Researchers set out to crack
ice fracture modeling puzzle
Posted October 26, 2010
Four frames of a simulation of stress in an ice sheet Click here to open video in a separate window.
The ice sheets are unraveling. At the edges of Antarctica and Greenland and across the arctic, great swaths are breaking into icebergs and miles-wide floes.
The Larsen B Antarctic ice shelf, which covered more than 1,200 square miles and was 650 feet thick, disintegrated in 2002. Six years later the nearby Wilkins ice shelf, more than 5,000 square miles, partially collapsed. They were only the latest of ice shelves to retreat or give way over the past few decades. After taking thousands of years to form, some ice shelves are falling apart in weeks.
In each case, fractures preceded failure, says Haim Waisman, a Columbia University assistant professor of Civil Engineering and Engineering Mechanics. Waisman heads a Department of Energy-supported project to improve computer simulation of land ice cracking and how global warming accelerates it. “If we can model and understand some of these fractures, maybe this can be coupled with ice sheet simulations and give a better understanding of what’s happening.”
Waisman’s project is part of ISICLES (see sidebar), the Ice Sheet Initiative for CLimate ExtremeS, supported by DOE’s Office of Advanced Scientific Computing Research (ASCR).
Ice sheet models have not explicitly considered ice fracture, a complex phenomenon that demands large computer resources. Yet fractures play a pivotal role in regional behavior and influence ice dynamics over larger areas in ways scientists don’t understand.
There are ice fracture models available, Waisman says, but none are definitive, failing to capture the real scenario of the ice shelf.
That’s understandable. Ice is nearly as common in the world as liquid water, yet “it has its own challenges which are different from the usual material,” Waisman says. Ice is heterogeneous, with widely varying qualities; it can be viscous, with characteristics lurking between liquid and solid; and it compresses under its own weight, so its density can vary.
The Intergovernmental Panel on Climate Change, a group of scientists from around the world charged with assessing global climate science, defines an ice sheet as land ice thick enough to hide the underlying topography. There are only three large ice sheets today: one on Greenland and the other two on Antarctica. Ice sheets can flow into ice shelves – thick slabs of floating ice extending from coastlines and often filling inlets or bays. Nearly all ice shelves are in Antarctica.
The scale of ice sheets also poses a challenge. They cover an area the size of “the entire United States and even more, and the largest ice shelf is bigger than California,” Waisman says. And modelers want the shape of their simulated ice sheets to mimic reality. “We want to do something that’s as close as possible. So we need to somehow obtain data from experimentalists on what is the real ice out there – which is a challenge by itself.”