I lost a lot of my emails when I left my job as editor of the magazine Natuurwetenschap & Techniek (NWT) in April 2008. One of the emails I was ‘missing’ was a reply of Keith Briffa to questions of mine about the divergence problem. Thanks to climategate 2.0 the email is back. It’s number 2990 and it even contains some interesting correspondence between Rob Wilson and Keith Briffa regarding my questions.

I was writing a news article about the divergence problem. The review comments on AR4 had become available and as regular readers of Climate Audit know, McIntyre had as a reviewer of AR4 asked the lead authors to deal with the divergence problem:

Show the Briffa et al reconstruction through to its end; don’t stop in 1960. Then comment and deal with the “divergence problem” if you need to. Don’t cover up the divergence by truncating this graphic. This was done in IPCC TAR; this was misleading. (Reviewer’s comment ID #: 309-18)]

The answer of the IPCC was (Briffa being a lead author of the chapter):

Rejected — though note divergence’ issue will be discussed, still considered inappropriate to show recent section of Briffa et al. series.

So I asked him the following:

For me as a journalist the comment that it is “inappropriate” to show the reconstruction after 1960 makes no sense. The reconstruction is real and therefore should have been shown. You then can try to explain why things happen.

Briffa then responds:

We can talk later also as for the “innapropriate” response to the suggestion to show the Briffa reconstruction “after 1960” – there is no reconstruction from these data after 1960 to show.

I found this answer pretty amazing. Briffa did show the reconstruction in two of his papers, the first one in 1999 if I remember well. Now saying there is no reconstruction, makes little sense.

In the same email 2990 is also an extensive reply of Rob Wilson. He speculates about the causes of the divergence problem:

The divergence phenomenon, whether for RW or MXD is complex. To date, no one cause has been implicated and it is likely that the divergence issue is a phenomenon which is related to multiple factors over different regions and species. I could quote you as many studies that show no divergence as those that do.

7) Some possible hypothesised causes are: (1) non linear effects – e.g. there is a thermal threshold (i.e. it is getting to warm and some other parameter (e.g. precipitation) becomes limiting to growth; (2) anthropogenic affects (e.g. effects of pollution etc); (3) increasing CO2 may not result in fertilisation but the opposite effect as it may reduce the water use capacity of the tree and result in moisture stress; (4) issues related to the statistically processing of the tree-ring data – i.e. end effect issues when the TR data are detrended to remove biological age biases in the series; (5) Urban Heat Effects. If there is a positive bias in the instrumental data, then this could affect the calibration with tree-ring data; (6) wrong target season – some studies at large scales target annual temperature when in reality tree-ring data portray a summer temperature signal. Reconstructions also calibrate against mean temperatures although the bulk of cambial activity is in the day time. Day time maximum temperatures might therefore be a more realistic parameter to target. Increasing temperatures are observed more at night than the daytime.

Then at the top of the email thread we see a short correspondence between Wilson and Briffa:

Hi Keith,

thanks for forwarding your message.

I tried to ignore Marcel’s initial e-mails but he phoned me at home so I said I would write back.

He was really fishing for me to criticise your truncation. Hopefully my answer was diplomatic enough.

Instead of answering whether the truncation was scientifically sound Wilson was trying to be ‘diplomatic’.

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