In response to the intense debate engendered by the significant differences that exist between the Esper et al. temperature history and that of Mann et al. (1998, 1999) -- which exhibits neither a Medieval Warm Period nor a Little Ice Age (which, of course, the Esper et al. history does) -- Cook et al. felt it necessary to (1) carefully review what Esper et al. had done, and (2) conduct some further analyses of the data they had employed in their reconstruction effort.  After all they did in this regard, however, their conclusions were not much different from those of Esper et al.  They concluded from their reanalysis of the Esper et al. reconstruction, for example, that (1) "its strongly expressed multi-centennial variability is highly robust over the AD 1200-1950 interval, with strongly expressed periods of 'Little Ice Age' cooling indicated prior to AD 1900," and that (2) "persistently above-average temperatures in the AD 960-1050 interval also suggest the large-scale occurrence of a 'Medieval Warm Period' in the Northern Hemisphere extra-tropics."

It is interesting to note, in this regard, that Cook et al. concluded the latter in spite of what they describe as strong criticism personally communicated to them by R.S. Bradley of Mann et al. fame (or infamy, as the case may be).  And why is this point so hotly debated?  Because if it was as warm as it is today a thousand or more years ago, when the air's CO2 concentration was fully 100 ppm less than it is today, there is no compelling reason for believing that the 100-ppm higher concentration of today has necessarily had anything to do with the global warming of the past century.

Another important finding of the Cook et al. study is that, like the reconstruction of Esper et al., their reanalysis of the data reveals that the "beginning of the end" of the Little Ice Age started nearly a century earlier than what the Mann et al. curve suggests, which is well before the lion's share of the historical increase in the air's CO2 content occurred, which clearly indicates that the bulk of the planet's recovery from the final cold spell of the Little Ice Age had to have been caused by something other than rising atmospheric CO2 concentrations.

We do, however, have one bone to pick with Cook et al., but it has nothing to do with their analysis of the data.  Rather, it has to do with their interpretation of the divergence between their reconstruction and the instrumental record over the last two decades of the 20th century.  Specifically, they report that their reconstructed temperature history "does very well at tracking the instrumental data on inter-decadal and longer timescales up to about 1982, after which the tree-ring estimates systematically under-estimate the actual warming."  We suggest that just the opposite is true, i.e., that over the latter part of the 20th century, the instrumental data systematically over-estimate the actual warming, which is faithfully reproduced in the tree-ring data.  However, since we discussed this topic in detail only two weeks ago in our analysis of the study of Briffa et al. (2002), we will say no more about it here.

In conclusion, we note that the case for the hockeystick temperature reconstruction of Mann et al. grows ever weaker by the week.  Someone needs to tell them and their IPCC friends that they are fighting a losing battle of logic with inferior scientific armaments, and that it will not be long before the political will to fight against reality collapses as well.

References
Briffa, K.R., Osborn, T.J., Schweingruber, F.H., Jones, P.D., Shiyatov, S.G. and Vaganov, E.A.  2002.  Tree-ring width and density data around the Northern Hemisphere: Part 1, local and regional climate signals.  The Holocene 12: 737-757.

Cook, E.R., Esper, J. and D'Arrigo, R.D.  2004.  Extra-tropical Norhtern Hemisphere land temperature variability over the past 1000 years.  Quaternary Science Reviews 23: 2063-2074.

Esper, J., Cook, E.R. and Schweingruber, F.H.  2002.  Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability.  Science 295: 2250-2253.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1998.  Global-scale temperature patterns and climate forcing over the past six centuries.  Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1999.  Northern Hemisphere temperatures duing the past millennium: Inferences, uncertainties, and limitations.  Geophysical Research Letters 26: 759-762.