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.
How soon could that happen? When it comes to understanding the implications of ice-sheet collapse, the speed of that breakdown is everything. It could mean sea levels that rise slowly and steadily, perhaps a foot or two per century, which might allow coastal communities to adapt and adjust. Or it could mean levels that rise at an accelerating pace, perhaps five feet or more per century — forcing the evacuation of the earth’s great coastal cities and producing millions of refugees and almost unimaginable financial costs. The difference between slowly and rapidly is a crucial distinction that one scientist recently described to me as ‘‘the trillion-dollar question.’’