What was done
In 1980, according to the authors, a 103-m ice core was retrieved from a high elevation site on Mount Logan - Canada's highest mountain - which is located in the heavily-glaciated Saint Elias region of the Yukon.  From this core, as well as subsequent shallow coring and snow-pit sampling, they derived a snow accumulation record stretching from 1693 to 2000, after which they demonstrated that "heavy snow accumulation at the site is associated with warmer tropospheric temperatures and higher geopotential heights over northwestern North America."  Hence, their snow accumulation record is a proxy for near-surface air temperature over the past three centuries in this part of the world.

What was learned
Over the first half of the record, there was no significant trend in the data.  From 1850 onward, however, there was a positive trend, significant at the 95% confidence level, which appears to be associated with both the Pacific North America pattern (Wallace and Gutzler, 1981; Barnston and Livezey, 1987) and the Pacific Decadal Oscillation (Zhang et al., 1997).

What it means
This important new data set casts ever more doubt upon the IPCC-endorsed "hockey stick" temperature history of the Northern Hemisphere derived by Mann et al. (1999), while at the same time supporting the more realistic record derived by Esper et al. (2002).  In the Mann et al. temperature history of the past millennium, for example, there is neither a Medieval Warm Period nor a Little Ice Age, only a gradual 900-year cooling trend that is followed by a rapid warming that does not begin until about 1910.  The Esper et al. record, on the other hand, includes both of these widely recognized climatic features and additionally indicates that the recovery from the Little Ice Age began much earlier in the early to mid-1800s, which is what the data of Moore et al. also show.

References
Barnston, A.G. and Livezey, R.E.  1987.  Classification, seasonality and persistence of low-frequency atmospheric circulation patterns.  Monthly Weather Review 115: 1083-1126.

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.  1999.  Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations.  Geophysiocal Research Letters 26: 759-762.

Wallace, J.M. and Gutzler, D.S.  1981.  Teleconnections in the geopotential height field during the northern hemisphere winter.  Monthly Weather Review 109: 784-812.

Zhang, Y., Wallace, J.M. and Battisti, D.S.  1997.  ENSO-like interdecadal variability: 1900-93.  Journal of Climate 10: 1004-1020.