From the question place, where a reader noted a high traffic item at Watt's Up With That and asked for a science response. Where to begin? First, I guess I'll note that most of the post is bluster and personal attack. Once you cross out those parts, it's a much shorter article.
Second, as always, go back to the original source. In this case, it is a Mann et al. 2008 paper Proxy-based reconstructions of hemispheric and global surface temperature variations over the past two millennia, with supplementary material.
Then, consider exactly what the claims (in this case, at WUWT) are, and just what evidence is produced for it.
The fundamental claim at WUWT is that the entire reconstruction is upside down. (We're treated to pictures of other things that are upside down.) Right off, we know WUWT is wrong.
There are three major features of temperature over the past 1000 or so years -- the 'Medieval Warm Period', the 'Little Ice Age', and the warming of the past century. We've known about the first two since at least the 1970s -- Hubert H. Lamb's Climate: Past, Present, and Future. The Mann and others reconstruction shows a Medieval Warm Period and a Little Ice age.
If WUWT were correct about Mann et al. having the curve upside down, then they (WUWT) must be insisting that it was a Medieval Cold Period and Little Warm Period -- which we've known for decades it wasn't, irrespective of anything that Mann or coworkers have done. WUWT is simply wrong from the get go.
There is then a lot of sound and fury regarding 'Tiljander'. This turns out to mean a paper by Tiljander and others in 2003, cited in the supplementary material of Mann and others. WUWT cites CA citing personal communication claiming that Mann et al. used this data set upside down. This looks more like a game of 'whispers' or 'telephone' than a serious scientific claim. If Tiljander (then CA, then WUWT) had serious evidence of error by Mann et al., the scientific literature is the place for it. Or, at the very least, Tiljander et al. could place a short note on their own blog/web site/university press release/.... Neither CA nor WUWT seem to cite any such thing, so I will draw the inference that this is because it doesn't exist.
Still, there might be a question as to whether the data were used correctly. A more significant question is whether the data and its usage materially affect the reconstruction. If, for instance, the only reason for showing a warming in the 20th century is Mann and others' use of this data set, we might be more concerned about whether there's been such a warming. Or at least it becomes a much more important question whether they did use the data set properly. So let's go back to the original source and see what usage was made, how, and what effects it has.
Page 2 of the supplementary material, under Sensitivity Analysis (NH Temperatures)Potential data quality problems. First, we'll note that the authors do indeed consider the possibility of data quality problems. I'll quote that paragraph here (all typos mine, see the original):
In addition to checking whether or not potential problems specific to tree-ring data have any significant impact on our reconstructions in earlier centuries (see Fig. S7), we also examined whether or not potential problems noted for several records (see Dataset S1 for details) might compromise the reconstructions. These records include the four Tiljander et al. (12) series used (see Fig S() for which the original authors note that human effects over the past few centuries unrelated to climate might impact records (the original paper states "Natural variability in the sediment record was disrupted by increased human impact in the catchment area at A.D. 1720." and later, "In the case of Lake Korttajarvi it is a demanding task to calibrate the physical varve data we have collected against meteorological data, because human impacts have distorted the natural signal to varying extents."). These issues are particularly significant because there are few proxy records, particularly in the temperature-screned dataset (see Fig. S9) available back through the 9th century. The Tiljander et al. series constitute 4 of the 15 available Northern Hemisphere records before that point.
They also note 3 other data sets with problems.
So, do the authors proceed blindly, pretending that all data are good (and equally good, at that)? No, there's the reconstructed figure in S7, using all data, and another using all data except for the tree rings. And then in figure S8, they show what happens after removing the 7 problematic data sets (The Tiljander 4 plus 3 from elsewhere). Same kinds of curves either way -- still a Medieval Warm Period, a Little Ice Age, and a warm recent century. As Mann and others note that before the 9th century there are few data if one withdraws Tiljander, I'm ignoring that part of the reconstructions.
From just reading the paper, we don't know whether the Tiljander data were used correctly. We do, however, know that the answers are quite similar whether they're used or not (S7 vs. S8). If there's an error, in other words, it's an error with little effect.
Contrast that with WUWT's initial claim of the whole reconstruction being upside down -- thereby turning the Medieval Warm Period into an ice age, and the Little Ice Age into a Warm Period. They give no evidence at all that this is the case. Irrespective of whether the Tiljander data were used wrongly, WUWT is wrong in their main claim.