Posted by: David Lungren


It is the iconic email of Climategate.  Phil Jones, one of the world’s top climate scientists, discussed a “trick” he used to “hide the decline.”  It has sparked hermeneutical warfare, with some, including President Obama’s Science Adviser, Dr. John Holdren, claiming the “trick” merely means “a clever way to tackle a problem.”  Others think it’s “a crafty or underhanded device.”  Below, in our review of the emails released as part of Climategate, the Holdren interpretation becomes more difficult to justify. 

Our friends in the majority on the House Select Committee on Energy Independence and Global Warming are siding with Dr. Holdren.  Though we are not taking sides—the EPW minority is conducting an investigation into Climategate—we question whether the trick, as the majority staff wrote in a recent report, was simply a straightforward “technique” to “combine direct thermometer readings with non-thermometer data, in this case from tree rings, to complete a full picture of temperature history.”

The problem with this interpretation is that it overlooks the context in which the “trick-to-hide-the-decline” statement was made.  The context was this.  In 1999, several scientists preparing for the third climate report by the UN’s Intergovernmental Panel on Climate Change (IPCC)—including Phil Jones, former head of the Climatic Research Unit (CRU), and author of the infamous email—were concerned about a proxy temperature reconstruction of the Northern Hemisphere showing declining temperatures in the late 20th century.  They were concerned, because the decline threatened to “dilute” the message that 20th Century temperatures were unprecedented in the last 1,000 years.

Authored by Keith Briffa of the CRU, the decline began after 1960.  How did Briffa come up with a decline?  Reliable thermometer data go back only to 1880, so scientists use ‘proxy data’ (tree rings, ice cores, boreholes, etc.) to reconstruct annual temperatures over long periods (i.e., 1000 years).   In this case, Briffa relied on tree rings, which showed a sharp and steady decline in temperature after 1960.   This seemed to conflict with actual temperature readings that showed a steep rise.   This discrepancy, according to one scientist, was “awkward.”

Indeed it was.  According to Steve McIntyre, the Canadian mathematician who debunked Michael Mann’s “hockey stick” temperature graph, several IPCC lead authors met in Arusha, Tanzania from September 1-3, 1999.  There they discussed the Briffa decline.  Briffa’s graph was, according to Michael Mann’s email, a “problem”:

"Keith’s series…differs in large part in exactly the opposite direction that Phil’s does from ours.  This is the problem we all picked up on (everyone in the room at IPCC was in agreement that this was a problem and a potential distraction/detraction from the reasonably consensus viewpoint we’d like to show w/ the Jones et al and Mann et al series (Mann email, 9-22-99)." 

After it was determined that the IPCC would include a proxy diagram in its Third Assessment Report, IPCC author Chris Folland wrote in an email that Briffa’s decline “somewhat contradicts the multi-proxy curve and dilutes the message rather significantly…This is probably the most important issue to resolve in Chapter 2 at present (Folland, 9-22-99).” 

The message apparently was very clearly delivered, as Briffa himself recounted.  “I know there is pressure to present a nice tidy story as regards ‘apparent unprecedented warming in a thousand years or more in the proxy data…,’” he wrote (Briffa 9-22-99).  Here is the email from Briffa in full:

"I know there is pressure to present a nice tidy story as regards ‘apparent unprecedented warming in a thousand years or more in the proxy data but in reality the situation is not quite so simple.  We don't have a lot of proxies that come right up to date and those that do (at least a significant number of tree proxies) some unexpected changes in response that do not match the recent warming. I do not think it wise that this issue be ignored in the chapter.  For the record, I do believe that the proxy data do show unusually warm conditions in recent decades. I am not sure that this unusual warming is so clear in the summer responsive data. I believe that the recent warmth was probably matched about 1000 years ago. I do not believe that global mean annual temperatures have simply cooled progressively over thousands of years as Mike appears to and I contend that that there is strong evidence for major changes in climate over the Holocene (not Milankovich) that require explanation and that could represent part of the current or future background variability of our climate."

Mann was nervous that skeptics would have a “field day” with the Briffa decline and that “he’d hate to be the one” to give them “fodder.”  As Mann wrote:

"We would need to put in a few words in this regard. Otherwise, the skeptics have a field day casting doubt on our ability to understand the factors that influence these estimates and, thus, can undermine faith in the paleoestimates.  The best approach here is for us to circulate a paper addressing all the above points. I'll do this as soon as possible.  I don't think that doubt is scientifically justified, and I'd hate to be the one to have to give it fodder! (Mann, 9-22-99)"

Yet, fodder was exactly what they got.  But what, exactly, was the “trick”?  As the Daily Mail reports, “All [Jones] had to do was cut off Briffa’s inconvenient data at the point where the decline started, in 1961, and replace it with actual temperature readings, which showed an increase.”  Below is the graph that was eventually included in the IPCC’s 2001 science assessment.  And it shows that Jones’s trick was successful: Briffa’s line in green is cutoff and obscured by the other lines.  (Courtesy of McIntyre at

This "trick" is, to say the least, somewhat controversial.  According to Philip Stott, emeritus professor of biogeography at London's Oriental and African Studies, "Any scientist ought to know that you just can't mix and match proxy and actual data.  They're apples and oranges.  Yet, that's exactly what [Jones] did."

More investigation of Climategate is needed.  The context surrounding the "trick" to "hide the decline" does, however, present a convincing challenge to those who believe it to be without guile.  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.


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