Greenland’s 3-million-year-old ice is disappearing, with extreme melt events that even include rain at the highest peak on the island. The 1 millimeter the meltwater adds to sea level every year makes it “by far the most rapidly increasing contribution” to sea level rise, said Poul Christoffersen, a professor of glaciology at the University of Cambridge.
Christoffersen is part of a research team that investigated what happens when meltwater drains through fractures in the ice and falls a kilometer or more to the bottom of the ice sheet. “We haven’t really understood the overall implication of this transfer of what ultimately are huge volumes of meltwater,” said Christoffersen.
To better investigate those implications, Christoffersen and his colleagues installed an autonomous phase-sensitive radio echo sounding (ApRES) instrument 30 kilometers inland on Sermeq Kujalleq (Store Glacier) and drilled a borehole alongside it. Using data collected from both the radar and the borehole, they tracked the water underneath the ice sheet and how it interacts with the ice above. The research was published in the Proceedings of the National Academy of Sciences of the United States of America.
They found melt rates on par with those measured at the surface and average melt rates of 14 millimeters per day. The most extreme measurement was 57 millimeters, recorded during a rainfall event when temperatures in the borehole hit 0.88℃. During that event, said Christoffersen, “82 million cubic meters of water were delivered to the base of this glacier in a single day.”
The researchers claimed that the energy generated by the meltwater’s dramatic drop is on par with the amount of energy produced by the world’s largest hydropower stations. This energy contributes significantly to the melt rate experienced by the glacier or ice sheet. In fact, Christoffersen said, the melt rates are 100 times higher than when the process isn’t included in models of sea level rise.
The idea of that amount of energy being generated at the bottom of an ice sheet was initially met with skepticism, said Christoffersen. “It’s just that we forgot to calculate the potential energy of that elevation drop,” he explained.
It’s generally been accepted that water forms channels as it flows through glaciers (the Röthlisberger theory), but Christoffersen and his colleagues observed a hydrological system spread over a wide area of the glacier’s bed. Even though the water heats as it travels, instead of forming channels it spreads out and heats the ice over a larger area. This mechanism could cause the ice sheet to leave a thawed bed with a small frozen core at its center, said Christoffersen. That pattern would result in faster ice flow, more ice displacement, and cracks and fractures at the surface. “It’s a super powerful mechanism that’s going to alter the thermal state of the interior of the ice sheet as we get meltwater at higher and higher elevations,” he said.
Christoffersen pointed out that the mechanism is applicable only to Greenland’s subglacial drainage areas and not the entire ice sheet. It’s also less important for smaller glaciers and ice sheets with thicknesses of less than a kilometer, he said.
The study marked the first time ApRES radar, which was developed for use on floating ice shelves, was deployed inland. This use of the instrument, which has millimeter-range precision, was interesting, said Christian Schoof, a professor at the University of British Columbia who wasn’t involved in the study.
“It would be really interesting to see a study like this done where you take not one of these instruments, but you put up an array of them, and you actually get an idea of how spatially variable this melt rate is as well,” he said.
—Danielle Beurteaux (@daniellebeurt), Science Writer