Background
The authors write that "it is still uncertain whether the magnitude and rate of 20th century warming exceeds natural climate variability over the last millennium," citing as evidence for this statement the studies of Esper et al. (2002, 2005a, 2005b), Moberg et al. (2005), D'Arrigo et al. (2006), Frank et al. (2007) and Juckes et al. (2007). Consequently, they conducted yet another study to further address this hot-point issue, which is of crucial importance to the CO2-climate debate.

What was done
Treydte et al., as they describe it, "present a millennium-long (AD 828-1998), annually resolved δ¹³C tree-ring chronology from high-elevation juniper trees in northern Pakistan [35.74-36.37°N, 74.56-74.99°W] together with three centennial-long (AD 1900-1998) δ¹³C chronologies from ecologically varying sites," in the process of which they "define an 'optimum' correction factor that is best suited to remove non-climatic trends from [their] high-elevation trees in the Karakorum," in order to "provide new regional temperature reconstructions derived from tree-ring δ¹³C, and compare those records with existing regional evidence."

What was learned
The end result of the six scientists' analysis was that "the 1990s are substantially below MWP [Medieval Warm Period] temperatures," and they state that their reconstruction "provides additional suggestions that High Asian temperatures during the MWP might have exceeded recent conditions," which finding, in their words, is also suggested by "ring-width data from living trees (Esper et al., 2007)." Consequently, as they state in the abstract of their paper, they "find indications for warmth during the Medieval Warm Period" that imply summer temperatures "higher than today's mean summer temperature."

What it means
As ever more carefully analyzed data become available, it is becoming ever more difficult for climate alarmists to deny that the Medieval Warm Period was probably slightly warmer than the Current Warm Period has been to date. And this evolving consensus suggests that the magnitude and rate of 20th-century warming likely did not exceed the natural climate variability of the last millennium, which further suggests there is no compelling reason to attribute 20th-century warming to 20th-century anthropogenic CO2 emissions, especially since even greater warmth likely existed a thousand years ago, when there was over 100 ppm less CO2 in the air than there is today.

References
D'Arrigo, R., Wilson, R. and Jacoby, G. 2006. On the long-term context for late twentieth century warming. Journal of Geophysical Research 111: 10.1029/2005JD006325.

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

Esper, J., Frank, D.C., Wilson, R.J.S. and Briffa, K.R. 2005a. Effect of scaling and regression on reconstructed temperature amplitude for the past millennium. Geophysical Research Letters 32: 10.1029/2004GL021236.

Esper, J., Frank, D.C., Wilson, R.J.S., Buntgen, U. and Treydte, K. 2007. Uniform growth trends among central Asian low- and high-elevation juniper tree sites. Trees 21: 141-150.

Esper, J., Wilson, R.J.S., Frank, D.C., Moberg, A., Wanner, H. and Luterbacher, J. 2005b. Climate: past ranges and future changes. Quaternary Science Reviews 24: 2164-2166.

Frank, D., Esper, J. and Cook, E.R. 2007. Adjustment for proxy number and coherence in a large-scale temperature reconstruction. Geophysical Research Letters 34: 10.1029/2007GL030571.

Jukes, M.N., Allen, M.R., Briffa, K.R., Esper, J., Hegerl, G.C., Moberg, A., Osborn, T.J. and Weber, S.L. 2007. Millennial temperature reconstruction intercomparison and evaluation. Climates of the Past 3: 591-609.

Moberg, A., Sonechkin, D.M., Holmgren, K., Datsenko, N.M. and Karlen, W. 2005. Highly variable Northern Hemisphere temperatures reconstructed from low and high-resolution proxy data. Nature 433: 613-617.