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Are glacial ice streams contributing to the rise of ocean levels?

A new study of an ancient ice sheet shows that even as the Earth warmed, the amount of ice lost through its ice streams decreased, which could mean ice streams pose less of a threat to ocean level rise than was previously thought.

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    Surface melt water rushes along the surface of the Greeland Ice Sheet through a supra-glacial stream channel to the southwest of Ilulissat, Greenland in 2012.
    Ian Joughin/AP/File
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A study released Wednesday in the journal Nature suggests that the recent thinning behavior of ice streams in some of the world's largest glaciers may not point to as grim a climate scenario as was once thought.

Ice streams, or fast-moving regions of massive ice sheets – glaciers larger than 19,000 square miles – have become a cause for concern in recent years as their retreat in the world's only major ice sheets in Greenland and Antarctic has been linked to sea level rise. But the new study may show that ice streams will have less of an impact on ice sheet deglaciation over time.

“Ice sheets and the global sea are locked in a tug of war for water in which climate change dictates which side gains or loses ground,” Jason Briner, an associate geology professor at the University at Buffalo, explained in a response to the research paper. “Mindful that tug-of-war contests often end with the catastrophic collapse of one side, climate scientists are deeply concerned about the manner in which ice sheets are currently declining.”

To reach conclusions about the behaviors of the world’s two contemporary ice sheets, the researchers analyzed 117 ice streams that appeared throughout the ancient Laurentide Ice Sheet, which melted during a period from around 22,000 to 7,000 years ago. That icy mass covered millions of square miles throughout most of the northern region of North America, about as much area as Antarctica occupies today.

While a complete collapse of a contemporary ice sheet would likely not occur for thousands of years, the recent uptick in ice-stream activity, potentially contributing to sea level rise and the destabilization of ice sheets, has driven fears of imminent danger. However, the study’s researchers are suggesting that ice stream activity may not be as detrimental to oceans as was once thought, “contrary to the view that sees ice streams as unstable entities that can accelerate ice-sheet deglaciation.”

“We find no evidence for major ice sheet instabilities linked to ice stream activity that is reflected in the spatial re-organization of their drainage network,” Durham University geology professor Chris Stokes wrote in the research paper about the Laurentide sheet. “Rather, we find that the overall number of ice streams decreased and they occupied a progressively smaller percentage of the ice sheet perimeter. This finding implies that the final 4–5 [thousand years] of deglaciation... was largely driven by surface melt.”

“Stokes and colleagues find that ice-stream activity decreased as the planet warmed,” Briner wrote in his piece.

This discovery paints a different picture than the one scientists previously had on what ice-stream activity means for the sheets and the surrounding water amid ongoing global warming. However, there are significant differences between the Laurentide’s surroundings and today’s that could make direct comparison of the two eras’ swiftly moving icy regions more difficult. Most significantly, the Laurentide retreated onto a surface of bedrock and its ice streams led to land, while the modern sheets end in the sea. Some of the current sheets should eventually occupy space on firm ground, but that contrast may mean that modern streams will behave differently than the Laurentide’s did.

Even so, the new research should establish a starting point for more informed analysis of the Earth’s ice sheets, despite the epochal differences. And the bigger variable affecting the sheets' deglaciation, surface melt, is still an issue today. But scientists will now be looking ahead to what ice streaming could mean for the world’s oceans in relation to its biggest glaciers.

“There is still a lot we don’t know about how these ice streams behave,” Briner said in a UB release on the study, adding that “understanding their behavior is crucial for accurate modeling of future ice sheet decline.”

While the study's findings should curb some concerns over ice sheet decline, other facets of worldwide ice activity in relation to climate change continue to draw scientists' attention. For example, recent measurements show that polar sea ice is at an all-time low despite a slowing in the ice loss process, and some researchers still fear that factors aside from ice streaming could be leading to the collapse of the ice sheets in Greenland and Antarctica.

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