The accelerating pace of climate warming in the earth’s polar regions is spurring a new sense of scientific urgency. This past February 28 a camera onboard the NASA satellite Aqua caught a Manhattan-size floating piece of ice shelf in the act of disintegrating. Slabs continued to calve and break up throughout the next 10 days; by March 8 the Wilkins ice shelf, comprising some 5,000 square miles of floating ice off the coast of the Antarctic Peninsula, had lost 160 square miles of ice to the Pacific Ocean.

\The breakup is the latest of seven major Antarctic ice-shelf collapses in the past 30 years, after some 400 years of relative stability. They include the detachment of a 1,300-square-mile chunk from the Lars­en B ice shelf, the disintegration of giant ice shelves in the Prince Gustav Channel and the Larsen Inlet, and the disappearance of ice shelves known as Jones, Larsen A, Muller and Wordie. All of them corroborate temperature measurements showing that the western Antarctic Peninsula—now known to insiders as the Banana Belt—is warming up faster than anyplace else on earth.

The Wilkins event—serendipitously caught on video by a team from the British Antarctic Survey just days after it was discovered—has rallied scientists around the world. “You have closer communication than ever among the global science community now,” says Robin E. Bell, a polar research scientist at the Lamont-Doherty Earth Observatory of Columbia Uni­versity. “We’re more sensitive that change is really happening quickly.” Relatively warm air seems to be the main culprit. As ice melts in the austral summer, pools of water fill the cracks that inevitably develop in any floating ice shelf as a result of bending and squeezing by the surrounding ocean. In a colder climate those fractures would be nothing more than shallow surface scars. But liquid water in the cracks can drill like a hot knife to the base of an ice shelf, snapping it in two.

The breakup and melting of floating ice has no direct effect on global sea levels. But an ice shelf is thought to act as a “cork in the bottle,” damming the flow of the land-based glacier that slowly feeds the shelf in the sea. When such a “cork” is removed, the glacier lurches forward. “Within a few months” of a breakup, explains glaciologist Ted Scambos of the National Snow and Ice Data Center at the University of Colorado at Boulder, the glacier “accelerates significantly, and within a year or two, it can be moving [toward the ocean] up to four times as fast as it moved when the ice shelf was intact.” As Bell puts it, the result is that “more ice cubes get into the ocean,” which does raise sea level.

In the near term, however, the biggest concerns are the changes in the north: the declines of Arctic sea ice and the Greenland ice sheet. Warm air and surface water are melting the summer polar ice cap. The shrinking sea ice drives a classic positive feedback loop: as more ice melts, fewer patches of white snow reflect solar energy, and larger regions of dark, sunlight-absorbing seawater open up—both causing the ice to melt even faster. That runaway effect, Scambos says, could quickly lead to a warmer climate along the Arctic perimeter and to the loss of Arctic permafrost.

In Greenland the story of not so glacial changes in the outlet glaciers is much the same. Their seaward edges are speeding up, and the ice sheet behind them is thinning. Measurements of local gravitational anomalies by the GRACE satellites show that the Greenland ice sheet, particularly in its southern reaches, is rapidly losing mass. “The ice sheet is on a diet,” Bell says. A lot of Greenland ice is slipping into the Atlantic Ocean.

Do all those effects add up to a tipping point? No one really knows. Investigators are anxiously seeking the answers to two great unknowns about the changes in polar ice. How fast will the ice sheets continue to slide into the sea, and how much more warming will it take to melt the Arctic permafrost? If the permafrost melts, prodigious amounts of trapped methane gas will burp out of the once frozen ground. Twenty years after such a release, methane is 72 times more potent than carbon dioxide (CO2) as a greenhouse gas (after 100 years it remains 25 times more potent than CO2), so if the methane is released, the planet risks a runaway climate catastrophe.

Note: This story was originally printed with the title, "Polar Express".