Rumblings in the Ice

Glaciologists have long been aware that ice sheets do change; investigators simply assumed that such changes were gradual, the kind you infer from carbon 14 dating - not the kind, such as the breakup of the Larsen B ice shelf, that you can mark on an ordinary calendar. In the idealized view, an ice sheet accumulates snow - originating primarily in evaporated seawater - at its center and sheds a roughly equal mass to the ocean at its perimeter by melting and calving icebergs. In Antarctica, for instance, some 90 percent of the ice that reaches the sea is carried there by ice streams, giant conveyor belts of ice as thick as the surrounding sheet (between 3,500 and 6,500 feet) and 60 miles wide, extending more than 500 miles "upstream" from the sea. Ice streams moving through an ice sheet leave crevasses at their sides as they lurch forward. Near the seaward margins of the ice sheet, ice streams typically move between 650 and 3,500 feet a year; the surrounding sheet hardly moves at all.

But long-term ice equilibrium is an idealization; real ice sheets are not permanent fixtures on our planet. For example, ice-core studies suggest the Greenland ice sheet was smaller in the distant past than it is today, particularly during the most recent interglacial period, 120,000 years ago, when global temperatures were warm. In 2007 Eske Willerslev of the University of Copenhagen led an international team to search for evidence of ancient ecosystems, preserved in DNA from the base of the ice sheet. His group's findings revealed a Greenland that was covered with conifers as recently as 400,000 years ago and alive with invertebrates such as beetles and butterflies. In short, when global temperatures have warmed, the Greenland ice sheet has shrunk.

Today the snowfall on top of the Greenland ice cap is actually increasing, presumably because of changing climatic patterns. Yet the mass losses at its edges are big enough to tip the scales to a net decline. The elevation of the edges of the ice sheet is rapidly declining, and satellite measurements of small variations in the force of gravity also confirm that the sheet margins are losing mass. Velocity measurements indicate that the major outlet glaciers - ice streams bounded by mountains—are accelerating rapidly toward the sea, particularly in the south. The rumblings of glacial earthquakes have become increasingly frequent along the ice sheet's outlet glaciers.

Like the Greenland ice sheet, the West Antarctic ice sheet is also losing mass. And like the Greenland ice sheet, it disappeared in the geologically recent past - and, presumably, could do so again. Reed P. Scherer of Northern Illinois University discovered marine micro-fossils at the base of a borehole in the West Antarctic ice sheet that only form in open marine conditions. The age of the fossils showed that open-water life-forms might have lived there as recently as 400,000 years ago. Their presence implies that the West Antarctic ice sheet must have disappeared during that time.

Only the ice sheet in East Antarctica has persisted through the earth's temperature fluctuations of the past 30 million years. That makes it by far the oldest and most stable of the ice sheets. It is also the largest. In many places its ice is more than two miles thick, and its volume is roughly 10 times that of the ice sheet in Greenland. It first formed as Antarctica drew apart from South America some 35 million years ago and global levels of carbon dioxide declined. The East Antarctic ice sheet appears to be growing slightly in the interior, but observers have detected some localized losses in ice mass along the margins.