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We try an alternate route, following a ridge of ice that parallels the river. This time we make good headway and march across the ice sheet for miles. We can't find the moulin on foot, but we make an intriguing observation: On the journey out, the holes we were jumping were separate, circular bowls, but on the way back, just half a day later, there's been enough melting so that the holes are connected by swift-running creeks.

At camp that night we find out what Tedesco and Steiner have confirmed about the bottom of the meltwater lake. It is mottled with cryoconite.

Cryoconite begins as airborne sediment spread over the ice by wind. It is composed of mineral dust sucked up from as far away as Central Asian deserts, particles from volcanic eruptions, and soot. The soot particles come from fires both natural and man-made, diesel engines, and coal-fired power plants. Cryoconite is not a new phenomenon: Arctic explorer Nils A. E. Nordenskiöld discovered and named the fine brown silt during his visit to the Greenland ice sheet in 1870. Human activities have increased the amount of black soot in cryoconite since Nordenskiöld's day, and global warming has given it new importance.

Carl Egede Bøggild is a native Greenlander and geophysicist who has spent the past 28 years studying the ice sheet. Recently Bøggild has focused on cryoconite. "Even though cryoconite is composed of less than 5 percent soot," he says, "it is the soot that causes it to turn black." The darkness decreases the albedo, or reflectivity, of the ice, which increases the absorption of heat; that in turn increases the amount of melting.

Snow falls each year on the ice sheet along with a dusting of cryoconite. As each year's snow cover hardens, it traps the dust. When summers are particularly warm, as they have been in recent years, multiple layers of ice melt, releasing extra amounts of trapped cryoconite, which creates a more concentrated, darker layer of the substance at the surface. "What we have is a vicious, constantly accelerating cycle," Bøggild says. "It's like pulling a black curtain over the ice."

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