Geologic change is usually measured in millimeters and takes place over millenniums. But Alaska's Bering Glacier, the largest on the North American continent, is fracturing, retreating and sometimes surging at record speeds.
Its skittishness may be a sign that the planet is in for a rough ride.
In the past few years, the Bering -- roughly the size of Delaware -- has triggered mammoth floods, carved away hillsides, bulldozed bird colonies and calved icebergs as large as small islands. The cause of all this instability is simple: This enormous glacier is shrinking.
The Bering Glacier is 140 miles long, an average of about 10 miles wide and a half-mile deep.
Beginning at an altitude of 18,000 feet in the St. Elias Mountains, the glacier covers about 2,000 square miles and contains 1,000 cubic miles of ice. But during this century, the Bering has lost about 3 percent of its surface size, and about 600 feet in thickness near its terminus.
Most of the world's subpolar glaciers have been dwindling since about the turn of the century. And some climatologists and geologists think the thawing is another important signal that Earth's climate is becoming warmer.
Off and on since 1974, Bruce F. Molnia of the U.S. Geological Survey has been flying a stretch of the Alaskan coast to study the restless Bering.
Molnia and his colleagues have found evidence that about 5,000 years ago, the Bering was much smaller, with its terminus many miles from the Pacific Ocean.
Then, more snow began to fall on the ice field in winter than melted in the summer, causing the glacier to inch down fjords until it reached the coastline and its maximum size in about 1100 A.D.
For the next 800 years, the glacier slumbered, neither advancing nor retreating. Then in 1900, something happened. The summer thaw began to claim more of the behemoth's annual coat of snow. The 2,000-square-mile block of ice began to shrink, and gradually lost 600 feet of thickness in its pancake-shaped lower end.
When growing, a glacier moves slowly but inexorably -- advancing as its tremendous weight forces the ice crystals to deform, causing the seemingly rigid ice to flow like a block of extremely dense syrup.
When shrinking, it becomes far more active. It can gush rivers of water, splinter into icebergs at its terminus, retreat up the path it scours through the terrain and then slide back down in what is called a surge.
In April 1993, the Bering began what was probably its fourth or fifth surge in this century, advancing an average of a half-mile a month.
For two years it slid along, at one point creeping at a rate of more than 300 feet per day. (While a giant tortoise walks about 100 times faster, this is lightning speed for a block of ice larger than the Great Salt Lake.)
White thunder -- the sound of the ice breaking up -- echoed across the landscape. Water roared off the glacier in a single stream, carrying far more water than Niagara Falls. "It's a deafening roar," Molnia says. "There were these explosive cannon shots where big blocks of ice were being thrown off the glacier."
"Imagine a wall of ice [nearly] stretching from Baltimore to New York City, 10 miles wide and a half a mile thick," he says. Now imagine it moving a third of a city block a day as pieces of its terminus break up in very slow motion.
"It was one of the more spectacular glaciological events of this century," he says.
A surge occurs when meltwater becomes trapped under the glacier and in hidden pools and lakes. As the water rises, it presses against the ice above. At a critical point, it overcomes friction and gravity takes over, causing the glacier to hydroplane along its own base. In the Bering's case, water collected under its enormous belly to an average depth of about 3 1/2 feet before the glacier began to slip and slide.
A surging glacier can do serious damage. During its recent rampages, the Bering bulldozed through one of the largest rookeries in the central gulf region of Alaska, on islands in Vitus Lake. It pushed over the eggs of thousands of birds, including a population of threatened Dusky Canada geese. A burst ice dam flooded the land to the west, washing out a valley that teemed with moose and other wildlife.
Humans, though, weren't threatened; the nearest town is about 80 miles away. The only potential threats to safety are the huge icebergs that calve off the face of the glacier. For now, they are about as dangerous as rubber ducks in a bathtub, because they are trapped in Vitus Lake.
The lake is separated from the Pacific by a ring of gravel and rocks dumped by the advancing glacier and breached at one point by a relatively shallow river. Only smaller icebergs can escape via the river into the Pacific, where they melt quickly in the warmer ocean water.
"But we're looking at the long-term possibility that we could get a 100-year storm that opens up the width and depth of this river," Molnia says. If that happened, larger icebergs could escape and begin drifting into shipping lanes, disrupting oil tanker traffic coming out of Prince William Sound to the northwest.
But the most important reason for studying glaciers may be for what they tell us about global warming and the theory that humans could be causing it.
By itself, of course, the shrinking of the Bering doesn't prove anything about global weather patterns. It might have been triggered by natural, short-term changes in the region's climate.
But the timing of the glacier's restlessness is intriguing.
The Bering began to shrink about a half-century or so after the Industrial Revolution began, when people began burning large amounts of wood, coal and oil. Combustion of these fossil fuels releases carbon dioxide and other "greenhouse gases" into the atmosphere. Scientists fear that rising levels of these gases may have magnified the atmosphere's ability to trap the heat from sunlight, which in turn causes the Earth's temperature to rise.
As Molnia told Alaska Airlines Magazine not long ago: "It's like a natural refrigerator -- if you leave the door open, it's going to melt."
But there is a big problem with using the planet's ice -- the cryo- sphere -- as global thermometers. No one knows for certain what is happening to the two largest stretches of ice on Earth -- one covering Greenland, the other covering Antarctica.
Richard S. Williams of the U.S. Geological Survey in Woods Hole, Mass., points out that all the world's glaciers, taken together, account for only about 0.7 percent of the ice on the Earth's surface, while the Greenland ice sheet accounts for almost 8 percent and Antarctica more than 91 percent.
For the past couple of decades, satellites have been keeping close track of changes in the surface area of the Greenland and Antarctic ice sheets. But researchers don't know how thick those sheets are. Until they do, they won't know if the world's cryosphere is growing, shrinking or holding its own.
Williams is working with scientists at the National Aeronautics and Space Administration to test the first practical technology for measuring that height. The technique uses the satellite Global Positioning System and a device called a laser altimeter. He expects to achieve accuracy to within about a yard.
Even without more accurate data, though, Williams says there are worrying signs of shrinkage in both Greenland and Antarctica. "And it seems to be accelerating," he says.
"There's something going on here that seems to be other than cyclical," he says. "The canary in the mine may be the tropical glaciers and the temperate glaciers."
Pub Date: 2/10/97