Fwd: Re: [Take two]: Proposed GLIMS definition of "glacier" and ice shelf

Wed Mar 1 17:03:57 MST 2006

Bruce and others,

I have one query:

Why "two or more" glaciers in:

> > An ice shelf -- floating ice downstream of the grounding zone of 
> two or more glaciers --
> > shall be considered as a separate glacier.

The basic criteria is one of floating and the characteristics that 
follow from that.

I wonder if we/you are trying to separate items that have been named 
as "glacier" or "glacier tongue" by historical / geographic / 
cartographic accident, from features named ice shelf(ves), by similar 
accident.  Names were often applied with little knowledge of the 
physical characteristics of the feature.  The floating parts of some 
glaciers are far larger than some ice shelves.  Some ice shelves 
appear and behave as separate ice streams that happen to be 
adjacent.  How does one decide if there is one or more glaciers that 
merge to form the shelf.  By what is named or by ???   There is no 
mention of merging or combining to form a single whole entity.  There 
is always going to be a broad spectrum of these characteristics, 
especially if a shelf / glacier is in the process of change, which 
then comes to the issues Jeff raised regarding the impact of the 
changing location of the grounding line (actually zone).

Regards,
         Neal

>Date: Wed, 01 Mar 2006 16:25:26 -0700
>Subject: Re: [Take two]: Proposed GLIMS definition of "glacier"
>From: Jeff Kargel <jkargel1054 at earthlink.net>
>To: GLIMS Mailing List <glims at flagmail.wr.usgs.gov>
>
>Bruce and all,
>
>This definition seems functional for GLIMS.  I would add one more
>consequence, which is perhaps problematic in some respects, but maybe less
>problematic not doing it this way.  Comment concerns this part of the
>definition:
>
> > An ice shelf -- floating ice downstream of
> > the grounding zone of two or more glaciers -- shall be considered as a
> > separate glacier.
>
>First a question or clarification.  It seems that your definition of ice
>shelf means any floating part of the glacier, as stated.  Does this include
>glaciers floating on lakes?  I guess so by definition.  Hey, it's just
>semantic for the wider world of glaciology, but we need a pragmatic
>definition for GLIMS, and I think this does it. But it, too has bizarre
>consequences.
>
>Consequences #1:
>A glacier having a receding grounding line (due to glacier thinning or sea
>level rise or lake level rise) may register a considerable shortening.  The
>floating part may register are large lengthening or shortening depending on
>its dynamics.   A glacier that thickens and has its grounding line advance
>seaward will register a lengthening, and the ice shelf will register
>shortening (unless it, too advances).   A glacier terminating in a floating
>glacier on a lake may experience lengthening and shortening related to lake
>level rise or fall, in addition to thinning, thickening of the glacier.
>
>Why I lean toward your definition and can swallow the consequences: The
>floating part of a glacier (ice shelf) does not contribute to sea level when
>it melts.
>
>Consequence #2: Improved analysis (or different analysis) may register and
>advance or retreat of the grounding line even if there is no actual advance
>or retreat.
>
>Why I can accept that consequence: It's all part of the analysis error.
>
>--Jeff K
>
>On 3/1/06 3:35 PM, "Bruce Raup" <braup at nsidc.org> wrote:
>
> > Hello all,
> >
> > Thanks for the feedback on how to define "glacier" for GLIMS.  Below is the
> > next draft, after thinking about all the comments.  Originally, the need
> > for a new definition was driven by the need to solve these two issues:
> >
> > 1) define what to do about connected snowfields above the bergschrund
> > 2) define how to treat merging tributaries
> >
> > Of course, more issues have come up in the process.  The definition below
> > generally specifies the minimum of what should be included in a "glacier",
> > under a single glacier ID.  In some cases, more may be included.  I think
> > it addresses the two issues above, plus some.  Please let me know what you
> > think of this version.  Again, please send comments to the list.
> >
> > Bruce
> >
> >
> > $Id: glacier_definition.txt,v 1.2 2006/03/01 22:26:52 braup Exp $
> >
> > Summary of comments posted to the GLIMS mailing list and my notes:
> >
> > Andrew Fountain:  don't have an artificial lower-bound on
> > size; let the imaging technology determine what gets included, and
> > definitely include the small snow patches, since they are important
> > hydrologically.
> >
> > Graham Cogley:  What do to about ice shelves?  He proposes to treat them
> > separately, and define an ice shelf as "An ice shelf consists of the
> > floating parts of two or more glaciers."  This is consistent with the
> > historical view of GLIMS to treat ice shelves as separate entities.
> >
> > My comment on observation D:  Such a snowfield should not be included as
> > part of the glacier that dominates the basin lower down, but it well
> > could be included as a "glacier" with its own ID.
> >
> > Vladimir Konovolov brings in all kinds of other stuff into the discussion,
> > including "subjective" reasons.  It is these subjective differences that
> > necessitate a precise definition of glacier for use within GLIMS.  Most of
> > the differences in GLACE results are from (1) lack of topo info, and (2)
> > differing ideas about what should be included.
> >
> > Hugh Kieffer:  Somehow retain old names of feeder glaciers, even if they
> > are lumped together with the main trunks.  [One solution:  include all
> > relevant names in the "name" field for that "glacier".  That way it would
> > be searchable by name.]
> >
> > -------------------------------------------------------------------------
> >
> > Through experience with data submitted to the GLIMS Glacier Database, as
> > well as the GLACE experiments, it has become apparent that a more precise
> > practical definition of "glacier" is needed within the GLIMS project.  Such
> > a definition will help ensure consistency of analysis results in the
> > database, facilitating more meaningful multi-temporal comparisons and
> > change-detection results in the future.
> >
> > The following candidate definition is the result of discussion at the GLIMS
> > Workshop in Twizel, New Zealand and subsequent discussion on the GLIMS
> > mailing list.
> >
> > Candidate practical definition of "glacier" for the GLIMS Project:
> >
> > A glacier, identified by a single GLIMS glacier ID, consists of a body of
> > ice and snow that persists through the end of the melt season, or, in the
> > case of tropical glaciers, after transient snow melts.  This includes, at
> > a minimum, all tributaries and connected feeders that contribute ice to
> > the main glacier, plus all debris-covered ice.  Excluded is all exposed
> > ground, including nunataks.  An ice shelf -- floating ice downstream of
> > the grounding zone of two or more glaciers -- shall be considered as a
> > separate glacier.
> >
> > Consequences and observations:
> >
> > A. Snowfields immediately above the accumulation zone of a glacier shall be
> >  considered part of the glacier, because they contribute snow (through
> >  avalanches) and ice (through creep flow) to the glacier.
> >
> > B. A tributary in a glacier system that has historically been treated (and
> >  named) as a separate glacier should, within the GLIMS framework, be
> >  included as part of the glacier into which it flows.  The name field for
> >  the glacier should be populated with all relevant names of tributaries.
> >
> > C. Any steep rock walls that avalanche snow onto a glacier but do not
> >  retain snow themselves are NOT included as part of the glacier.
> >
> > D. [deleted]
> >
> > E. A stagnant ice mass still in contact with a glacier is part of the
> >  glacier, even if it supports an old-growth forest.
> >
> > F. If no flow takes place between separate parts of a continuous ice mass,
> >  they should, in general, be treated as separate units, separated at the
> >  topographic divide.  However, for practical purposes, such an ice mass
> >  may be analyzed as a unit at the analyst's discretion, if delineation of
> >  the flow divides is impossible or impractical.  If the same system is
> >  analyzed in the same way later, it will have the same glacier ID, and
> >  can therefore be compared.  If the system is analyzed in more detail
> >  later by breaking it into its component glaciers, those pieces will get
> >  new IDs (ID of system will be "parent icemass" ID), and future analyses
> >  of those pieces, if done in the same way, will be comparable.
> >
> > G. Marginal and terminal moraines should be included (as debris-covered
> >  glacier) if they contain ice.
> >
> > H. It is possible that an ice body that is detached from another may still
> >  contribute mass to the latter through ice avalanches, or it may no
> >  longer do so.  It is practically impossible to tell which is the case
> >  from a single satellite image.  Therefore, within GLIMS, adjacent but
> >  detached snow and ice areas should, in general, be considered as
> >  different "glaciers", regardless of whether they contribute mass to the
> >  main glacier through snow or ice avalanches.  However, at the analyst's
> >  discretion, detached ice masses may be included as parts of one glacier.
> >  This is similar to the situation described in F above.  If the pieces
> >  are analyzed separately later, each piece should be given a new ID, the
> >  old one being used as the "parent icemass" ID for all the pieces.
> >
> > I. What about the lower parts of lateral snowfields, whose extent varies
> >  from year to year?  Map only at the end of summer to exclude seasonal
> >  snow.  Then map everything that is connected to the glacier.
> >
> > For details on how to produce outlines consistent with this definition,
> > see the GLIMS Glacier Analysis Tutorial at
> > http://www.glims.org/MapsAndDocs/assets/GLIMS_Analysis_Tutorial.pdf
> > [to be updated soon].

__________________________________________________________________

Neal Young
ACE CRC
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ACE CRC:  Antarctic Climate & Ecosystems Cooperative Research Centre
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