By Kevin Krajick
As climate warms, the surface of the Greenland ice sheet is melting, and all that meltwater ends up in seasonal rivers that flow to the sea. At least that is what scientists have assumed until now. Now, a new study has shown that some of the meltwater is actually being soaked into porous subsurface ice and held there, at least temporarily. The finding is part of the first comprehensive on-the-ground study of such a river system. The research could alter calculations of how ongoing melting might fuel global sea-level rise.
Declining ice sheets have been playing an growing role in sea-level rise in recent years. Satellite measurements show that Greenland is losing an average of 260 billion tons of ice a year to the ocean. Something less than half of that appears to come from icebergs falling off the end of the sheet into the sea; the rest is presumably meltwater. But it is not clear exactly how much meltwater actually reaches the sea and where it comes from, because most of the plumbing system is hidden under ice. Lacking direct measurements, computer models have generally used the assumption that meltwater just flows directly out. “It’s always treated as a parking lot, water runs straight off,” said Laurence Smith, a geographer at the University of California, Los Angeles who led the new study.
The study indicates that instead, “the ice acts as a sort of sponge, in which part of the water is trapped,” said Marco Tedesco, a glaciologist at Columbia University’s Lamont-Doherty Earth Observatory and coauthor of the paper. “Instead of behaving like a rigid slab, the ice can rot, creating pores and interstices that are filled with water.” Writing in the Italian newspaper La Repubblica, Tedesco said, “The inclusion of this phenomenon reduces discrepancies between satellite data and model outcomes and may alter Greenland’s projected contributions to sea-level rise.” Tedesco said that meltwater sponged up by various parts of the sheet will eventually make its way to the sea, but the timetable is unknown.
The scientists gathered their data in summer 2015 from a 27-square mile watershed on the ice surface, consisting of a complex system of lakes, tributary creeks and a substantial river. The river culminated at a moulin–a giant hole where the water plunged down, presumably all the way to the rocky bed of the ice. The team mapped the watershed with a drone. Then, anchored to icy stream banks to avoid a fatal fall in, team members took turns lowering instruments down to take hourly readings of the water’s volume, velocity, temperature and depth. They also dropped free-floating sacrificial instruments into the current that took continuous readings until they were swept into the depths of the moulin. The work was documented in a multimedia package by The New York Times.
The scientists determined that up to 430,000 gallons per minute flowed into the moulin. But this did not account for all the water being fed by the watershed. The rest, they concluded, was being soaked into cracks and pore spaces of so-called rotten ice along the route, and probably stored there. Previous models have assumed that there was about 20 to 60 percent more runoff than what the scientists actually measured.
“After eliminating all the other possibilities, we deduced that the disagreement in our data could be attributed to subsurface melting and meltwater storage, said coauthor Dirk van As, a researcher at the Geological Survey of Denmark and Greenland.
“If there’s a mismatch between observation and model, that means the model is moving the mass in one way or another and not respecting the way things happen in the real world,” said Tedesco.
Tedesco and colleagues plan to return to Greenland this summer to investigate other phenomena, including the widespread growth of algae within the ice that is darkening the near-surface and apparently increasing the ice’s uptake of solar energy. This in turn may be accelerating melting in many areas.