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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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It's also worth noting that the phenomenon is hardly a secret. It's been discussed at length in the scientific literature for at least 15 years — basically, since scientists first recognized it. Here's a 1995 article from the journal Science, one of many.

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It's also discussed in Chapter 6 of the Intergovernmental Panel on Climate Change's most recent report, pages 472-473.

Here's another more recent, and more thorough, review of the phenomenon [PDF].

So why does divergence happen? First, divergence doesn't happen in all tree-ring records. In some, trees respond positively to warming temperatures just as scientists would have predicted. Generally, scientists find that the divergence phenomenon shows up in trees from the far northern hemisphere, but not the southern, although that may be due to a paucity of samples from the southern hemisphere.

Even in the northern hemisphere, some tree rings don't show divergence.

Where they do diverge, one crucial factor seems to be the micro environment of the sampled tree. If temperatures are rising there, but moisture isn't, then higher temperatures lead to water stress, which retards, rather than enhances, growth.

In an e-mail, Rosanne D'Arrigo, senior research scientist at the Tree-Ring Lab at Columbia University's Lamont-Doherty Earth Observatory, explains: "[B]eyond a certain threshold level of temperature the trees may become more stressed physiologically, especially if moisture availability does not increase at the same time."

As it turns out, there's some independently verifiable evidence of this droughtlike stress on northern forests; it's visible from space. Says Dr. D'Arrigo: "[S]atellite vegetation data, another entirely independent source of information from tree rings, shows evidence of browning in some northern vegetation despite recent warming, supporting some tree-ring evidence for divergence effects in the far north."

And there's another intriguing possibility for the divergence phenomenon, one that explains why, at least so far, it seems restricted to the northern hemisphere: global dimming from pollution. Here's an explanation, including references, from the review cited above:

Global dimming is defined as a measured decline in solar radiation reaching the ground, which has been observed since the beginning of routine measurements over approximately the past half century (Stanhill and Cohen, 2001). The identified causes are a combination of cloud changes and air pollution (e.g., Russak, 1990; Liepert, 2002). The combination of more cloud water and more aerosols effectively decreases incoming solar radiation (Cohen et al., 2004; Liepert et al., 2004). It is estimated that the average amount of sunlight reaching the ground has declined by 4–6% over 1961–1990, although the estimated effects can vary from region to region (Stanhill and Cohen, 2001; Liepert, 2002; Che et al., 2005), and there can be considerable disagreement between instrumental measurements at the ground and satellite estimates of surface solar radiation (e.g. Xia et al., 2006). A decline in solar radiation of this magnitude can potentially have a profound impact on climate, the hydrological cycle (Liepert et al., 2004), and ecosystems worldwide (Stanhill and Cohen, 2001).
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