Clearing up misconceptions regarding 'hide the decline'

What The Science Says:
The "decline" refers to a decline in northern tree-rings, not global temperature, and is openly discussed in papers and the IPCC reports.

Climate Myth: Scientists tried to 'hide the decline' in global temperature
'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)

There are a number of misconceptions concerning Phil Jones' email. These are easily cleared up when one takes the time to read Jones' words in context.

The "decline" is about northern tree-rings, not global temperature

Phil Jones' email is often cited as evidence of an attempt  to "hide the decline in global temperatures". This claim is patently false and shows ignorance of the science discussed. The decline actually refers to a decline in tree growth at certain high-latitude locations since 1960.

Tree-ring growth has been found to match well with temperature. Hence, tree-rings are used to plot temperature going back hundreds of years. However, tree-rings in some high-latitude locations diverge from modern instrumental temperature records after 1960. This is known as the "divergence problem". Consequently, tree-ring data in these high-latitude locations are not considered reliable after 1960 and should not be used to represent temperature in recent decades.

The "decline" has nothing to do with "Mike's trick".

Phil Jones talks about "Mike's Nature trick" and "hide the decline" as two separate techniques. However, people often abbreviate the email, distilling it down to "Mike's trick to hide the decline". Professor Richard Muller from Berkeley commits this error in a public lecture:

"A quote came out of the emails, these leaked emails, that said "let's use Mike's trick to hide the decline". That's the words, "let's use Mike's trick to hide the decline". Mike is Michael Mann, said "hey, trick just means mathematical trick. That's all." My response is I'm not worried about the word trick. I'm worried about the decline."

Muller quotes "Mike's nature trick to hide the decline" as if its Phil Jones's actual words. However, the original text indicates otherwise:

"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."

It's clear that "Mike's Nature trick" is quite separate to Keith Briffa's "hide the decline". "Mike's Nature trick" refers to a technique (a "trick of the trade") by Michael Mann to plot recent instrumental data along with reconstructed past temperature. This places recent global warming trends in the context of temperature changes over longer time scales.

There is nothing secret about "Mike's trick". Both the instrumental and reconstructed temperature are clearly labelled. Claiming this is some sort of secret "trick" or confusing it with "hide the decline" displays either ignorance or a willingness to mislead.

Hockey Stick

Figure 1: Northern Hemisphere mean temperature anomaly in °C (Mann et al 1999).

The "decline" has been openly and publicly discussed since 1995

Skeptics like to portray "the decline" as a phenomena that climate scientists have tried to keep secret. In reality the divergence problem has been publicly discussed in the peer-reviewed literature since 1995 (Jacoby 1995). The IPCC discuss the decline in tree-ring growth openly both in the 2001 Third Assessment Report and in even more detail in the 2007 Fourth Assessment Report.

The common misconception that scientists tried to hide a decline in global temperatures is false. The decline in tree-ring growth is plainly discussed in the publicly available scientific literature. The divergence in tree-ring growth does not change the fact that we are currently observing many lines of evidence for global warming. The obsessive focus on a misquote taken out of context, doesn't change the scientific case that human-caused climate change is real.

Basic rebuttal written by doug_bostrom

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial


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