Glaciologist William Pfeffer from the University of Colorado Boulder has been taking photos of the Columbia Glacier in Prince William Sound on Alaska’s south central Pacific coast since 2004 and has created a time-lapse video that documents the glacier’s rapid retreat and subsequent ice discharge.
The video shows large chunks splitting off from the glacier as it flows out to sea. Pfeffer presented the animated glacier flow at the International Polar Year 2012 scientific meeting in Montreal, Canada, on 25 April.
For a typical mountain glacier, the ice flows faster as it gets thicker and steeper. But the dynamics of glaciers that advance into coastal waters are different: as ice is pushed into the ocean, it thins and becomes buoyant, lifting the weight off its bottom face and reducing friction with the ground below.
“Think of yourself in a pool. As you walk into the water, it gets deeper and deeper, so more of your weight is being carried by the buoyancy forces of the water, so your feet start to lose contact with the bottom of the pool. If that’s the only thing that is keeping you from sliding forward, then as the water gets deeper, you’re going to slip forward more easily,” says Pfeffer.
According to scientists the Columbia Glacier became ungrounded in 2007 and presented scientists with their first opportunity to observe the transition between a glacier’s grounded and floating states in temperate tidewater glacier.
When Pfeffer compared the thickness of the ice at the terminus of the glacier with the depth of the water, he found that the glacier became unstable when the ratio dipped below four-thirds. Pfeffer hopes that further study will allow him to estimate when other tidewater glaciers will become unstable, start to shed ice from the terminus, and contribute to sea level rise.
“These glaciers are melting fast,” says Pfeffer. “They’re small buckets with very large holes in them.”