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)
It's been many years since 'climategate' - when in 2009, the email server at the Climate Research Unit, University of East Anglia, was hacked. The unidentified hacker helped themselves to thousands of emails. These were sifted through and a selection was in due course made available for public download on a Russian server.
What followed was typical of the tactics used in the campaign to deny the existence of human-caused climate change and is illustrative for that reason. A lot of this manufactured controversy centred on one email, written by Dr Phil Jones, in which the following sentence was picked out and trumpeted all over the media:
"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."
"Gotcha!", the deniers proclaimed.
Climategate was marketed to the public by encouraging them to take such sentences at face value - without any idea of what they actually meant. The notion being planted was that climate scientists were busy covering up declining temperatures.
It meant nothing of the sort. The people in that email were not talking about temperatures. They were talking about tree-rings.
"Mike's Nature trick" referred to a technique described in a 1998 Nature paper. The paper presented a 600 year-long global temperature reconstruction by Michael Mann and colleagues. Michael has long used tree-ring growth patterns in ancient wood to reconstruct conditions at the time those rings formed. The basic idea is that in cold, dry years, trees grow more slowly so their rings are relatively narrow and densely-spaced. In warm wet years, it's the opposite.
The "trick" is the technique of plotting recent instrumental data - in other words direct temperature measurements - alongside the reconstructed tree-ring data for the time they overlap. Why? Because it's one way of checking if the reconstructed tree-ring data are a good proxy for temperatures of the past. It therefore makes entire sense to run such checks. Now to the 'decline'. What declined?
Beyond a certain point on the timeline, around 1960, the temperature reconstructions based on some tree-ring data fell away and downwards from observed thermometer records. They declined from reality. This only happened with certain tree-ring datasets from specific places. Known as the 'divergence problem', it had been discussed in the scientific literature since the mid 1990s - 15 years before 'climategate'. However, not a lot of people realised that. How lucky for the deniers. "Hide the decline!", they chanted feverishly, to anybody who would listen.
Everything that occurs in the physical world has a cause. The divergence of temperature reconstructions, based on tree-ring growth, from the observational temperature record is a case in point. The loss of tree-ring sensitivity to temperatures kicked in around 1960. It only affected certain tree-species at certain locations. However, in those cases the post-1960 tree-ring datasets do not reflect the actual conditions. In other words, the data are useless. That's the decline for you. Much ado about nothing.
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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.
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.
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.
Figure 1: Northern Hemisphere mean temperature anomaly in °C (Mann et al. 1999).
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 & D'Arrigo 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.
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