The Greenland ice sheet is melting, pushing more water out into the ocean, and contributing to global sea level rise—one of the major risks of climate change.
Now, NASA scientists have found that pockets of liquid water trapped inside granular snow can drain into the ocean, which is an important piece of information for modelling how we predict future sea level rise.
Greenland's thick ice sheet isn't really solid ice. It contains pockets of liquid water, lead author Kristin Poinar, a postdoctoral fellow at NASA's Goddard Space Flight Center in Maryland, told me over the phone. These are called "firn aquifers," and occur when surface-level glacier ice melts in the summer, then keeps its liquid state year-round—thanks to snow that creates a blanket on top of it, trapping it beneath the surface, Poinar said.
The total volume of all this water is probably close to the volume of Lake Tahoe, she said, representing about 140 cubic km.
"We weren't sure what was happening before in these reservoirs," she said. "We knew from previous NASA data that there was a large volume of liquid water [trapped] inside some locations of the ice sheet," but it wasn't clear whether that water stayed there, or found its way out to the global ocean.
The recently published paper marks the first time scientists have examined what happens to water entrapped in firn aquifers. Measurements were collected using airborne radars flown on airplanes as part of NASA's Operation IceBridge.
To measure the movement of water, the team studied the crevasses, which Poinar said were about the width of a helicopter, or about 10 meters.
Poinar and others at NASA saw three possibilities: all this water locked beneath the surface might eventually freeze, stay where it was in liquid form, or bore down through a crevasse and reach the ocean (thereby pushing sea levels higher). Their research on firn aquifers in Southeast Greenland, published in Frontiers in Earth Science, shows that the water in the firn aquifer they studied did eventually reach the ocean.
While these reservoirs also exist outside of Greenland—in the Arctic and in mountain glaciers around the world—Poinar described Greenland as "more of a wildcard" in terms of how its melting ice could ultimately contribute to sea level rise, so understanding these firn aquifers is very important. Future studies will be needed to determine their impact on rising sea levels, Poinar said.
This water could also be lubricating the ice sheet beneath the ice and facilitating its flow to the ocean, she said, although this has yet to be studied in detail. "In some cases, you can make the ice flow faster, and therefore contribute more to sea level [rise]."
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