What was done
Using new tree-ring data from the Columbia Icefield area of the Canadian Rockies, Luckman and Wilson present a significant update to a millennial temperature reconstruction published for this region in 1997.  The new update employed different standardization techniques, such as the regional curve standardization method, in an effort to capture a greater degree of low frequency variability (centennial to millennial scale) than reported in the initial study.  In addition, the new data set adds over one hundred years to the chronology and now covers the period 950-1994.

What was learned
The tree-ring record was found to explain 53% of May-August maximum temperature variation observed in the 1895-1994 historical data and was thus viewed as a proxy indicator of such temperatures over the past millennium.  Based on this relationship, the record showed considerable decadal- and centennial-scale variability, where generally warmer conditions prevailed during the 11th and 12th centuries, between about 1350-1450 and from about 1875 through the end of the record.  The warmest reconstructed summer occurred in 1434 and was 0.23°C warmer than the next warmest summer that occurred in 1967.  Persistent cold conditions prevailed between 1200-1350, 1450-1550 and 1650-1850, with the 1690s being exceptionally cold (more than 0.4°C colder than other intervals).

What it means
The revised Columbia Icefield temperature reconstruction provides further evidence for natural climate fluctuations on centennial-to-millennial time scales and demonstrates, once again, that temperatures during the Modern Warm Period are no different from those observed during the Medieval Warm Period (11-12th centuries) or the Little Medieval Warm Period (1350-1450).  And since we know that atmospheric CO2 concentrations had nothing to do with the warm temperatures of those earlier periods, we cannot rule out the possibility that they also have nothing to do with the warm temperatures of the modern era.

But if not CO2, then what?  According to Luckman and Wilson, the Columbia Icefield reconstruction "appears to indicate a reasonable response of local trees to large-scale forcing of climates, with reconstructed cool conditions comparing well with periods of known low solar activity," which is a nice way of suggesting that the sun is the main driver of these low frequency temperature trends.