckitri/research/WegmanReport.pdf      or here    Local

 

Analysis of the Wegman Report by the blog that broke the hockey stick