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

[Frank Paul]

Dear all

Please apologize for this rather long email!

Let me give a few comments on the glacier definition problems. First of
all I would suggest to follow the naming conventions from the UNESCO
document that are also included in the `Illustrated GLIMS Glacier
Classification Manual' which is available from the GLIMS website. Based on
this document I would separate the discussion on (1) ice-sheets/ ice
shelves, (2) valley-/mountain glacier/glacierets, and (3) ice caps/ice
fields. We have two ice sheets on our planet, covering Antarctica and
Greenland and the term ice shelf should be geographically restricted to
Antarctica. All other ice terminating in the ocean or in lakes is just
`floating' ice from glaciers and treated under digit 3 (frontal
characteristics) of the WGI classification (calving) and the extended
GLIMS classification (floating). The classification problems for (3) ice
caps and ice fields are very different from (2) as they are more related
to the definition of drainage divides in the accumulation area and not so
much on contributing snow/ice fields above the accumulation area. At this
point I have to admit that I do NOT agree with Fig. 1 on page 6 of the
GLIMS classification manual as this figure shows an ice cap and not an ice
field (that has to be separated by rock outcrops). For this reason Figs. 3
and 6 (the background ice mass) on page 7 are NOT ice fields but ice caps
(draining into outlet glaciers). And, to close this, I would not term the
ice mass visible in Fig. 13 (page 9) a mountain glacier as it is connected
at a floating terminus with adjacent ice masses. I would term it
`miscellaneous' as it seems to be an ice mass that contributes to an ice
shelf. Of course, we might have a separate discussion on this topics later
on.

At this point we are right in the discussion of the next important issue:
What is the actual and later purpose of the definition? With this respect,
the glacier parts that will get an individual ID might be very different.
Here are some examples:

(a) If we want to calculate topographic glacier parameters (min/max
elevation, steady state ELA0, slope, aspect, length) from the outlines
(and a DEM) we should exclude all snow/ice couloirs above the main
accumulation area, perhaps excluding all areas above the bergschrund as
well (?), exclude any dead ice and exclude all glaciers with a
disconnection inbetween (calving/regenerated) completely (e.g. ELA cannot
be determined in such cases).

(b) The points in (a) are strongly related to the climate change
assessment / modelling purpose, e.g. even small but calving ice caps will
not change size over decades and got too high ELA0 values while they feed
(maybe several) regenerated glacier parts with comparably low ELA0 values.
Moreover, if we calculate mean slope from length and elevation range or
basal shear stress from elevation range we should only include those
glacier parts that take part on the glacier flow. This is also important
for the still most popular volume-area scaling: glacier volume will be
strongly overestimated if all these small and mostly very thin
contributing ice fields will be accounted for in the total area. But if
they are treated separately, you will have many (very) small glaciers
without any change for decades and, of course, there sensitivity with
respect to overall changes in ELA is completely different from the
`regular' glaciers.

(c) Following change in glacier geometry through time is another important
purpose (also related to (b)). Although the concept of a parent glacier is
capable of tracing such changes, it is lost for the temporal evolution
when it is used for the spatial summary of individual glacier parts at one
point in time. And, if you use a parent ice mass in a temporal sense, you
have to be sure about the starting point. Although most glacier
inventories will not go back until the Little Ice Age (LIA) maximum
extent, you will get problems if you want to include changes since the
LIA, but your numbering scheme starts in the 1970s (e.g. with the WGI).
Moreover, the separation of a larger tributary has several implications
for the further evolution of the new components as the entire set of
modelling parameters might change (ELA, length, slope, elevation range,
etc). Thus, there is some need to trace temporal changes in the data base.

(d) A different counting and labelling scheme can also be mandatory for
GIS-based computation of topographic parameters from zonal statistics. For
instance, if two individual glacier parts should be counted as one for
specific zonal calculations, they have to get a specific and identical
label for this purpose. The decision on which parts are important for
which parameter and a consistent application of a related labelling scheme
is very time consuming or may even not be solved.

(e) And a last point is related to the data source used for the
compilation. There might be several parts of a glacier that should be a
part of the main glacier but cannot be recognized on satellite imagery
(ice-cored moraines, debris-covered tongues, etc.) or were missed due to
the low spatial resolution or filtering (e.g. perennial snow patches or
thin couloirs). This is of major importance if comparisons with former
inventory data that originate from different source data (e.g. aerial
photography) are performed. And, of course, it underlines the necessity to
generate a (global) 2D inventory (polygons) as it is almost impossible to
decide which glacier is behind the point information provided in the WGI
without having the related base maps or photographs.

In summary: First, if we talk about how to define a glacier we should
first clarify which type of glacier is meant [(1), (2) or (3)] and maybe
also agree about what we would like to do with the data [(a)-(e)].
Secondly, due to the fact that the answer to the last question is indeed
not easy, I would like to ask for a special discussion on these topics at
the GLIMS meeting connected to the IGS conference in Cambridge and of
course for further comments from this list. Finally, I added some comments
to the definition and their implications provided by Bruce & others below.

Best regards, Frank



> 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
see the discussion on UNESCO classification above

> accident.  Names were often applied with little knowledge of the
> physical characteristics of the feature.  The floating parts of some
yes, this is indeed true! but this implies that at least ourselves
should agree upon the naming conventions

> 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
I propose to restrict ice shelf geographically to Antarctica. All other
floating ice is only floating (digit 3, frontal characteristics)
>
> Regards,
>          Neal
>
> > > 1) define what to do about connected snowfields above the bergschrund
depends on what you would like to do with the data afterwards (see above)

> > > 2) define how to treat merging tributaries
if they have separate accumulation areas AND there is only very little
contribution to the ice mass of the main glacier: make two of them.

if they originate in the same basin OR make a substantial contribution
to the main glacier: make one of them


> > > 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.
ok, but in this case they have to be identified by a unique label that
they easily can be excluded from the sample for modelling and other
purposes (see above)

> > > 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.
At least they should be named only ice shelf if they are connected to
Antarctica (see above). And here I would count all tributary ice masses
(e.g. those depicted in Fig. 13 of the illustrated GLIMS manual)
and ice streams (yet another name :) as part of the ice shelf (e.g. for
modelling purposes and the consequences Jeff mentioned)


> > > 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.
see my comments above

> > > 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.]
good idea, but it will be difficult (or very long) to document which name
belongs to which glacier part.


> > > 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.
ok, but what about introducing 'must flow due to the accelaration of gravity'?
this would allow to exclude all the perennial stuff that makes modelling
difficult and to give both proper names


> > >  An ice shelf -- floating ice downstream of
> > > the grounding zone of two or more glaciers -- shall be considered as a
> > > separate glacier.
Skip this ice shelf thing if we here talk about mountain and valley glaciers
+ glacierets/cirques


> > > 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.
Exclude snowfields, include ice fields only if they make a contribution
to the ice below

> > > 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.
YES!

> > > 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.
YES! (see A)

> > > 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.
YES! (it should be termed dead ice if the contact to the main glacier is
disrupted, if it is not flowing and not feeded by ice avalanches).
If a glacier advances beyond a steep slope by forming an ice avalanche
cone I would not count this cone before it is connected to the main glacier

> > > 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.
ok, but please remember that we may have to use two 'parent icemass IDs'
one for following the temporal evolution and one for representing the
spatial connectivity (see above)

> > >
> > > G. Marginal and terminal moraines should be included (as debris-covered
> > >  glacier) if they contain ice.
maybe difficult to determine by remote sensing from one image. in
particular i would NOT count the terminal moraine as it is not longer
connected to the glacier and without flow (so it is dead 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
yes, but only if both 'ice areas' are flowing! (this excludes any 'snow
areas' and ice cones from advancing glaciers)

> > >  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.
ok, but here again the 'parent icemass' is only related to the spatial
component not to the temporal one (that might show a different evolution)

> > > 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.
ok, but also use only end of melt period images from years without any snow
outside of glaciers! (Indeed, this will reduce the number of scenes
available from ASTER or Landsat for the purpose of a glacier inventory
drastically and exceptions have to be made!)