The Hockey Stick Data in the 2007 UN IPCC Report

One of the two principal arguments used to justify the claim that the global temperature increase of the late 20th Century was due to man's emissions of CO2, is a number of sets of hockey stick data derived from tree ring type studies.  Among the data sets was the already thoroughly discredited Mann hockey stick data which showed a land surface temperature record in which the Medieval Warming and the Little Ice Age were of muted scale and were followed by a huge increase suddenly in the late 20th Century.  With much fanfare, the Mann hockey stick data was withdrawn from the Summary Report of the 2007 report because Steve McIntyre of Climate Audit discovered the serious statistic errors that gave rise to the artifice hockey stick.  In addition to that hockey stick data, there are a number of other surface temperature records based on tree growth which are also hockey stick data sets.  These data sets are all in use in the heart of the report as part of the argument that solar activity cannot explain the unique, suddenly increasing temperatures of the late 20th Century, so the cause must be man's emissions of carbon dioxide.

Recently, Steve McIntyre has found that there are severe problems with the temperature record in the Yamal Tree Ring Cores from Siberia used by Briffa.  The global warming alarmists have long claimed that the temperature increase in Siberia was especially large and that this is what you would expect given increasing CO2 concentrations in the atmosphere.  It turns out that the Yamal data set plays an especially important role in the surface temperature record, just as the Mann data set still does.  More on that later.  The entire tree core set for the Yamal data set is 10 cores in 1990 and 5 cores in 1995.  Much larger core sets exist, such as the Polar Urals core set, and that set shows the Medieval Warming Period to be as warm as the late 20th Century warming.  But, the 2007 UN IPCC report ignores these other data sets.

Steve McIntyre examined the larger Schweingruber Khadyta River tree core data set from Siberia and compared it to the Yamal data set used by the UN IPCC report and which is in the East Anglia Climate Research Unit archive.  He tried very hard for a long time to get access to the Yamal data set, as did others.  This East Anglia CRU group is that same one which is both the world's primary keeper of the land surface temperature records and the has a history of denying others access to the data.  They also were recently found to have lost the original data in many cases and to have only retained highly modified data.  The result of McIntyre's analysis is plotted below.

The hockey stick sudden rise in temperature at the end of the 20th Century has disappeared.  It was an artifact of the very small core data set given such prominence by CRU and the UN IPCC.  It was a huge mistake.  To see how huge a mistake, we must go into this a bit deeper.

Steve McIntyre says that the IPCC AR4 (2007) said that the late 20th Century was the warmest period in 1300 years based on 10 tree growth data sets including the Mann Hockey Stick data and reconstructions of the surface temperature record by a relatively small group of authors.  9 of the 10 tree histories go back to the Medieval Warm Period, that same period a number of global warming alarmists have said they need to eliminate.  The plot of these surface temperature histories is given below.

The Mann hockey stick data and the Yamal data set give the most pronounced hockey stick results of these 10 data sets, but the other data sets are nonetheless also hockey stick temperature records, so maybe the UN IPCC conclusions are still right?  Well, no they are not.  The reason is that almost all of these data sets are dependent upon the Mann and/or the Yamal data sets being right.  Again, this is a point that McIntyre has been making for a long time, but could not really clearly demonstrate until he was able to get the Yamal data.

McIntyre says:

You will frequently see apologists state that these reconstructions provide "independent" evidence. However, this is not the case on two counts. The same proxies [procedure to convert observed core characteristics into temperatures] are used over and over again - a point reported at CA [Climate Audit] on many occasions and confirmed in Wegman et al 2006. Thus, if problems arise with (say) bristlecones or (say) Yamal, this will affect multiple studies and not be contained to one result. More on this below. In addition, there are not really 10 "independent" groups, as "independence" is understood in the real world. Mann and Jones 2003 is not independent of Mann, Bradley and Hughes (1998) or Jones (Briffa) et al 1998 or Rutherford et al 2005 (Mann, Osborn, Bradley, Briffa, Hughes and Jones).

 The other point - which I will state here but not prove - is a point about reconstruction methodologies. It is well known in the statistical climate blogs (CA, Jeff Id, David Stockwell, Lubos) that typical paleoclimate operations applied to red noise will yield HS [Hockey Stick] patterns - a line of argument that Ross and I discussed in detail in connection with MBH PC methods, but the same problem arises when you cherry pick from red noise or do ex post correlation weighting. To my knowledge, the climate "community" is in denial on this issue. There is a corollary to this "theorem" - a point that we discussed in our Reply to von Storch and Zorita, which is very important, but which hasn't got as much attention. If you manually include a huge HS-shaped series in a bunch of red noise and apply standard paleoclimate methods to the network - CPS, as well as Mannian PCs - you get an enhanced HS back with a a minor amount of static.

Rather than paleoclimate methods being "robust" as they self-proclaim, they are profoundly non-robust as this term is used in statistics - denoting the lack of stability of results to individual series. There are two series that play a particular role in the current spaghetti graph population: strip-bark bristlecones/foxtails (especially as Mann's PC1) and Briffa's Yamal (and its predecessor).....

In summary, the apparent problems with Briffa's Yamal series impact multiple other studies:
Briffa 2000, Mann and Jones 2003 (used in the recent UNEP graphic), Mann et al (EOS 2003), Jones and Mann 2004, Osborn and Briffa 2006, D'Arrigo et al 2006, Hegerl et al 2007, Kaufman et al 2009 (and of course, Briffa et al 2008).

Once again we find that a key part of the land surface temperature history used by the man-made global warming alarmists to make their case is unreliable and indeed wrong.

Elsewhere at Climate Audit, Finnish Professor Atte Korhola is quoted, in translation, as saying

I put immediately forward a thesis that I'm glad to expose to public criticism: when later generations learn about climate science, they will classify the beginning of 21st century as an embarrassing chapter in history of science. They will wonder our time, and use it as a warning of how the core values and criteria of science were allowed little by little to be forgotten as the actual research topic — climate change — turned into a political and social playground.

Another example is a study recently published in the prestigious journal Science. It is concluded in the article that the average temperatures in the Arctic region are much higher now than at any time in the past two thousand years. The result may well be true, but the way the researchers ended up with this conclusion raises questions. Proxies have been included selectively, they have been digested, manipulated, filtered, and combined, for example, data collected from Finland in the past by my own colleagues has even been turned upside down such that the warm periods become cold and vice versa. Normally, this would be considered as a scientific forgery, which has serious consequences.