New
iceberg theory points to areas at risk of rapid disintegration
23
July, 2013
In
events that could exacerbate sea level rise over the coming decades,
stretches of ice on the coasts of Antarctica and Greenland are at
risk of rapidly cracking apart and falling into the ocean, according
to new iceberg calving simulations from the Univ. of Michigan.
"If
this starts to happen and we're right, we might be closer to the
higher end of sea level rise estimates for the next 100 years,"
said Jeremy Bassis, asst. prof. of atmospheric, oceanic and space
sciences at the U-M College of Engineering, and first author of a
paper on the new model published in Nature Geoscience.
Iceberg
calving, or the formation of icebergs, occurs when ice chunks break
off larger shelves or glaciers and float away, eventually melting in
warmer waters. Although iceberg calving accounts for roughly half of
the mass lost from ice sheets, it isn't reflected in any models of
how climate change affects the ice sheets and could lead to
additional sea level rise, Bassis said.
"50%
of the total mass loss from the ice sheets, we just don't understand.
We essentially haven't been able to predict that, so events such as
rapid disintegration aren't included in those estimates," Bassis
said. "Our new model helps us understand the different
parameters, and that gives us hope that we can better predict how
things will change in the future."
The
researchers have found the physics at the heart of iceberg calving,
and their model is the first that can simulate the different
processes that occur on both ends of the Earth. It can show why in
northern latitudes—where glaciers rest on solid ground—icebergs
tend to form in relatively small, vertical slivers that rotate onto
their sides as they dislodge. It can also illustrate why in the
southernmost places—where vast ice shelves float in the Antarctic
Ocean—icebergs form in larger, more horizontal plank shapes.
The
model treats ice sheets—both floating shelves and grounded
glaciers—like loosely cemented collections of boulders. Such a
description reflects how scientists in the field have described what
iceberg calving actually looks like. The model allows those loose
bonds to break when the boulders are pulled apart or rub against one
another.
The
simulations showed that calving is a two-step process driven
primarily by the thickness of the ice.
"Essentially,
everything is driven by gravity," Bassis said. "We
identified a critical threshold of one kilometer where it seems like
everything should break up. You can think of it in terms of a kid
building a tower. The taller the tower is, the more unstable it
gets."
Icebergs
do have a tendency to form before that threshold though, Bassis
suspects, due to cracks that are already there—either formed when
capsizing bergs crash into the water and send shockwaves through the
surrounding ice, or when melted water on the surface cuts through.
The former is believed to have led to the Helheim Glacier collapse in
2003. The glacier had begun to retreat slowly in 2002, but suddenly
gave way the following year when the thinner ice had broken away,
exposing a thicker ice coast.
The
latter—melted water pools—are occurring more frequently due to
climate change, and they're believed to have played a role in the
rapid disintegration of the Antarctica's Larsen B ice shelf, which
crumbled over about six weeks in 2002.
When
the researchers added random cracks to their model, it could mirror
both Helheim and Larsen B.
A
third feature is also required for the most dramatic ice collapses to
occur. Icebergs can't float away and make room for more icebergs to
break off the main sheet unless the system has access to open water.
So areas that border deep, unobstructed ocean rather than fjords or
other waterways are at greater risk of rapid ice loss. The
researchers point to the Thwaites and Pine Island glaciers in
Antarctica and the Jakobshavn Glacier in Greenland, which is already
retreating rapidly, as places vulnerable to "catastrophic
disintegration" because they have all three components.
"The
ice in those places gets thicker as you go back. If our threshold is
right, then if these places start to retreat as you expose the
thicker calving font, they're susceptible to catastrophic breakup,"
Bassis said.
Retreat
of the current ice coasts in these places areas could occur via
melting or iceberg calving.
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