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Going with the Flow: Predicting Future Sea Level Rise

Going-with-the-Flow

Icebergs in the Jakobshavn Ice Fjord in western Greenland. With no sense of scale, it’s hard to believe; but some of the ice blocks here are as big as battle ships. Click on the photo to see a video of icebergs calving off the Greenland Ice Sheet into the fjord.

photo by Daniel Grossman

Because polar ice sheets are so massive, the rate at which they melt has serious implications fpr future sea level rise.  If ice sheets behaved like an ice cube dropped out of a freezer tray on a summer day, predicting how fast they’d waste away—and how fast sea level would rise—would be relatively easy.  Ice cubes melt from the outside inward. As the exterior dribbles off, inner ice appears in an orderly fashion, like layers of an onion. To predict the fate of ice melting this way requires taking into account factors like ambient temperature and the movement of air currents. Scientists know how to perform such calculations for ice cubes as well as for ice sheets. They’ve estimated, roughly, that if all the world’s glaciers melted this way, sea level would rise by about 15 inches by the end of the century. Sea level rise of this magnitude can’t be ignored, but it’s relatively small, considering that it would occur over many decades.

But, while behaving in part like ice cubes, ice sheets also waste away in a manner unlike any ice cube: from the inside out. Frozen ice streams convey an ice sheet’s bulk from it’s interior, at the highest elevations to it’s lower-elevation perimeter, sometimes hundreds of miles away.  At the edge of an ice sheet, great blocks shear off and fall into the sea. It’s possible that an ice sheet could lose volume much faster by this process than by surface melting.

In the late 1990s, ice flowing from the Jakobshavn Glacier on Greenland’s west coast sped up dramatically, startling glacier researchers and increasing concern that global warming could cause ice sheets to disintegrate rapidly. Already one of the world’s fastest moving glaciers, Jakobshavn doubled the speed of ice flowing into the sea. Before it slowed back down, the glacier had expelled 20 billion tons of icebergs a year into the Atlantic.

Scientists don’t know why glaciers like Jakobshavn speed up at times or slow down. They don’t know what controls an ice stream’s speed or how much ice such streams could dump into the sea. However, many researchers believe the removal of solid ice in such streams could dwarf the quantity of water that melts on the surface.  Until these questions are resolved some researchers consider it possible that sea level rise later this century could be measured in many feet, not inches.