(CN) – Upside-down "rivers" of warm ocean water are eroding the fractured edges of thick, floating Antarctic ice shelves from below, helping create conditions that lead to ice-shelf breakup and rising sea levels, according to a new study.
Findings published Wednesday in Science Advances describe a process that affects Antarctica's ice and the continent's contribution to rising seas. Models and forecasts do not yet account for the newly understood and troubling phenomenon.
"Warm water circulation is attacking the undersides of these ice shelves at their most vulnerable points," said the study’s co-author Karen Alley, a visiting assistant professor of earth sciences at The College of Wooster in Ohio.
"These effects matter," she said. "But exactly how much, we don't yet know. We need to."
Ice shelves float out on the ocean at the edges of land-based ice sheets, extensions which surround about three-quarters of the Antarctic continent.
The shelves can be hemmed in by canyon-like walls and bumps in the ocean floor. When restrained by these bedrock obstructions, ice shelves slow down the flow of ice from the interior of the continent toward the ocean. But if an ice shelf retreats or falls apart, ice on land flows much more quickly into the ocean, increasing rates of sea-level rise.
The scientists' work focuses on two factors conspiring to weaken ice shelves. First, flowing ice often stretches and cracks along its edges or "shear margins," especially when it is flowing quickly, Alley said, adding that crevasses are abundant and easy to spot in satellite imagery.
As those craggy features flow toward the ocean and become part of floating ice shelves, they are vulnerable to erosion from below by warm plumes of ocean water, the researchers reported.
Warm and fresh water is more buoyant than cold and salty water, so it tends to find high spots in floating ice, sometimes forming a type of upside-down river that can grow miles wide and tens of miles long.
Alley and her colleagues first mapped those rivers or "basal channels" a few years ago, spotting them as wrinkles or sags in otherwise smooth ice surfaces.
Now, they have put it all together, showing that large basal channels are more likely to form at the shear margins – the weakest parts – of fast-flowing ice shelves.
While the ice is still on land, large troughs form in the shear margins, becoming thin spots when the ice flows onto the ocean. Warm ocean water finds those thin spots along the base of the ice shelf, further eroding and weakening margins, making ice shelves more vulnerable to retreat and collapse.
In the past, researchers did not know that warm plumes were so common beneath ice-shelf margins. Alley's team used satellite imagery to show that, at the ends of shear margins on many of Antarctica's fastest-changing glaciers, warm water rises to the surface, melting sea ice and forming areas of open water called "polynyas."
The study found these polynyas forming year after year in the same spots, which means that warm water is indeed channelizing beneath thin, weak ice-shelf shear margins.
These processes appear to happen on ice shelves in both Antarctica and Greenland, Alley said, although the new work focuses on Antarctic glaciers.
The research team published earlier work focused on the damaging effects of meltwater on the surface of the ice shelves.
"Now we're seeing a new process, where warm water cuts into the shelf from below," said co-author Ted Scambos, a researcher at the University of Colorado Boulder. "Like scoring a plate of glass, the trough renders the shelf weak, and in a few decades, it's gone, freeing the ice sheet to ride out faster into the ocean."
Scambos and Alley are heading back to Antarctica this fall, to continue work on the continent's ice dynamics. Scambos is a lead scientist in the International Thwaites Glacier Collaborative.
Richard Alley of Penn State University and Nick Holschuh of the University of Washington also co-authored the study. The work was funded by the U.S. Geological Survey and the National Science Foundation.
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