Geoscience Reference
In-Depth Information
history behind the IPCC Third Assessment Report published in 2001. It was in that publication that
our work truly entered onto the world stage. There is a pyramid-like hierarchy of IPCC report
authorship. For any given chapter, there are dozens of contributing authors, fewer than a dozen lead
authors, and, among the latter, two convening authors who have primary responsibility for the chapter.
While I had been nominated to be an author of some sort prior to the publication of our work on
paleoclimate reconstructions, I was still relatively fresh out of graduate school, and I was surprised
when I learned in late 1998 that I had been selected as a lead author for the new report—a choice
presumably related to the scientific attention our recent work had received.
My task as a lead author was to work with the numerous contributing authors in assessing the
state of knowledge regarding evidence from the paleoclimate record, which—as in the previous IPCC
reports—was part of the “observations” chapter. The first job was to solicit input from the leading
experts in the field, including Keith Briffa and Phil Jones of the University of East Anglia's Climate
Research Unit, who had performed their own proxy reconstructions of past climate, and Henry
Pollack, a researcher at the University of Michigan who had derived an independent assessment of
past temperatures from boreholes.
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Boreholes are deep holes in the ground generated for purposes of
geophysical exploration. Under the right circumstances (when there is minimal intrusion of fluid flow,
the properties of the bedrock are reasonably uniform, changes in seasonal snow cover are minimal,
etc.), the penetration of heat from the surface down into the upper layer of Earth can reasonably be
assumed to follow a simple diffusion process. The physics of diffusion dictates that the vertical
temperature profile down the hole can then be used to provide an estimate of how temperatures at the
surface changed back in time.
To be credible as a true assessment of prevailing scientific understanding, IPCC reports must
accurately reflect the diverse views within the scientific community on any particular issue—no
simple task when it comes to contentious disciplines such as climate change. In addition, since the
IPCC report is an assessment report, not simply a literature review, it is necessary that the IPCC
evaluate the collective work of the scientific community in such a way that it is clear to readers on
what points scientists agree, and where there is still active debate.
There were several rounds of discussion among the lead authors regarding each of the various
drafts of our chapter. The lead authors included scientists with a wide range of views such as John
Christy, a scientist with a somewhat contrarian outlook on climate change. In the end, whatever was
said in the chapter would have to meet with John's approval. Moreover, whatever would be
concluded in our chapter would have to withstand the scrutiny of the rigorous IPCC review process.
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My primary responsibilities involved the assessment of the paleoclimate records of past
centuries.
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It would be essential in this work to reflect the diverse range of views in the recent
scientific literature and to present the range of estimates that had been published. Any key conclusions
arising from the chapter could not rely on one study or one group's findings. They had to reflect a
consensus of recent studies, if indeed that existed. The MBH99 hockey stick was shown in a plot by
itself for two important reasons: (1) It was the only reconstruction done at the level of individual
years rather than decadal or longer-term averages, and (2) it came with error bars, which the other
reconstructions didn't. Thus, unlike other studies, it spoke to whether recent years, such as 1998,
stood out as unusual against the backdrop of the longer-term reconstruction and its uncertainties.
An additional plot in the chapter compared three different independent or largely independent
quantitative reconstructions of Northern Hemisphere temperatures over the past millennium. One was
