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Climategate, global warming, and the tree rings divergence problem

Much discussion of the Climategate e-mails has centered on "tricking" tree ring data that may not confirm global warming. What's the divergence of data all about and does it really confirm cooling instead of warming?

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In other words, tree growth may have slowed because the amount of sunlight reaching trees, which is especially critical to growth at high latitudes, has diminished since the mid-20th century.

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If global dimming is a major factor in the divergence of the past 50 years, should we expect to see the opposite of divergence — realignment? — now that the skies are cleaner?

D'Arrigo responds: "I think this would be difficult to detect and tease apart due to the many competing factors potentially impacting tree growth – hard to separate this type of effect from other environmental factors but an intriguing possibility."

All of this does raise another question, of course. Given the divergence of some tree-ring records from observed temperature during the past 50 years, how do we know that when we interpret growth patterns from deeper in the past, we're not actually seeing drought or abrupt warming similar to today's –- that is, how do we know divergence is only a recent phenomenon?

In an e-mail, Rob Wilson, a tree ring scientist at the University of St. Andrews in Fife, Scotland, and a coauthor on above-cited review, responds to this question: "[D]o not forget that tree-rings are not the only proxy source of past climatic information. If the divergence issue had been relevant in the past, we would see the TR [tree ring] data divergence from other proxy records (e.g. lake sediment, ice cores etc). We do not see this and on the whole all the multi-proxy records agree. Current evidence points to the fact that the divergence phenomenon is a phenomenon of the recent period."

And finally, speaking to the allegations of manipulated data directed at Jones, D'Arrigo says:

[I]n my mind the phenomenon has been discussed in the literature for at least the past – 15 years – e.g. a paper by Jacoby and D’Arrigo (1995 GBC and related note by G. Taubes, Science) and in 1998 K. Briffa et al. Nature described it extensively in a circumpolar boreal tree ring width and density data set. There has been no specific attempt to hide anything -- quite the opposite. Scientists have been publishing about it, and have simply been trying to better understand the issue and that the trees appear in some cases to not be responding as positively to temperature in recent decades, and to try to reconcile that observation with the need for accuracy in the reconstructions, and one way of dealing with this in good faith has been to truncate the most recent tree-ring values in the interest of caution and the need for accuracy, and to avoid being potentially misleading.

As Monitor colleague Pete Spotts mused when the purloined e-mails first came to light, at least the tree ring aspect of Climategate may turn out to be little more than a tempest in a teapot.

Editor’s note: For more articles about the environment, see the Monitor’s main environment page, which offers information on many topics. Also, check out our Bright Green blog archive and our RSS feed.

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