Scientists have made a breakthrough in determining what actually causes ice shelves in the Antarctic and Greenland to break apart, resulting in icebergs and contributing to increased sea levels.
The findings, reported in the latest edition of the Science journal, could lead to improved climate change models, as scientists will now be able to predict more accurately where icebergs will “calve off” from their parent ice shelves.
Scientists have discovered the main factor to cause ice to calve away from ice sheets, creating icebergs, and resulting in higher sea levels. Photo: Kim Hansen.
The term “calving” in this context, refers to ice breaking off the ice shelves and landing in the ocean, causing icebergs to form. Typically, a shelf front will extend forward for years or decades between major calving events.
Until now, the main problem for scientists was determining where an ice shelf was to calve. At what point does an ice shelf have so much ice hanging over the ocean that it starts to break off?
What made this a particularly difficult question was that, there didn’t seem to be a common size between ice shelves. For example, the Ross Ice Shelf in Antarctica hangs over the ocean for as much as 500 miles. Other ice shelves only extend for a mile or two before breaking up.
The Need For A Law For Ice Shelf Calving
To date there hasn’t been a law based on physical principles that explains ice shelf calving.
“To predict the future of the ice sheet and to understand the past, we have to put the information into a computer,” says Richard B. Alley, the Evan Pugh professor of geosciences. “The models we have do not currently have any way to figure out where the big ice sheets end and where the ice calves off to form icebergs.”
Because of this, the scientists searched for the most important variable that determines where an ice shelf will break off into the ocean - not an easy task according to professor Alley.
“Fracture-mechanics problems are invariably difficult,” he says.
“Earthquake prediction comes to mind, or guessing whether a tea cup pushed off the table will break or bounce upon hitting the floor. With the tea cup, a drop from 1 mm high won’t break it, and a drop from 100 m almost surely will — one term, the height of the drop, explains a whole lot of the behavior.” he added.
“Our hope was to find such a dominant term in calving of bergs from ice shelves.”
Simple Law for Ice Shelf Calving
The scientists believe they have found a dominant term that can be used to forecast ice shelf calving.
In the tea cup example, the height of the tea cup was the dominant term. With ice shelves, the scientists found that the spreading tendency in the direction of ice and berg motion was the dominant term.
The equation is the rate of spreading times the width of the shelf times thickness multiplied by a constant.
In other words, it is the rate at which ice shelves spread that is the most important variable that determines when an ice shelve is about to calve.
“The spreading rate can be calculated from ice thickness and a few other things that are already solved for in numerical models, so we have provided a practicable calving law,” said Alley. “At present, models rarely if ever calculate physically where the ice ends, instead stopping the model before the ice ends or using some other relation that is not fully physical.”
Global Warming
Armed with this knowledge, scientists will be in a better position to forecast the impact of global warming on sea levels.
Computer models will be able to use this information to better predict how ice sheets will behave in warmer temperatures.
Scientists recently predicted that sea temperatures would rise by over a meter by the end of this century.
