There have been many studies refuting the science behind the both Mann's analysis of tree ring studies (which the "hockey stick" graph is based) and of the underlying tree ring studies themselves. This latest study from the University of Pennsylvania and printed in the journal Nature calls all tree ring studies in to doubt.

The Mann Hockey Stick has served as a rallying point for those that believe in Anthropogenic Global Warming. According to them this one chart clearly shows current temperatures well above normal anything we've seen in a thousand years. This is the chart made famous by Al Gore in the Inconvenient Truth and in the 2001 IPCC Third Assessment Report (it was taken out of the 2007 report).

Mann Hockey Stick - from Wikipedia

“The claimed robustness to presence/absence of tree rings was also untrue. Sensitivity analysis showed that the reconstruction was not only highly sensitive to the presence/absence of bristlecone pines, but indeed the shape of the early part of the reconstruction was entirely dependent on bristlecones. We also observed that they had modified the principal components calculation so that it intentionally or unintentionally mined for hockey stick shaped series. It was so powerful in this respect that I could even produce a HS from random red noise.”

“This last observation has received much publicity. However, we did not and do not argue that this is the only way that a HS series can be obtained from red noise: there is the old fashioned method - manually select series with a hockey stick shape and then average.” - Steve McIntyre

The following is an abstract from "Subtropical to boreal convergence of tree-leaf temperatures" - Nature.com

The oxygen isotope ratio (δ¹⁸O) of cellulose is thought to provide a record of ambient temperature and relative humidity during periods of carbon assimilation. Here we introduce a method to resolve tree-canopy leaf temperature with the use of δ¹⁸O of cellulose in 39 tree species. We show a remarkably constant leaf temperature of 21.4 ± 2.2 °C across 50° of latitude, from subtropical to boreal biomes. This means that when carbon assimilation is maximal, the physiological and morphological properties of tree branches serve to raise leaf temperature above air temperature to a much greater extent in more northern latitudes. A main assumption underlying the use of δ¹⁸O to reconstruct climate history is that the temperature and relative humidity of an actively photosynthesizing leaf are the same as those of the surrounding air. Our data are contrary to that assumption and show that plant physiological ecology must be considered when reconstructing climate through isotope analysis. Furthermore, our results may explain why climate has only a modest effect on leaf economic traits in general.

AFP did an article on this study and interviewed the lead researcher Brent Helliker:

The new findings not only challenge long-held precepts in plant biology, but could upend climate models that use tree rings to infer or predict past and present temperature changes.

Oxygen atoms within water molecules evaporate more or less quickly depending on the number of neutrons they carry, and the ratio between these differently weighted atoms in tree trunk rings has been used as a measure of year-to-year fluctuations in temperatures and rainfall.

"The assumption in all of these studies was that tree leaf temperatures were equal to ambient temperatures," lead researcher Brent Helliker told AFP. "It turns out that they are not." - "Hot climate or cold, tree leaves stay in comfort zone: study" Yahoo News

If you want to learn more about the poor science behind the Mann Hockey Stick chart and tree ring studies as temperature proxies you should read Steve McIntyre's presentation (11mb) that he gave at Ohio State University (5/2008). It's an excellent primer. Steve also has a page of links devoted to the Hockey Stick Studies, several are peer reviewed, all are very informative.   Source