Among the really, really bad possibilities with climate change is that as the world warms, ice sheets in Greenland and Antarctica will shrink at increasingly fast rates. That would be driven in large part by streams of ice carrying frozen water from a sheet's interior out to the sea at an ever faster speed. But a new study splashes a bit of cold water on those fears: As it turns out, ice streams on the ice sheet that once covered much of North America actually slowed down over time.
Durham University professor of geography Christopher Stokes and his colleagues caution that their results may or may not apply to modern glacial ice sheets. "However, contrary to the view that sees ice streams as unstable entities that can accelerate ice-sheet deglaciation, we conclude that ice streams exerted progressively less influence on ice sheet mass balance during the retreat of the Laurentide Ice Sheet," they write. That finding suggests modern ice streams in Antartica and Greenland might do the same, the team writes in Nature.
Similarities between the Laurentide Ice Sheet and Antarctica in particular suggest ice streams may not accelerate as others had thought.
In general, ice sheets are particularly fast-moving portions of a glacier, traveling roughly a kilometer per year, or several meters a day. (In contrast, Mount Rainier's Nisqually Glacier, which literally falls off the side of a cliff at one point, moves less than one meter per day.) In part because of their speed, ice streams are responsible for about 90 percent of the ice loss in Antarctica—making them a pretty important event in nature. A common view is that ice streams are accelerating, bringing us closer to collapsing glaciers and accompanying sea-level rise. But that view is based largely on evidence from just the last few decades, Stokes and his team argue.
Their new research began instead with an extensive database of ice streams on the Laurentide Ice Sheet, which peaked some tens of thousands of years ago but left behind clues in the landscape as to where, when, and how fast it traveled. Using that database in conjunction with data on how the Laurentide Ice Sheet shrank over time, the team could estimate how much ice was being lost over time.
Surprisingly, the number of ice streams and the amount of ice lost to them declined over time. From about 22,000 to 15,000 years ago, losses held steady at around 1,500 cubic kilometers of ice per year, roughly the amount it would take to cover Alaska with a one-meter thick layer of ice. But then, the losses dropped off. By about 9,000 years ago, the Laurentide Ice Sheet was losing less than 100 cubic kilometers per year.
Of course, very little remains of the Laurentide Ice Sheet today, and likewise the results don't mean that the ice of Greenland and Antarctica isn't shrinking—it clearly is. Instead, the researchers write, similarities between the Laurentide Ice Sheet and Antarctica in particular suggest ice streams may not accelerate as others had thought, and that researchers will need to understand ice streams better in order to understand what's next for the world's iciest places.
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