BENEATH THEIR FEET When a multidisiplinary team of hydrologists, climate scientists and modelers led by UCLA geographer and geologist Laurence C. Smith compared model predictions with field measurements of a river on the surface of the Greenland Ice Sheet, they found that the models overestimated runoff by 21 to 58 percent. That's a huge discrepancy, and it raises big questions about what's missing from the models. Matthew Cooper, a UCLA graduate student who conducted fieldwork for the study, realized the answer could be right at their feet. Literally. "The so-called 'solid ice' isn't solid like an ice cube, it's decomposed and rotten," he explains. "It's not until you're standing right on top of the ice and looking down that you see that the entire surface of the ice is covered with these cryoconite holes, from as small as a pencil to as large as a meter across, all filled with water." Cryoconite holes form as sunlight penetrates the ice in the ablation zone, the region where Cooper and his colleagues worked. The bare ablation zone lies below the elevations covered in snow and crusty firn, but above the edge of the ice, where melting is so rapid that the surface of the glacier is dense and polished. In the ablation zone, sun penetrates the ice to a depth of a couple of meters, raising the temperature above the freezing point. The upper layer melts in a pattern of vertical shafts, forming cryoconite holes an average of 25 centimeters (10 inches) deep that fill about halfway with water. Below the holes, to a depth of 1 to 1.5 meters (40 to 60 inches), the ice is porous and weathered. "That's a pretty small amount of water, but scaled up across the whole surface area, that can add up to a non- trivial volume," Cooper says. In fact, he and his colleagues estimated the volume of water held in the porous ice of the small catchment they studied and calculated that it represented 10 million metric tons (8,266 acre-feet) of meltwater. So while models assume that meltwater flows across the glacier's surface like a rivulet across a sheet of bedrock, glacial meltwater in the ablation zone may actually travel more like groundwater in the shallow soils of mountain watersheds—held in pockets and pores, transiting slowly towards streams. "We don't actually demonstrate that yet," Cooper notes. "We haven't quantitatively tied it to the numbers in Larry [Smith]'s study, but there are promising links." Complicating the reality even further, Cooper and the team also noticed that the upper centimeter or so of the water in the cryoconite holes froze every night and melted in the morning. "There's a substantial amount of energy that goes into melting that frozen water every day," says Cooper. "Some energy has to be allocated [in the model] to melt that ice. If the models don't simulate the freezing at night, they aren't seeing that energy." Cryoconite Holes − Courtesy: Matt Cooper Geologists, climate scientists and modelers teamed up to ground-truth models in Greenland. Photo: Lincoln Pitcher 29 FEATURE STORY Who’s Minding the Planet?