Just Google "hide the decline" or "Hockey Stick briffa" to see some of the arguments. In a nutshell the Mann hockey graph obscured the fact that the proxies have risen 50% (or less) than the instrumented temperature since 1960. That's a fact and not a debating point. The arctic (Greenland) and antarctic ice cores show the MWP as being warmer and in theory these measures should be more stable than the tree rings. Before 1998 the MWP was believed to be much warmer. If the Medieval temperature anomaly is doubled (to reflect recent proxy performance) it would clearly be warmer. "Sigh" however what this really means is the variability of tree ring response to temperature is so great we can't tell accurately how warm the MWP was and this is just a pissing contest.
More evidence of error comes from the Wegman Report which reviewed the hockey stick and said:
Of the approach used by Mann: "...it is unlikely that the
temperature records and the data derived from the proxies can be adequately modeled
with a simple temperature signal with superimposed noise."
On the hockey stick: "The controversy of Mann’s methods lies in that the
proxies are centered on the mean of the period 1902-1995, rather than on the whole time
period. This mean is, thus, actually decentered low, which will cause it to exhibit a larger
variance, giving it preference for being selected as the first principal component. The net
effect of this decentering using the proxy data in MBH98 and MBH99 is to produce a
“hockey stick” shape."
On his statistics: "We note that there is no evidence that Dr.
Mann or any of the other authors in paleoclimatology studies have had significant
interactions with mainstream statisticians."
On the social network and independence of authors: "Our findings from this analysis suggest that authors in the area of paleoclimate studies are closely connected and thus ‘independent studies’ may not be as independent as they might appear on the surface."
Here are some more interesting arguments:http://joannenova.com.au/2009/12/fraudulent-hockey-sticks-and-hidden-data/
This question brings Ayn Rand and “Why don’t you believe in housewives?” to mind. I don't know why they are doing it - but they are (at least in the case of Mann).
The problem is that the proxies peaked in 1940. The Mann "Hockey Stick" graph used strategically placed instrument data to obscure this fact. For a discussion of the proxy problems:
http://icecap.us/images/uploads/Holocene,Historicandrecentglobaltemperatures.pdf
From a proxy perspective 1940 may/may not be warmer than the MWP. The 21st century definitely isn't. Given that the Vikings aren't growing grain in Greenland like they were during the MWP, the MWP was probably warmer.
On rereading the source discussion - he appears to argue that there are enough errors in the data that we don't know for sure when the warmest period was. Sounds good - but Mann did try to eliminate the MWP.
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