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an estimate by Keith Briffa and collaborators based on tree ring density (as opposed to ring width)
millennium—were the Jones et al. multi-proxy estimate discussed earlier in the chapter and the
MBH99 hockey stick. Each showed recent warming to be anomalous in the context of the longer-term
temperature history of the record. Another reconstruction of Northern Hemisphere temperatures over
the past millennium by Crowley and Lowery was published too late to be included in the comparison
figure, but its conclusions, similar to those of the other three reconstructions, were summarized in the
IPCC chapter.
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Yet another figure in the chapter depicted the Pollack et al. estimate of past temperature change
from borehole records, an estimate that was entirely independent of the other proxy estimates. Though
the borehole temperature reconstructions came with their own caveats, they provided independent
evidence that recent warming was unusual in at least a five-hundred-year time frame—as far back as
reliable borehole temperature estimates went. A separate plot in the chapter showing glacier mass
balance records spanning the past five hundred years indicated that the melting of mountain glaciers
worldwide too was unprecedented over at least this time frame.
Collectively, the data clearly indicated that the modern warming was unprecedented in a long
time frame. But only two estimates (the Jones estimate and the MBH99 estimate) gave a quantitative
depiction of hemispheric-scale temperature changes for the full millennium, and only one (the
MBH99) had error bars attached to it. After much discussion among all the lead authors, a consensus
was reached on a tentative conclusion. The word
likely
, the group decided, would be attached to the
conclusion that recent warming for the Northern Hemisphere on the whole was anomalous in a
millennial context. In the parlance of the IPCC, this careful phrasing indicated confidence of about 67
percent, that is, a two-out-of-three chance that the conclusion was correct. That is a far cry from the
90 percent threshold required for “confident” inferences—what in IPCC parlance was referred to as
“very likely.” Like MBH99 itself, the IPCC Third Assessment Report was extremely cautious in the
level of confidence it attached to its conclusion that recent warming was anomalous in a millennial
context.
