In the “climategate” din, one accusation is shouted perhaps louder and more often than any other: the so-called “trick” supposedly played by IPCC lead author Michael Mann to “hide the decline” in Keith Briffa’s Northern Hemipshere extra tree-ring proxy-based reconstruction, featured along with three others in a key chart in the 2001 WG1 Third Assessment Report (TAR).
According to self-appointed climate science auditor Steve McIntyre, Mann took it on himself to truncate Briffa’s data set and replaced the deleted data with instrumental temperature data, thus creating less of a drop at the end of the resulting smoothed chart. But a closer examination shows the evidence for instrumental “padding” is far from certain. And the difference engendered between different possible “padding” values is minimal in any case.
Even worse, the key element of McIntyre’s narrative, namely the claim that Mann himself truncated Briffa’s data set, is false. In fact, it turns out that the actual data set used by Mann (as sent by Tim Osborn on Keith Briffa’s behalf) contains values only up to 1960 – exactly as in the figure produced in TAR. So once again, we have another egregiously false accusation from Steve McIntyre, one that has been echoed by McIntyre acolytes and CruTape Letters authors Steven Mosher and Thomas Fuller.
In this installment, I’ll look at another technique in the climate auditor’s toolbox, namely selective quotation. Once again, our example case study will involve accusations by Steve McIntyre concerning the use of paleoclimatologist Keith Briffa’s tree-ring based reconstruction in a key figure from the IPCC Third Assessment Report.
Arguing from a cherrypicked selection of quotes from the “Climategate” emails, McIntyre has claimed that IPCC authors Chris Folland and Michael Mann pressured Briffa to submit a reconstruction that would not “dilute the message” by showing “inconsistency” with multi-proxy reconstructions from Mann and Briffa’s CRU colleague Phil Jones. Briffa “hastily re-calculated his reconstruction”, sending one with a supposedly larger post-1960 decline before. According to McIntyre, Mann resolved this new “conundrum” and simply “chopped off the inconvenient portion of the Briffa tree-ring data”.
But a review of the emails – including some that have never been quoted before – clearly contradicts McIntyre’s version of events:
- Jones and Briffa were concerned that Mann had an outdated version of the Briffa reconstruction, and both urged the adoption of the newer “low frequency” one, more appropriate for comparison with other multi-century reconstructions.
- Far from pressuring Briffa to change his reconstruction right away, Mann questioned whether an immediate change was required, or even possible, and counselled waiting for the next revision.
- CRU colleague Tim Osborn advised Mann that he and Briffa “usually stopped” the “low frequency” reconstruction in 1960, and went one better in his later “resend” to Mann, by explicitly removing the post-1960 data.
I’ll also show how McIntyre has changed his narrative along the way , in an effort to prove that the true “context” of the famous “trick” to “hide the decline” is somehow an indictment of the IPCC. (Speaking of which, be sure to take the poll at the end about McIntyre’s next move). But first, once again, here is the cause of all the fuss, namely Figure 2-21 from Chapter 2 of the IPCC Third Assessment Report – Working Group I: The Scientific Basis (2001).