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

The skeptic argument...

Mike's Nature trick to 'hide the decline'
'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...

The Independent Climate Change Email Review investigated CRU's handling of tree ring data. The "decline" refers to the divergence of tree ring proxies from temperatures after 1960, which is not hidden but openly discussed in the literature. While the inquiry did criticize the individual graph mentioned in the "trick" email, it found no evidence of CRU manipulating tree ring data or downplaying the uncertainties.

Exhibit No. 1 of the climate conspiracy theory is a collection of emails stolen from the Climatic Research Unit (CRU) of the University of East Anglia (UEA), which appeared on the internet in November 2009. Though some of these "Climategate" emails can sound damning when quoted out of context, several inquiries have cleared the scientists. The most comprehensive inquiry, the Independent Climate Change Email Review, did something the media completely failed to do: it put the emails into context by investigating the main allegations. Its general findings (summarised here) were that the scientists' rigour and honesty are not in doubt, and their behaviour did not prejudice the advice given to policymakers, though they did fail to display the proper degree of openness.

One set of allegations relate to CRU’s paleoclimate reconstructions, which use tree rings as a proxy for temperature change. Although this is a relatively obscure branch of climate science, it was prominently featured in the Summary for Policymakers of the IPCC Third Assessment Report (TAR). Consequently, it has become a prime target for contrarians, who have long argued that CRU manipulated tree ring data to make modern warmth look more unusual.

In what is probably the most notorious of the CRU emails, dated 16/11/1999, Jones wrote:

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.

This email is often quoted by commentators with little or no understanding of what it refers to, so it is worth taking some time to explain the context. Jones was discussing a graph for the cover of an obscure 1999 World Meteorological Organisation (WMO) report, which depicted both instrumental temperature data and reconstructed temperatures based on tree rings. The “decline” refers to the fact that some tree ring series (though not all) diverge from instrumental records in recent decades, for reasons that are not fully understood (although there are grounds for believing it is only a recent phenomenon). The “trick” was a way of presenting the data in this one particular graph, namely to truncate the tree ring data at the point when it diverged.

Anyway, contrarians take the use of the words “trick” and “hide the decline” as evidence that this was done to deceive, and this was the allegation that the inquiry examined. More generally, the contrarians claim (in the Review's words) that the divergence problem “may not have been properly taken into account when expressing the uncertainty associated with reconstructions”.

Contrarians have also accused CRU researchers (in particular their leading expert on tree ring reconstructions, Keith Briffa) of cherry-picking tree ring series that would produce a favoured result, namely that the late 20th century was warmer than the Medieval Warm Period. They allege that Briffa’s selection of an obscure tree ring chronology from the Yamal peninsula in Siberia “had an undue influence on all of the lines appearing in Chapter 6 of the 4th IPCC Report” (AR4); and that CRU withheld access to the Yamal data. And they use all these alleged flaws in CRU’s work to claim that less confidence should be placed in the conclusions of that AR4 chapter (specifically, the conclusion that current temperatures are likely the warmest in 13 centuries).

Regarding the “hide the decline” email, Jones has explained that when he used the word “trick”, he simply meant “a mathematical approach brought to bear to solve a problem”. The inquiry made the following criticism of the resulting graph (its emphasis):

[T]he figure supplied for the WMO Report was misleading. We do not find that it is misleading to curtail reconstructions at some point per se, or to splice data, but we believe that both of these procedures should have been made plain — ideally in the figure but certainly clearly described in either the caption or the text. [1.3.2]

But this was one isolated instance that occurred more than a decade ago. The Review did not find anything wrong with the overall picture painted about divergence (or uncertainties generally) in the literature and in IPCC reports. The Review notes that the WMO report in question “does not have the status or importance of the IPCC reports”, and concludes that divergence “is not hidden” and “the subject is openly and extensively discussed in the literature, including CRU papers.” [1.3.2]

As for the treatment of uncertainty in the AR4’s paleoclimate chapter, the Review concludes that the central Figure 6.10 is not misleading, that “[t]he variation within and between lines, as well as the depiction of uncertainty is quite apparent to any reader”, that “there has been no exclusion of other published temperature reconstructions which would show a very different picture”, and that “[t]he general discussion of sources of uncertainty in the text is extensive, including reference to divergence”. [7.3.1]

Regarding CRU’s selections of tree ring series, the Review does not presume to say whether one series is better than another, though it does point out that CRU have responded to the accusation that Briffa misused the Yamal data on their website. The Review found no evidence that CRU scientists knowingly promoted non-representative series or that their input cast doubt on the IPCC’s conclusions. The much-maligned Yamal series was included in only 4 of the 12 temperature reconstructions in the AR4 (and not at all in the TAR).

What about the allegation that CRU withheld the Yamal data? The Review found that “CRU did not withhold the underlying raw data (having correctly directed the single request to the owners)”, although “we believe that CRU should have ensured that the data they did not own, but on which their publications relied, was archived in a more timely way.” [1.3.2]

In summary, while the inquiry did criticize an individual graph, it found no evidence of CRU intentionally manipulating tree ring data or downplaying the associated uncertainties to mislead the public.

Despite being heralded as “the final nail in the coffin of anthropogenic global warming”, Climategate has not even invalidated CRU's results, let alone the conclusions of the climate science community. In any case, the entire work of CRU comprises only a small part of the large body of evidence for anthropogenic global warming. That mountain of evidence cannot be explained away by the behaviour of a few individuals.

Rebuttal written by James Wight. Last updated on 24 December 2010.

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Comments

Comments 1 to 14:

  1. "The “trick” was a way of presenting the data in this one particular graph, namely to truncate the tree ring data at the point when it diverged."

    This is incorrect. It's "Mike's Nature trick". Mike (=Michael Mann) did not truncate any tree ring data in his publications (not specifically in his infamous 1998 Nature paper). Instead the "trick" is to add instrumental temperature series to the end of the reconstruction (to the truncated reconstruction in the case of Briffa's series) prior to smoothing. This should be clear as the sentence continues "of adding in the real temps to each series for the last 20 years (ie from 1981 onwards) and from 1961 for Keith’s". Finally, the effect of this "trick" is to turn the end of the smoothed series upwards (instead of downwards as they would without adding in the instrumental series), and thus "to hide the decline".
  2. "While the inquiry did criticize the individual graph mentioned in the "trick" email, it found no evidence of CRU manipulating tree ring data or downplaying the uncertainties."

    So what does 'hide the decline' mean in scientific terms then??

    We know what 'the decline' is. It is the decline in tree ring proxy temperatures after about 1960.

    So why would anyone want to hide that fact which is supposedly well discussed in the specialist literature?

    So what would a reasonably intelligent layman make of these facts? Well how about this:

    CRU's Jones used the word 'hide' in a private communication because that was his intent - to hide the awkward bit of data which did not fit the upward trajectory of the warming chart.

    "But this was one isolated instance that occurred more than a decade ago."

    Right - one little bit of deceit a decade ago - and we did nothing else - honest!!
  3. Fred Staples at 09:34 AM on 4 January, 2011
    "The “trick” was a way of presenting the data in this one particular graph, namely to truncate the tree ring data at the point when it diverged"

    A quite incredible comment. The infamous "hockey stick" graph grafted post 1978 instrumental data (the blade) onto proxy data (teh stick). It failed to display the post 1977 proxy data, which shows a blade pointing in teh opposite direction.

    Hence the song, "hide the decline".
    Response: [muoncounter] See the thread Is the hockey stick broken? for a thorough discussion of these graphics.
  4. Bibliovermis at 09:52 AM on 4 January, 2011
    When proxy readings are validated by multiple, independent proxy methods. When a proxy record starts to conflict with the observational records, which should be considered valid - indirect proxy or direct observation? Should the whole proxy record be discarded when its validity starts to decline due to extraneous factors?
  5. Bibliovermis at 09:56 AM on 4 January, 2011
    oops... first sentence was chopped: When proxy readings are validated by multiple, independent proxy methods, they can be viably compared.
  6. Fred Staples at 09:05 AM on 5 January, 2011
    Phil Jones 16th November 1999
    “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.”

    Michael Mann December 2004
    “No researchers in this field have ever, to our knowledge, “grafted the thermometer record onto” any reconstrution. It is somewhat disappointing to find this specious claim (which we usually find originating from industry-funded climate disinformation websites) appearing in this forum”.

    As a direct result of this hockey-stick nonsense, George Monbiot in the Guardian ( a paper with a well respected “bad science” column) announced the elimination of both the Medieval Warm Period and the Little Ice Age with a single stroke.

    The problem with the relentless propaganda which alarmists purvey is that non-scientists believe it.
    Response: [Daniel Bailey] Please refrain from accusations such as: "The problem with the relentless propaganda which alarmists purvey is that non-scientists believe it." Unless, of course, you can provide linked quotes from reputable sources (i.e., not a blog) to prove your assertions. Thanks!
  7. Fred,

    Why don't you post the entirety of Mr. Mann's comment?
  8. The entire Dr. Mann comment can be found here on RealClimate - search for "graft" to see it.

    To quote one of the next lines:

    "Often, as in the comparisons we show on this site, the instrumental record (which extends to present) is shown along with the reconstructions, and clearly distinguished from them (e.g. highlighted in red as here)...(again see the comparisons here, with the instrumental record clearly distinguished in red, the proxy reconstructions indicated by e.g. blue or green, and the uncertainties indicated by shading)"

    The extra detail is quite informative.
  9. #6: "the relentless propaganda... "

    If quotes from 1999 and 2004 are all you've got to show relentless propaganda, you don't have much. FYI, relentless means (of pace or intensity) sustained; unremitting.

    Meanwhile, nothing about these 'tricks' changed any of the basic facts. See, for example, Did global warming stop in 1998? and the more recent Did global warming stop in ____fill in the blank with year of your choice___?

    If it's propaganda in today's world you want, look here. The lead article has tropical cyclone activity reaching a new low (ask our friends in Queensland how that's working out).

    Even better, look here. Dr. Singer is at it again: “the number of skeptical qualified scientists has been growing steadily; I would guess it is about 40% now.”
  10. KL #2

    "We know what 'the decline' is. It is the decline in tree ring proxy temperatures after about 1960."

    No it is not. It's the decline in reliability of the tree ring proxy temperatures when compared to the instrumental record after about 1960.
  11. I like the part where Fred Staples completely ignores counter-arguments in order to repost a variation of his original (incorrect) statement.

    Contrarians are like broken records.
  12. kdkd #10

    Happy new year.

    So some tree ring proxies showed warming and some showed cooling after 1960. And what is the exact date which these became 'unreliable'? 1950?, 1940?

    It sounds odd that these proxies become unreliable within a relatively short period of time.

    Logic would dictate that the same factors which caused the 'unreliability' were were working all through the time record. A likely explanation is that these proxies were never reliable.
  13. Bibliovermis at 14:06 PM on 6 January, 2011
    Please refer to argument #107.

    Tree-rings diverge from temperature after 1960
    The divergence problem is a physical phenomenon - tree growth has slowed or declined in the last few decades, mostly in high northern latitudes. The divergence problem is unprecedented, unique to the last few decades, indicating its cause may be anthropogenic. The cause is likely to be a combination of local and global factors such as warming-induced drought and global dimming. Tree-ring proxy reconstructions are reliable before 1960, tracking closely with the instrumental record and other independent proxies.
  14. KL #12

    Seeing as the tree rings are validated against the thermometer record for much of recent history, and against other proxies prior to this availability, it's really very difficult for me to understand the substance of the point that you're trying to make here. It certainly doesn't support a so-called sceptic agenda.

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