At one point several hundred thousand years ago, snow began falling over the center of the earth’s largest island. The snow did not melt, and in the years that followed, storms brought even more. All around Greenland, the arctic temperatures remained low enough for the snow to last past spring and summer. It piled up, year after year, century after century, millennium after millennium. Eventually, the snow became the Greenland ice sheet, a blanket of ice so huge that it covered 650,000 square miles and reached a thickness of 10,000 feet in places. Meanwhile, in Antarctica, a similar process was well underway. There, as snow fell upon snow for years without end, the ice sheet spread out over a much vaster area: 5.4 million square miles, an expanse far larger than the lower 48 states. By the start of the modern era, when power plants and electric lights began illuminating the streets of Manhattan, about 75 percent of the world’s freshwater had been frozen into the ice sheets that lay over these lands at opposite ends of the earth.
The ice sheets covering Greenland and large areas of Antarctica are now losing more ice every year than they gain from snowfall. The loss is evident in the rushing meltwater rivers, blue gashes that crisscross the ice surface in warmer months and drain the sheets’ mass by billions of tons annually. Another sign of imbalance is the number of immense icebergs that, with increasing regularity, cleave from the sheets and drop into the seas. In late August, for instance, a highly active glacier in Greenland named Jakobshavn calved one of the largest icebergs in its history, a chunk of ice about 4,600 feet thick and about five square miles in area.
If the ice sheets on Greenland and Antarctica were to collapse and melt entirely, the result would be a sea-level rise of 200 feet or so. This number, though fearsome, is not especially helpful to anyone but Hollywood screenwriters: No scientist believes that all that ice will slide into the oceans soon. During the last year, however, a small contingent of researchers has begun to consider whether sea-level-rise projections, increased by the recent activity of collapsing glaciers on the periphery of the ice sheets, point toward a potential catastrophe. It would not take 200 feet to drown New Orleans. Or New York. A mere five or 10 feet worth of sea-level rise due to icebergs, and a few powerful storm surges, would probably suffice.