OVER THE PAST several months, scientists working in Antarctica have been watching—with a mixture of professional fascination and personal horror—a fissure growing in the continent’s fourth-largest ice shelf. Since last November, the crack has lengthened by some 90 miles. It has 13 miles more before it rends completely, and a chunk of ice the size of Delaware goes bobbing into the Weddell Sea. The calving chunk could be a sign that the entire Larsen C ice shelf—nearly twice the size of Massachusetts—is breaking apart.