Glacier Geometry Linked To Diverse Calving Patterns
July 23, 2013

Researchers Model The Physics At The Heart Of Iceberg Calving

April Flowers for - Your Universe Online

Large stretches of ice on the coasts of Antarctica and Greenland are at risk of rapidly cracking apart and falling into the ocean in events over the coming decades that could aggravate sea level rise. The new study, published in Nature Geoscience, describes new iceberg calving simulations from the University 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, assistant professor of atmospheric, oceanic and space sciences at the University of Michigan College of Engineering.

Icebergs are formed when ice chunks break off larger shelves or glaciers and float away. This process is called calving. These icebergs eventually melt in warmer waters. Roughly half of the mass lost from ice sheets is accounted for by iceberg calving. The calving, however, isn't reflected in any models of how climate change affects the ice sheets and could lead to additional sea level rise, according to Bassis.

"Fifty percent 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 team of scientists discovered the physics underlying the heart of iceberg calving, allowing them to create the first model that can simulate the different processes that occur on both ends of the planet. The model is able to show why icebergs tend to form in relatively small, vertical slivers that rotate onto their sides as they dislodge in northern latitudes - where glaciers rest on solid ground. The model also shows why in the southernmost places - where vast ice shelves float in the Antarctic Ocean - icebergs form in larger, more horizontal plank shapes.

In the new model, ice sheets - both floating shelves and grounded glaciers - are treated like loosely cemented collections of boulders, which is how scientists in the field have described what iceberg calving actually looks like. Those loose bonds are allowed to break loose in the model when the boulders are pulled apart or rub against one another.

The team's simulations revealed 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."

Due to cracks already there, icebergs have a tendency to form before that threshold is reached. The cracks form when capsizing bergs crash into the water and send shockwaves through the surrounding ice, or when melted water on the surface cuts through. Scientists believe the Helheim Glacier collapse in 2003 was a result of the former mechanism. The glacier had already begun a slow retreat in 2002, but suddenly gave way in 2003 when the thinner ice had broken away, exposing a thicker ice coast.

Melted water pools, on the other hand, are occurring more frequently due to climate change. These pools are 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 random cracks were added to the model, it mirrored both Helheim and Larsen B.

For most dramatic ice collapses to occur, a third feature is required. Unless the system has access to open water, icebergs can't float away and make room for more icebergs to break off the main sheet. This means areas of the sheet that border deep, unobstructed ocean rather than fjords or smaller waterways are at greater risk of rapid ice loss. The Thwaites and Pine Island glaciers in Antarctica and the Jakobshavn Glacier in Greenland are already retreating rapidly, but the researchers have identified them as being vulnerable to "catastrophic disintegration" because all three components are present.

"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 front, they're susceptible to catastrophic breakup," Bassis said.

The model reveals the retreat of current ice coasts in these regions could happen because of either melting or iceberg calving.