Background
The authors write that prior efforts to estimate the mass loss of glaciers in Alaska and northwest Canada -- which they refer to as simply Alaskan glaciers -- "have relied on extrapolating site-specific measurements to the entire region," citing the studies of Arendt et al. (2002), Meier and Dyurgerov (2002) and Dyurgerov and Meier (2005). With respect to what they refer to as the "landmark study" of Arendt et al., for example -- which is entitled "Rapid wastage of Alaska glaciers and their contribution to rising sea level" -- they indicate that the latter researchers used laser altimetry to measure elevation changes on 67 glaciers, but that those glaciers represented only 20% of the area of the entire ice field.

What was done
In an attempt to greatly expand the areal coverage of these past analyses, as well as overcome several other methodological deficiencies, Berthier et al. say they calculated ice elevation changes for nearly three quarters of the ice-covered areas in the Alaskan glacier range by combining "a comprehensive glacier inventory with elevation changes derived from sequential digital elevation models," the first set having a median date of 1962 and the latter having a date of 2006.

What was learned
The five scientists report that "between 1962 and 2006, Alaskan glaciers lost 41.9 ± 8.6 km³ per year of water, and contributed 0.12 ± 0.02 mm per year to sea-level rise," which they note was 34% less than estimated by Arendt et al. (20002) and Meier and Dyurgerov (2002). And in discussing this large difference, they say the reasons for their lower values include "the higher spatial resolution of [their] glacier inventory as well as the reduction of ice thinning underneath debris and at the glacier margins, which were not resolved in earlier work."

What it means
In addition to significantly revising what was previously believed about the magnitude of ice wastage in Alaska and northwest Canada in recent decades, Berthier et al. say their results suggest that "estimates of mass loss from glaciers and ice caps in other mountain regions could be subject to similar revisions," all of which would tend to mitigate against the rapidity with which the world's climate alarmists have long contended earth's mountain glaciers and ice caps were wasting away and thereby contributing to global sea level rise.

References
Arendt, A.A., Echelmeyer, K.A., Harrison, W.D., Lingle, C.S. and Valentine, V.B. 2002. Rapid wastage of Alaska glaciers and their contribution to rising sea level. Science 297: 382-386.

Dyurgerov, M.B. and Meier, M.F. 2005. Glaciers and the Changing Earth System: A 2004 Snapshot. Instaar.

Meier, M.F. and Dyurgerov, M.GB. 2002. Sea level changes: How Alaska affects the world. Science 297: 350-351.