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Terminal Ice (Cont.)

SOME PEOPLE HAVE jobs that involve thinking about ice and icebergs all day long. A while ago I went to the National Ice Center in Suitland, Maryland, and met a few of them. The Ice Center is affiliated with the National Oceanic and Atmospheric Administration, the Coast Guard, and the Navy. The offices of the Ice Center are in one of the many long, three-story government buildings with extra-large satellite dishes on their roofs in a fenced-in, campuslike setting just across the

"Did you hear what just happened to Larsen B? One day the shelf was there, the next day it started to break up. OVER A THOUSAND SQUARE MILES OF ICE—POOF. This major geographic feature of Antarctica ceased to exist."

Potomac from Washington. Antennae poking up from behind clumps of trees add to the spy-thriller atmosphere. The Ice Center's supervisor, a lieutenant commander in the Navy—many of the people who work at the Center are military personnel—introduced me to Judy Shaffier, ice analyst. She is a slim woman in her mid-thirties with a shaped haircut and avid dark eyes accustomed to spotting almost invisible details. An ice analyst looks at satellite images of ocean ice on a computer screen, compares the images to other weather information, and figures out what they mean. "It's a great job, really neat to tell people about at a party or something," Shaffier said. "But it takes explaining. When I say I analyze ice, sometimes people don't get that I mean ice. They think it must be one of those government-agency acronyms."

Much of the Ice Center is closed to visitors. That's the part where it pursues its main purpose, which is to provide classified information on ice conditions to the military. For example, a nuclear submarine can break through ice three feet thick or less; the Center can tell a submarine how close to surfaceable ice it is. Many countries—Japan, Denmark, Great Britain, Russia, France, Sweden—have ice-watching agencies similar to the Ice Center. Any country involved in global ocean shipping needs ice information sometimes. Providing it to merchant ships, scientific expeditions, and the general public is the nonclassified part of the Center's job.

Shaffier led me into a room with darkened windows and computers all along the walls. The glow of screens in the dimness lit the faces of ice analysts tapping on keyboards, summoning up satellite pictures of ice-covered oceans and seas. "I was trained as a meteorologist originally," Shaffier said. "I started analyzing ice in '94. Mostly what we do here is sea ice. Each of us has areas we concentrate on. Mine are the Yellow Sea, the Sea of Okhotsk, the Ross Sea, and the Sea of Japan. I feel like I really know the ice in those seas. Tracking icebergs is kind of secondary. We do it to keep everyone informed about possible dangers in the shipping lanes—in the Southern Hemisphere only, because another agency handles North Atlantic bergs. Also, I guess we do it for scientific and geographic reasons, or because icebergs are just interesting.

"For us to track an iceberg it must be at least ten nautical miles long," she continued. "We label each one according to the quadrant of Antarctica where it broke off. The quadrants are A through D, and after the letter we add a number that's based on how many other bergs from that quadrant we've tracked since we started doing this back in 1976. The A quadrant, between 90 west longitude and zero, has shelves that calve big icebergs all the time, and we've tracked a lot of bergs from there. A-38 was a recent one. And if a berg breaks up into pieces, any piece that's bigger than ten nautical miles gets its own label, like A-38A, A-38B, and so on."

The subject turned to giant "celebrity" icebergs, and whether she had a favorite.

"In March of 2000 I was sitting at this computer," she said. "Another analyst, Mary Keller, was sitting at the next one, and suddenly she said, ÔMy God! It's a huge iceberg!' A huge piece had broken off a shelf in the B quadrant since we'd last checked a day or so before. There had been no stress fractures visible in the ice sheet; the calving was completely unexpected. This iceberg was the 15th in B, so we labeled it B-15. I had never seen a berg that size. It was awesome—158 by 20 nautical miles. After it broke off it kind of ratcheted itself along the coast, sliding on each low tide, slowly moving from where it began, and in the process it eventually split into two pieces, one of about a hundred miles long and another of about 80. B-15 was the most exciting iceberg I've watched since I've been here."

Shaffier and I spent hours looking at computer images that hopscotched the icy places of the globe. Some of the pictures were visible-light photographs; some were made by infrared imagery that indicated different ice temperatures by color. Iceberg ice is 20 or 30 Celsius degrees colder than sea ice, old sea ice is a few degrees colder than new sea ice; in general, the colder the ice, the more difficult it is to navigate through. In passing, we checked up on B-15A—the giant was partly blocking the entry to West Antarctica's McMurdo Sound, apparently stuck on underwater rocks.

"Let's look at the Larsen Ice Shelf, or what's left of it," Shaffier said. "Did you hear what just happened to Larsen B? The Larsen Shelf is in a part of West Antarctica where local temperatures have gone up four or five degrees over the past decades, and a few years ago a big part of the shelf, Larsen A, disintegrated almost completely. People said that the rest of the shelf, Larsen B, would probably go in the next two years. Well, a few weeks ago, it went. Over a thousand square miles of ice—poof. One day the shelf was there, the next day it started to break up, and 35 days later the satellite images showed nothing but dark water and white fragments where solid white ice used to be. In about a month this major geographic feature of Antarctica ceased to exist."

Later, a glaciologist I talked to explained how the breakup of Larsen B probably occurred: Higher surface temperatures created pools of meltwater that accumulated on the ice during the summer months until the water flowed into ice crevices; once in the crevices, the water became a hydraulic wedge, forcing its greater weight down through the ice and cracking it apart. When pieces of the ice broke off, they pushed over the pieces next to them, like books falling on a shelf. The process had a swiftness and a magnitude glaciologists had never seen before, and it created the largest movement of ice in a single event in recent times. Unlike the calving of giant icebergs, the Larsen Shelf breakup occurred because of a sharp rise in local temperatures, which scientists believe was almost certainly the result of global warming.

"Actually, I've never seen an iceberg except in pictures," Shaffier told me. "The only ice on my computer screen I've ever seen in person was ice on Lake Erie near Brook Park, Ohio, where I used to live. Next fall I might get a place on the supply ship going down to McMurdo. I'd love to do that. I want to see the ice up close, but even more, I want to hear it. The images on my screen are silent, of course. But think about when an iceberg 200 miles long by 21 miles wide breaks off Antarctica. Think what that sound must be."

According to scientists, probably no one has ever heard that sound, or been present when a giant Antarctic iceberg calved. Whether that event is even accompanied by sound audible to humans, no one can yet say. Instruments that listen for underwater oceanic sounds sometimes pick up vibrations like a cello bow across strings—only much lower, below human hearing—which are believed to come from the friction of giant icebergs, though none of the vibrations have yet been matched to a specific calving. When giant icebergs run into something, however—when they collide with the ocean bottom or the land—they cause seismic tremors that register at listening stations halfway around the world.

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