8/7/09, South Cascade Glacier has shrunk by half

Interesting article about glacier studies in the Seattle Times today:

seattletimes.nwsource.com/html/localnews...4459_glaciers07.html

The article describes how the South Cascade Glacier and others have shrunk in the past 50 years. This is no surprise to anybody who's been paying attention.



To me, the most surprising statement was this one, in the second paragraph of the story:

[size=10pt]One of the glaciers, the South Cascade Glacier in Washington, has lost nearly half its volume and a quarter of its mass since 1958, U.S. Geological Survey scientists said.
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My immediate question was how can a glacier lose half its volume but only a quarter of its mass? Then it occurred to me that the volume loss must be primarily due to melting of lower density snow and firn. The higher density ice in the core of the glacier must be the last to go. Am I interpreting this correctly? Or did the newspaper get it wrong?

My 2003 story about glaciers on the Ptarmigan Traverse can be found here:

www.alpenglow.org/climbing/ptarmigan-1953/index.html

Hmm -- time for some math.

If the 1/2 of the volume that disappeared contained 1/4 of the mass of the original glacier, then the 1/2 that remains contains 3/4 of the original mass.  If the water density of ice is 100% (which it isn't) then the melted snow averaged 33% moisture content.

Hey, this is the PNW!!  Our snow is around 33% when it falls.  Surely the melted snow was denser than new fallen snow.  The article is just plain wrong about this mass and volume stuff.

It would be easy math if the glacier has lost half of its areal extent and a quarter of its volume.

Several decades of air photos of this glacier are spliced together at:  wa.water.usgs.gov/projects/glacier/video.html

That fact must be true, as someone has added it to wikipedia already and cited Tankersley's article.

en.wikipedia.org/wiki/South_Cascade_Glacier

I think this is the original from USGS (but on a quick scan does not list the before nad after total volume number for the South Cascade Glacier):

Fact Sheet: pubs.usgs.gov/fs/2009/3046/
Report: pubs.usgs.gov/fs/2009/3046/pdf/fs20093046.pdf