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What does Mike's Nature trick to 'hide the decline' mean?

The skeptic argument...

'Perhaps the most infamous example of this comes from the "hide the decline" email. This email initially garnered widespread media attention, as well as significant disagreement over its implications. In our view, the email, as well as the contextual history behind it, appears to show several scientists eager to present a particular viewpoint-that anthropogenic emissions are largely responsible for global warming-even when the data showed something different.' (David Lungren)

What the science says...

"Mike's Nature trick" refers to the technique of plotting recent instrumental data along with the reconstructed data. This places recent global warming trends in the context of temperature changes over longer time scales. "Hide the decline" refers to a decline in the reliability of tree rings to reflect temperatures after 1960. This is known as the "divergence problem" where tree ring proxies diverge from modern instrumental temperature records after 1960, discussed in the peer reviewed literature as early as 1995

The most quoted email is from Phil Jones discussing paleo-data used to reconstruct past temperatures (emphasis mine):

"I've just completed Mike's Nature trick of adding in the real temps to each series for the last 20 years (ie from 1981 onwards) and from 1961 for Keith's to hide the decline."

"Mike's Nature trick" refers to a technique (aka "trick of the trade") used in a paper published in Nature by lead author Michael Mann (Mann 1998). The "trick" is the technique of plotting recent instrumental data along with the reconstructed data. This places recent global warming trends in the context of temperature changes over longer time scales.

The most common misconception regarding this email is the assumption that "decline" refers to declining temperatures. It actually refers to a decline in the reliability of tree rings to reflect temperatures after 1960. This is known as the "divergence problem" where tree ring proxies diverge from modern instrumental temperature records after 1960. The divergence problem is discussed in the peer reviewed literature as early as 1995, suggesting a change in the sensitivity of tree growth to temperature in recent decades (Briffa 1998). It is also examined more recently in Wilmking 2008 which explores techniques in eliminating the divergence problem. So when you look at Phil Jone's email in the context of the science discussed, it is not the schemings of a climate conspiracy but technical discussions of data handling techniques available in the peer reviewed literature. More on the hockey stick divergence problem...

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