Climategate: On the “Mike’s Nature trick” email

Regarding the leaked emails from the Climate Research Unit (CRU), one thing that is clear is that the “Mike’s Nature trick” email is genuine, as confirmed by the CRU’s Phil Jones himself. It is thus a good starting point for examining the charges laid against the CRU team.

Having studied the relevant publications, it seems to me that the email provides evidence of what may be nothing more than some sloppiness in producing a graph for this WMO Statement. The “divergence” talked about is openly discussed in papers published both before and after the WMO Statement and also in the IPCC AR4. I think the data in the WMO Statement should have been presented differently, in a more transparent manner, but I don’t think this is evidence of manipulation or deliberate hiding of the divergence.

The remainder of this article explains my reasoning.

The email, sent by Prof Phil Jones of the CRU in 1999, states:

Dear Ray, Mike and Malcolm,
Once Tim’s got a diagram here we’ll send that either later today or
first thing tomorrow.
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) amd from
1961 for Keith’s to hide the decline. Mike’s series got the annual
land and marine values while the other two got April-Sept for NH land
N of 20N. The latter two are real for 1999, while the estimate for 1999
for NH combined is +0.44C wrt 61-90. The Global estimate for 1999 with
data through Oct is +0.35C cf. 0.57 for 1998.
Thanks for the comments, Ray.


So what is the “trick” referred to? Is this evidence of the manipulation of data, as some claim? The first point to note is that Prof Jones claims otherwise:

The following email, which I can confirm is genuine, has caused a great deal of ill-informed comment, but has been taken completely out of context and I want to put the record straight.

“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) amd from 1961 for Keith’s to hide the decline. Mike’s series got the annual land and marine values while the other two got April-Sept for NH land N of 20N. The latter two are real for 1999, while the estimate for 1999 for NH combined is +0.44C wrt 61-90. The Global estimate for 1999 with data through Oct +is 0.35C cf. 0.57 for 1998.”
The first thing to point out is that this refers to one diagram – not a scientific paper – which was used in the World Meteorological Organisation’s statement on the status of the global climate in 1999 (WMO-no.913).

The diagram consisted of three curves showing 50-year average temperature variations for the last 1000 years. Each curve referred to a scientific paper and a key gives their details.

Climate records consist of actual temperature records from the mid-19th century and proxy data (tree rings, coral, ice cores, etc) which go back much further. The green curve on the diagram included proxy data up to 1960 but only actual temperatures from 1961 onwards. This is what is being discussed in the email.

The word ‘trick’ was used here colloquially as in a clever thing to do. It is ludicrous to suggest that it refers to anything untoward.

I can accept that “trick” was used in the sense discussed above, after all who am I to know what was in Prof Jones’ head at the time he wrote this email? But the use of the word “trick” is a side show. The real issue is the fact that data from proxy records and data from temperature records were combined to produce the graph on the front of this WMO Statement. This raises the following questions:

  1. Was it clear in the WMO document that the green curve had been constructed using a mix of proxy and direct temperature records?
  2. Is it valid to extend a curve produced using one set of data produced by certain means with a different set of data obtained by different means?

Regarding question 1, I first note that Prof Jones/the CRU have published the graph with the two curves plotted separately, for comparison with the original, alongside a more detailed explanation of the email. It seems to me the second graph would have been a better way of presenting the data, making it clearer what was going on. Anyway, the WMO document, page 2 says the following about the original graph:

Northern Hemisphere temperatures were reconstructed for the past 1000 years (up to 1999) using
palaeoclimatic records (tree rings, corals, ice cores, lake sediments, etc.), along with historical and long
instrumental records. The data are shown as 50-year smoothed differences from the 1961–1990 normal.
Uncertainties are greater in the early part of the millennium (see page 4 for further information). For more
details, readers are referred to the PAGES newsletter (Vol. 7, No. 1: March 1999, also available at and the National Geophysical Data Center (
(Sources of data: P.D. Jones, K.R. Briffa and T.J. Osborn, University of East Anglia, UK; M.E. Mann,
University of Virginia, USA; R.S. Bradley, University of Massachusetts, USA; M.K. Hughes, University of
Arizona, USA; and the Hadley Centre, The Met. Office).

Whilst it is clear that multiple data sources are used, it does not make it clear that the green curve is a mix of the two proxy records and temperature records, with the years 1961 onwards being based on temperature records. This seems sloppy to me, especially in light of the apparent divergence between the proxy records (green curve) and the temperature records (black curves) in the graph that plots the data from the different sources separately.  However the same press release that gives us the two versions of the graph also states:

Phil Jones comments further: “One of the three temperature reconstructions was based entirely on a particular set of tree-ring data that shows a strong correlation with temperature from the 19th century through to the mid-20th century, but does not show a realistic trend of temperature after 1960. This is well known and is called the ‘decline’ or ‘divergence’. The use of the term ‘hiding the decline’ was in an email written in haste. CRU has not sought to hide the decline. Indeed, CRU has published a number of articles that both illustrate, and discuss the implications of, this recent tree-ring decline, including the article that is listed in the legend of the WMO Statement figure. It is because of this trend in these tree-ring data that we know does not represent temperature change that I only show this series up to 1960 in the WMO Statement.”

The ‘decline’ in this set of tree-ring data should not be taken to mean that there is any problem with the instrumental temperature data. As for the tree-ring decline, various manifestations of this phenomenon have been discussed by numerous authors, and its implications are clearly signposted in Chapter 6 of the IPCC AR4 report.

The first point to note here is that the WMO Statement does not mention the decline. So one consequence of merging the data from proxy records with the temperature records in the WMO Statement’s graph is precisely to hide this decline from readers of that document. If what Prof Jones says above is correct, then readers of the WMO Statement will have to chase down references in order to find out about the decline at all. It would at least have been more transparent to have had a footnote or other explanation in the WMO Statement explaining these points about the graph and the divergence, referring the interested to other relevant sources. That said if it was considered common knowledge in the field and mention of it felt to be a distraction in the context of the WMO Statement, it may be nothing more than an oversight rather than a deliberate attempt to hide the decline. It would still be sloppy at best though.

The sources mentioned are Chapter 6 of the IPCC AR4 report and the article “listed in the legend for the WMO Statement figure” which I presume is Briffa (1999), Quaternary Science Review Vol 19, pp 87-105, cited in the legend in relation to the green curve in the original graph.

Taking the AR4 report first, pages 472 to 473 in Chapter 6 do indeed talk about the divergence between the tree-ring data and observed temperatures:

Several analyses of ring width and ring density chronologies, with otherwise well-established sensitivity to temperature, have shown that they do not emulate the general warming trend evident in instrumental temperature records over recent decades, although they do track the warming that occurred during the early part of the 20th century and they continue to maintain a good correlation with observed temperatures over the full instrumental period at the by low-resolution proxy records that cannot be rigorously calibrated against recent instrumental temperature data (Mann et al., 2005b). None of the reconstructions in Fig. 6.10 show pre-20th century temperatures reaching the levels seen in the instrumental temperature record for the last two decades of the 20th century.

It is important to recognise that in the NH as a whole there are few long and well-dated climate proxies, particularly for the period prior to the 17th century (Figure 6.11). Those that do exist are concentrated in extratropical, terrestrial locations, and many have greatest sensitivity to summer rather than winter (or annual) conditions. Changes in seasonality probably limit the conclusions that can be drawn regarding annual temperatures derived from predominantly summer-sensitive proxies (Jones et al., 2003). There are very few strongly temperature-sensitive proxies from tropical latitudes. Stable isotope data from high-elevation ice cores provide long records and have been interpreted in terms of past temperature variability (Thompson, 2000), but recent calibration and modelling studies in South interannual time scale (Briffa et al., 2004; D’Arrigo, 2006). This ‘divergence’ is apparently restricted to some northern, high-latitude regions, but it is certainly not ubiquitous even there. In their large-scale reconstructions based on tree ring density data, Briffa et al. (2001) specifically excluded the post-1960 data in their calibration against instrumental records, to avoid biasing the estimation of the earlier reconstructions (hence they are not shown in Figure 6.10), implicitly assuming that the ‘divergence’ was a uniquely recent phenomenon, as has also been argued by Cook et al. (2004a). Others, however, argue for a breakdown in the assumed linear tree growth response to continued warming, invoking a possible threshold exceedance beyond which moisture stress now limits further growth (D’Arrigo et al., 2004). If true, this would imply a similar limit on the potential to reconstruct possible warm periods in earlier times at such sites. At this time there is no consensus on these issues (for further references see NRC, 2006) and the possibility of investigating them further is restricted by the lack of recent tree ring data at most of the sites from which tree ring data discussed in this chapter were acquired.

So clearly AR4 does discuss these issues as claimed, but it was published 8 years after the WMO Statement. That leaves the Quaternary Science Review article, published in 1999 for the 2000 volume of QSR. (NB:This is an image PDF and thus not searchable and the text cannot be selected.) This also mentions the “divergence”, e.g. on page 90 it states:

It is notable that these density based reconstructions fail to emulate the magnitude of the positive growth anomalies (and inferred warming) seen in the 20th century ring-width series. We shall return to this point later.

Page 96/97 is where the divergence is discussed again, where it is stated that the reasons for the divergence are not known but possible explanations put forward include a loss of spring growth and changes in the timing of snow melts. It’s notable that several papers are referenced in this discussion, which may indicate the problem is well known in the field.

My conclusion then is that the “trick” email may merely expose sloppiness in failing to make it clear how the graph in the WMO Statement was constructed rather than a deliberate attempt to hide the divergence, since it is clear the divergence is discussed in other publications involving CRU members and other researchers in the field, both prior to and after the WMO Statement was published.If there was a deliberate attempt to hide this divergence you wouldn’t expect open discussion of it in the journals or in the IPCC’s reports after all! The fact that some of the tree-ring data diverges from the observed temperature records does not disprove AGW, and there are ongoing attempts to explain this, e.g. see the works referenced by Unity in his discussion of a different CRU email. The divergence is an acknowledged phenomenon in the field, and may simply be due to the fact that tree-ring density is affected by numerous factors other than temperature.

As to the validity of the technique, it seems to me that it may be valid if there is reason to regard the divergence of the tree-ring data as somehow being due to factors other than temperature, though I think it would be clearer to present the data as is and openly explain why the divergence of the tree-ring data does not invalidate the other records.

Of course far more is being discussed than the “Mike’s Nature trick” email, for example the code that was released is being treated as particularly damning by some bloggers, and there are emails that are being used as evidence of attempts to suppress dissenting scientists. I will address these in future articles.

3 Responses to “Climategate: On the “Mike’s Nature trick” email”

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