One of the purported revelations in the recently publicly-released e-mail correspondence among climate scientists — one that skeptics of human-induced climate change say shows that climate scientists are manipulating data — has to do with tree rings and their relationship to temperature.

Scientists use tree rings as one of many ways to reconstruct climate conditions of the past 2,000 years. Traditionally, they've looked at tree ring width and density, preferably from trees at the very edge of their comfort zone — at high latitudes, for example, or high up on mountains near the tree line — as an indicator of temperature.

The idea: The temperature signal will be strongest in trees living in extreme environments where cold is a major factor limiting growth. They'll very clearly grow more when it's warmer, and less when it's cold.

But climate skeptics have jumped on climate scientist Phil Jones’s use of the word “trick” in one of the hacked e-mails to “hide the decline” from the 1980s onward. (Dr. Jones, head of the University of East Anglia’s Climate Research Unit, has temporarily stepped down pending an investigation into the e-mails — what's been dubbed Climategate.)

Here's the skeptic site Climate Audit's take on the "trick." And here's Phil Jones's response, which also contains the e-mail in question.

What is Jones talking about? A well-known and long-documented (at least in the scientific literature) problem called "divergence."

Here's the issue: In recent decades, some — although not all — trees have stopped responding positively to higher temperatures. How do we know? For the past 150 years, we've been measuring temperatures directly with various instruments. And, indeed, trees seem to follow temperatures faithfully, growing more during warm years and less during cold, until around 50 years ago. Then, even as scientific instruments continued to register rising temperatures, some trees started growing less.

If you were to go solely by these tree rings — and if you were looking at just ring density and width — you'd erroneously conclude that temperatures were falling when, in fact, they were rising. That's why scientists sometimes omit tree-ring data from recent decades in favor of the more accurate instrumental data.

Here are two paragraphs from Jones's response to the recent controversy. One shows the tree-ring data separate from the instrumental data. The other shows a graph with instrumental data inserted for the last two decades. One key point: For more than 100 years, tree rings and instrumental data track each other closely. They only diverge significantly in the past 20 years. Why?

Scientists have several possible explanations for this divergence, none of them mutually exclusive, and all of which — drought, global dimming, ozone holes — fault human activity for the slowing growth rate of some trees. In other words, scientists generally take the divergence as further evidence that humans are changing Earth's climate and that the warming is stressing various life forms — including trees in environments that are already extreme.

This conclusion is precisely the opposite of that reached by authors of many climate-skeptic opinion pieces and blogs, who argue that if tree rings show cooling, earth cannot be warming.

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.

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).

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.

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.