What was done
The authors sampled trees of white spruce (Picea glauca (Moench) Voss) from fourteen sites near the elevational treeline on the eastern Seward Peninsula of Alaska, obtaining 46 cores from 38 trees, which they used to develop a maximum latewood density (MXD) chronology for the period AD 1389 to 2001.  Calibrating a portion of the latter part of this record (1909-1950) against May-August monthly temperatures obtained from the Nome meteorological station, they then converted the entire MXD chronology to warm-season temperatures.

What was learned
D'Arrigo et al. report that "the middle-20th century warming is the warmest 20-year interval since 1640."  In viewing their plot of reconstructed temperatures, however, it can readily be seen there is a nearly equivalent warm period near the end of the 1600s, as well as a two-decade period of close-to-similar warmth in the mid-1500s.  What is more, in the latter part of the 1400s, there is a decade of warmth that is actually warmer than that of the mid-20th century.

What it means
The new temperature reconstruction, which the authors describe as "one of the longest density-based records for northern latitudes," provides yet another indication that 20th-century warmth was by no means unprecedented when compared to the past millennium or two, contrary to the claims of Mann et al. (1998, 1999) and Mann and Jones (2003).  Quite to the contrary, in fact, it rather supports the findings of Esper et al. (2002, 2003), McIntyre and McKitrick (2003), and Loehle (2004), which indicate there were several periods over the past millennium or more when it was equally as warm as, or even warmer than, it was during the 20th century.

References
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., Shiyatov, S.G., Mazepa, V.S., Wilson, R.J.S., Graybill, D.A. and Funkhouser, G.  2003.  Temperature-sensitive Tien Shan tree ring chronologies show multi-centennial growth trends.  Climate Dynamics 21: 699-706.

Loehle, C.  2004.  Climate change: detection and attribution of trends from long-term geologic data.  Ecological Modelling 171: 433-450.

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 during the past millennium: Inferences, uncertainties, and limitations.  Geophysical Research Letters 26: 759-762.

Mann, M.E. and Jones, P.D.  2003.  Global surface temperatures over the past two millennia.  Geophysical Research Letters 30: 10.1029/2003GL017814.

McIntyre, S. and McKitrick, R.  2003.  Corrections to the Mann et al. (1998) proxy data base and Northern Hemispheric average temperature series.  Energy and Environment 14: 751-771.