Friday, December 11, 2009

Drip, Drip, Drip

The details continue to emerge in the Climategate story. Today we learn about the problematic "Briffa reconstruction." Steve McIntyre at tells the whole sordid tale, and you should definitely read the whole thing, but here are a few highlights.

First, some brief background. Keith Briffa is one of the climatologists who worked at the East Anglia CRU. He supplied much of the primary data, based on studies of tree rings from bristlecone pine trees, that form much of the rationale that led to the famous "hockey stick" model that shows a dramatic increase in global warming in recent years. Bristlecone pines are especially important, because they live for thousands of years and thus we could glean data about conditions going back to at least A.D. 1000.

There was a problem with the data, though: it was cherry-picked. This was not public knowledge for a long time, but McIntyre finally was able to go through the data here. And the problem is pretty simple: without the cherry-picked data, the assumptions about warming didn't hold up. The East Anglia CRU didn't want to release the information, despite repeated requests. McIntyre tells the story here:

CRU staunchly refused to provide the measurement data used in Briffa’s Yamal reconstruction. Science(mag) acquiesced in this refusal in connection with Osborn and Briffa 2006. While the Yamal chronology was used in a Science article, it originated with Briffa 2000 and Science(mag) took the position that the previous journal (which had a different data policy) had jurisdiction. Briffa used the chronology Briffa et al (Phil Trans B, 2008) and the Phil Trans editors finally seized the nettle, requiring Briffa to archive the data. As noted before, Briffa asked for an extension and, when I checked earlier this year, the Yamal measurement data remained unarchived. A few days ago, I noticed that the Yamal data was finally placed online. With the information finally available, this analysis has only taken a few days.

If the non-robustness observed here prove out (and I’ve provided a generating script), this will have an important impact on many multiproxy studies that have relied on this study. Studies illustrated in the IPCC AR4 spaghetti graph, Wikipedia spaghetti graph or NAS Panel spaghetti graph (consult them for bibliographic refs) that use the Yamal proxy include: Briffa 2000; Mann and Jones 2003; Jones and Mann 2004; Moberg et al 2005; D’Arrigo et al 2006; Osborn and Briffa 2006; Hegerl et al 2007, plus more recently Briffa et al 2008, Kaufman et al 2009. (Note that spaghetti graph studies not included in the above list all employ strip bark bristlecone pines – some use both.)

As it turns out, the data did not support the hypotheses of unprecedented warming. And this was a problem for the climatologists who have been driving the narrative. And now, thanks to the e-mail trail that has emerged from the CRU, we have an idea of how the climatologists dealt with the problem. McIntyre:

Much recent attention has been paid to the email about the “trick” and the effort to “hide the decline”. Climate scientists have complained that this email has been taken “out of context”. In this case, I’m not sure that it’s in their interests that this email be placed in context because the context leads right back to a meeting of IPCC authors in Tanzania, raising serious questions about the role of IPCC itself in “hiding the decline” in the Briffa reconstruction.

Relevant Climategate correspondence in the period (September-October 1999) leading up to the trick email is incomplete, but, in context, is highly revealing. There was a meeting of IPCC lead authors between Sept 1-3, 1999 to consider the “zero-order draft” of the Third Assessment Report. The emails provide clear evidence that IPCC had already decided to include a proxy diagram reconstructing temperature for the past 1000 years and that a version of the proxy diagram was presented at the Tanzania meeting showing the late twentieth century decline. I now have a copy of the proxy diagram presented at this meeting (see below).

The emails show that the late 20th century decline in the Briffa reconstruction was perceived by IPCC as “diluting the message”, that “everyone in the room at IPCC” thought that the Briffa decline was a “problem” and a “potential distraction/detraction”, that this was then the “most important issue” in chapter 2 of the IPCC report and that there was “pressure” on Briffa and other authors to show a “nice tidy story” of “unprecedented warming in a thousand years or more”.

We'll discuss how this was done in the next post. Meanwhile, read McIntyre's post in full. It's long but hugely significant.

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