Alpine Glacier Mass Balance

The North Cascade Glacier Climate Project measures the annual balance of 10 glaciers, more than any other program in North America.  These records extend from 1984 - 2006 and represent the only set of records documenting the mass balance changes of an entire glacier clad range.  Our government (USGS) has measured mass balance on three glaciers in North America for more than 30 years, two in Alaska (Gulkana and Wolverine), and one in Washington (South Cascade.  These are considered the benchmark glaciers.  There have been spotty programs of mass balance assessment on Blue Glacier, Olympics (U of Washington) and McCall Glacier Brooks Range (USGS).  Two glaciers with the longest records are Taku and Lemon Creek Glacier, Alaska monitored by the Juneau Icefield Research Program.  The National Park Service ,with the continued efforts of Jon Riedel, has been establishing a program to monitor the mass balance of glaciers in the North Cascades and Mount Rainier NP.  To monitor an entire glaciated mountain range was listed as a high priority of the National Academy of Sciences in 1983.  The cumulative balance has been significantly negative since 1984, as noted in the graph at left.

How is it measured?

Ablation is measured by emplacing stakes in the glacier at the end of the previous melt season or the beginning of the melt season.  As the glacier surface melts the amount of the stake emerging from the glacier is measured.  The total melt at each stake by the end of the melt season is the net ablation.  Most of the stakes must be reemplaced during the summer. 

Accumulation is measured by either probing or creavasse stratigraphy to determine the annual snowpack thickness at many locations.  Crevasse layering is evident in the picture at right.  It is similar to reading tree ring width for climate analysis.  These measurements are completed both in August and again in late September, the end of the hydrologic year, each summer on 10 glaciers in the North Cascades by NCGCP.  Measurements of ablation are made at 3-6 locations and accumulation at 60-200 locations.   The impact of the number of measurements is examined in The Impact of Sampling Density on Mass Balance.  The changes in accumulation and ablation with location and during recent years indicates the importance of monitoring multiple glaciers as they are unique.  Crevasse stratigraphy is used to determine annual snow layer thickness.  In the slide at  right you can see four annual layers.  A thin surface layer, and than two approximately one meter thick layers, and then a partially shown lowest layer that is much dirtier.  The continuity of the layer thickness provides a better measure than a point measure in a snowpit or with probing.  We utilize probing where crevasses do not exist, above right.  Snowpits such as seen at lower right are not used since the density of the snowpack by August has been found to be constant.  Snowpits provide a point measurement and are time consuming.

What has research proven?  
The mean cumulative annual balance for the 1984-2006 period is -12.4 m w.e, which represents a net loss of ice thickness exceeding 14 m.  This is a significant loss for glaciers that average 30-50 m in thickness, representing 20-40% of their entire volume lost in two decades.    Annual longitudinal profiles on nine glaciers North Cascade glaciers confirm this volume change indicating a loss of
-10.7 to -15.3 m in thickness (-9.6 to -13.8 m w.e) between 1984 and 2006, agreeing well with the cumulative mean annual balance of 
-12.4 m w.e  for that period.   The data indicate broad regional continuity in glacial response to climate.  Cross correlation values of annual balance between glaciers ranged from 0.73 to 0.98.  The record reflects less variability and a more negative trend from 1984-1995.  Since 1996, there has been increasing inter-annual variability with alternately extreme positive and negative years, with a dominantly negative trend.  The annual balance of individual glacier is in a Table below The chart at right shows the annual balance of each glacier and how they are closely correlated, following the same trend from year to year, but do have a significant range in annual balance during each given year. 

What are The Effects and Consequences?  The impact of negative mass balances is glaciers that are thinning, moving slower and in some case disappearing. 

Detailed research findings (Published in  Hydrologic Processes)