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Climategate, global warming, and the tree rings divergence problem

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(Read caption) Cross section of a large stump of a Douglas fir tree. Scientists use tree rings to study temperatures in the past.

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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."

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