What was done
As part of a larger goal, i.e., to determine the response of surface ground temperature to changes in seasonal snow cover, the authors set about to determine what changes might have occurred in the mean onset date of snow and its yearly duration in North America over the period 1950-2002.  In describing the data sets with which they worked, they say that "data for the contiguous United States come from the USHCN and include 1062 stations," that "data for Canada come from the Canadian Daily Climate Dataset which includes 3785 stations," and that "data for Alaska are taken from the National Weather Service cooperative network which includes 543 stations."

What was learned
Bartlett et al. report that "for the period 1961-1990 the mean snow onset date in North America [was] 15 December, with mean snow cover duration of 81 days."  In addition, they say there were "no significant trends in either onset or duration from 1950 to 2002."  Nevertheless, interannual variations of as much as 18 and 15 days in onset and duration, respectively, were present in the data; but for both parameters they report that "no net trend was observed."

What it means
We find it extremely interesting that from 1950 to 2002, during which time the air's CO2 concentration rose by fully 20% (from approximately 311 to 373 ppm), there was no net change in either the mean onset or duration of snow cover for the entire continent of North America; and to provide some context for this 62-ppm increase in atmospheric CO2 concentration, we note that it is essentially identical to the mean difference between the highs and lows of the three interglacials and glacials recently reported by Siegenthaler et al. (2005) for the period prior to 430,000 years ago.  Surely, such a change should have made some impact on North American snow cover, unless, of course, atmospheric CO2 enrichment has far less impact on climate than what climate alarmists claim it does.

Reference
Siegenthaler, U., Stocker, T.F., Monnin, E., Luthi, D., Schwander, J., Stauffer, B., Raynaud, D., Barnola, J.-M., Fischer, H., Masson-Delmotte, V. and Jouzel, J.  2005.  Stable carbon cycle-climate relationship during the late Pleistocene.  Science 310: 1313-1317.