GLIMS definition issues

[Jeffrey S Kargel]

Bruce,

I scrolled down and saw that you had more to say...

... and your discussion of the up-glacier end of things is fine.  (The
beautiful "organized mess" that nature is makes it hard to deal with, but
your's is a reasonable approach.)

The recent paper by Ted Scambos and coauthors on the structures in the Ross
Ice Shelf highlights the thorniness of dealing with that bush (since you
used the term "leaves") and the attractiveness of the ice-shelf "rose."
Basically, an ice shelf is like a giant (short and squat) trunk glacier,
with all the ice streams and smaller feeder glaciers being its tributary.
Commonly it is possible to trace the major tributaries to the ice wall, and
so it would be possible to track each large glacier to its farthest
floating end. Smaller glaciers, and the tributaries of the tributaries,
can't be so easily tracked.  So on one hand, it should be possible to treat
an ice shelf as a unitary glacier.  On the other hand, Ted's (and others')
work has shown that there's a lot of information within the ice shelf, and
it would be a pity to throw it away by a desire to look at the forest to
the exclusion of some rather large and impressive trees.  (Not to imply
that there are any trees, other than petrified ones, in Antarctica.)

If we were to cut the glacier off at the grounding line, we have to expect
that there will be sudden huge negative changes in ice mass for surging
tide-water glaciers.   Okay, I am not saying we can't do that, but a
glacier that surges on land loses little mass, whereas one that surges at
sea loses huge amounts of mass.  And if a surge just happens such that the
added mass at the terminus depresses the ice down and pushes the grounding
line outward, that glacier may have "gained" mass practically overnight.  I
don't think that these will be rare, isolated cases; they will be common
wherever you have calving glaciers.  I think we should deal with the
floating parts.   Doing so will not eliminate the reality of some glaciers
that suddenly lose a great deal of mass, like the Columbia recently.  But
it seems to me that we should have a means to track the loss (and growth of
ice shelves and ice stongues and other floating glaciers), even if for no
reason other than to lend a predictable quality to the truly land-based
part of the database.  If we don't deal with ice shelves, what about
grounded "islands" of ice in places like the Ross ice shelf?  Would a
non-glacier floating ice shelf suddnly turn into a glacier, and then stop
being one after it has passed the grounding line of the seaward side of the
island?

Okay, if we were to cover ice shelves, then for purposes of sea level
discussions, we would have to have in the database information on the
grounding line location (as best as it is known)


--Jeff K