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: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.
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).
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.