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decades of the 20th century than during any comparable period over the preceding millennium.”
Contrarians in the climate change debate tried to exploit the committee's use of the word
plausible
,
which might appear weaker than the word
likely
that the IPCC used in its report and we used in our
work. However, subsequent clarifications by the panel members indicated that indeed they had
reached a conclusion similar to ours and that of the IPCC.
5
In a press release, the NAS committee asserted that there was “high confidence that [the] planet
is warmest in 400 years,” “less confidence in temperature reconstructions prior to 1600,” and “little
confidence” prior to
A.D.
900. While deniers seized on every single qualifier to suggest that the NAS
was somehow retreating from our findings or those of the IPCC, the panel made it clear that their
conclusions were consistent with those of MBH99. They noted that our work was “the first to include
explicit statistical error bars” and reminded readers of the original MBH99 findings that “the error
bars were relatively small back to about
A.D.
1600, but much larger for
A.D.
1000-1600,” explaining
that “the lower precision during earlier times is caused primarily by the limited availability of
annually resolved paleoclimate data.”
6
There were some minor points of contention. For instance, the panel expressed some skepticism
about conclusions regarding the warmth of individual years and decades. The MBH99 conclusion that
1998 was likely the warmest year and the 1990s the warmest decade of the past millennium rested
largely, however, on the proposition—which the panel endorsed—that the past few decades were
likely the warmest in a millennium. The warmest year and decade of an unusually warm interval (the
past few decades) were, our reasoning went, likely unusually warm themselves. The panel also
seemed to neglect the most recent peer reviewed literature, such as the work of Wahl and Ammann
and others who demonstrated in greater detail than in previous work that the entire issue of centering
in PCA that McIntyre and McKitrick had raised was a red herring; it didn't make any difference to the
results obtained in the end.
7
Some of the more important of the panel's findings came to light during its press conference on
the date the report was released.
8
The report authors made clear that they backed the key conclusions
of our original work. Panel chair Gerald North stated that “We roughly agree with the substance of
their findings.”
New York Times
reporter Andrew Revkin, noting that we had indeed emphasized the
importance of uncertainties and caveats in our original millennial hockey stick analysis (MBH99),
asked the panel who, if anyone, may have been responsible for any overstating of our conclusions.
North opined that “the community probably took the results to be more definitive than Mann and
colleagues intended.”
The panel emphasized the seminal nature of our now nearly decade-old studies. Panel member
Curt Cuffey characterized that work as “the first analysis of its type … it was a really remarkable
contribution” that ended up “teaching us a lot about how climate has changed.” Like any
groundbreaking scientific work, he said, “it's not surprising that they could have probably done some
detailed aspects of it better.” Panel member Peter Bloomfield of North Carolina State University, a
fellow of the Royal Statistical Society, added that ours was “a first of its kind study.” We “had to
make choices” that included “how the data were processed,” the “initial preparation, the inversion of
the calibration equations,” and “the selection of variables that were to be used in those equations.”
Our “methods were all quite reasonable choices,” he said, and to the extent that some of the choices
might not have been the optimal ones, he added that they “didn't have a material effect on the final
conclusion.” Bloomfield volunteered he “would not have been embarrassed by that work” if he'd
