Climategate, global warming, and the tree rings divergence problem

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.

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