There’s no place on Earth that’s changing faster – and no place where that change matters more – than Greenland. Late last month, NASA reported that ice all across the vast glacial interior of the world’s largest island was melting – a “freak event” that hadn’t occurred for at least 150 years. The alarming discovery briefly focused the media’s attention on a place that rarely makes headlines. RAPID ICE MELT BAFFLES SCIENTISTS, The Wall Street Journal declared.

In fact, scientists weren’t baffled at all – a paper published just weeks before had predicted that an abrupt, islandwide melt was imminent. The rapid loss of ice is only the latest in a chain of events that have upended conventional understanding of how the Earth’s “cryosphere” – its frozen places – behave. Taken together, the events offer new insight into how fast the world’s seas are likely to rise as a result of global warming – and hence, the fate of major cities like New York and Miami and Mumbai.

Jason Box, a scientist at the Byrd Polar Research Center, has probably spent more time in Greenland than any American of his generation. He began his yearly treks to the island in the 1990s as an undergraduate at the University of Colorado, helping his professor install a series of automated weather stations; last month he was sleeping on a sailboat near the mouth of a huge glacier and traveling onto the ice by helicopter to install yet more sensors. The shift he and his team have measured over the course of the past two decades is startling. “When I took my first course in glaciology,” Box says, “conventional thought had the reaction time of the ice sheets to heating on the order of 10,000 years.” The ice sheet, scientists believed, was a mostly inert ice cube frozen fast at its bed; if the glaciers melted because of global warming, the process would be, well, glacial.

But in a series of scientific epiphanies beginning in 2002, researchers using GPS have found that melting on the ice’s surface can cause large sections of the ice sheet to break free of its moorings in hours, not millennia. In 2006, scientists discovered that ice was suddenly pouring into the ocean at twice the rate previously measured, spurred by a pulse of warm ocean temperatures that undercut the glaciers from below. In two separate instances, Box correctly predicted which sections of a glacier would soon break off – sections, in each case, that were many times larger than the island of Manhattan.

But Box’s most crucial contribution to ice science – and the scariest part of his new findings – involves his measurement of Greenland’s reflectivity, or “albedo.” We know that snow is white: When sunlight hits a glacier, most of it bounces back into space, instead of being absorbed by dark-blue oceans or green forests. But not all ice shines with the same brightness. As snow crystals warm – even before they melt – they lose their jagged edges and become rounder, reflecting less light. “You can see it with your naked eye,” says Box. “Think of the way wet sand is darker than dry sand.”

Fresh snow bounces back 84 percent of the light that hits it; warm, rounded crystals can reflect as little as 70 percent. Slushy snow saturated by water – which gives it a gray cast, or even a bluish tint – reflects as little as 60 percent. Add dust or soot, and the albedo drops below 40 percent. Box’s satellite data has shown a steady darkening in Greenland’s albedo, from a July average of 74 percent when the century began to about 68 percent last year.

And then came this summer: Without warning, the line on the albedo chart dropped deep into uncharted territory. At certain altitudes, the ice sheet in Greenland was suddenly four percent less reflective – in a single season. “I confess my heart skipped a beat when I saw how steep the drop was,” says Box. “I thought it meant the satellite sensor might have degraded.” Instead, weeks of “ground-truthing” – going out on the ice to gather data from imbedded sensors and inspect conditions – verified that it was the ice sheet itself that was darkening. The heat accumulating in the ice sheet year after warm, sunny year was suddenly making it far easier to melt the surface. What’s more, in a vicious feedback loop, soot from the wildfires raging in Colorado and Siberia – themselves spurred by climate change – may be helping to darken the surface of the ice. (Box hasn’t been able to raise the funds to send a graduate student to do the sampling that would provide a definitive answer.)

Box had conservatively predicted that it might take up to a decade before the surface of Greenland’s ice sheet melted all at once. That it actually happened in just a few weeks only underscores how consistently cautious ice scientists have been in forecasting the threat posed by global warming. Now, however, that caution is being replaced by well-founded alarm. “Greenland is a sleeping giant that’s waking,” says Box. “In this new climate, the ice sheet is going to keep shrinking – the only question is how fast.”

The new data from Greenland matters for every corner of the planet. Water pouring into the North Atlantic will not only raise sea levels, but is also likely to modify weather patterns. “If the world allows a substantial fraction of the Greenland ice sheet to disintegrate, all hell breaks loose for eastern North America and Europe,” says NASA’s James Hansen, the world’s foremost climatologist.

But the future, pressing as it is, sometimes gives way to sheer awe at the scale of what we’ve already done. Simply by changing the albedo of the Greenland ice sheet, Box calculates, the island now absorbs more extra energy each summer than the U.S. consumes in a year. The shape and color of the ice sheet’s crystals, in other words, are trapping more of the sun’s rays than all the cars and factories and furnaces produce in the world’s biggest economy. One of Box’s collaborators, photographer James Balog, puts it like this: “Working in Greenland these past years has left me with a profound feeling of being in the middle of a decisive historic moment – the kind of moment, at least in geologic terms, that marks the grand tidal changes of history.” Amid this summer’s drama of drought, fire and record heat, the planet’s destiny may have been revealed, in a single season, by the quiet metamorphosis of a silent, empty sheet of ice.

This story is from the August 30th, 2012 issue of Rolling Stone.