Researchers led by Richard Alley of Pennsylvania State University believe they have a way of predicting how fast “calving events” will occur on an ice shelf. Their model, briefly described in a press release from the university and reported in the November 28 issue of Science, will prove helpful as more “calves,” or icebergs, are produced when temperatures warm in the Arctic and Antarctic.

Ice shelves are the lips of glaciers that spread under their own weight and flow off land and over the ocean. The edges of the ice shelves break off and fall into the oceans as icebergs.

Ross Ice Shelf in Antarctica floats some 500 miles over the ocean before its edges begin to break off. Some ice shelves float over water for no more than a mile or two.

The researchers studied thickness, calving rate, and strain rates for 20 ice shelves. They found that the calving rate is primarily determined by the rate at which the ice shelf is spreading away from the land. The shelves crack under the stresses of spreading. If a shelf spreads slowly, it remains intact. Narrow shelves, buttressed from the sides or the seafloor, calve more slowly than wider ones.

 Alley cautions there are many variables in determining when things will break, but a basic equation for ice calving would read: the rate of spreading times the width of the shelf times the thickness times a constant. The equation may help scientists improve their climate models and add to the understanding of ice behavior. In its 2007 assessment, the Intergovernmental Panel on Climate Change noted the lack of understanding of ice sheet and ice shelf processes as a factor leading to uncertainty in sea-level rise projections.