[Take two]: Proposed GLIMS definition of "glacier"

[Jeff Kargel]

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].