The long goodbye

Alaska's glaciers appear to be disappearing before our eyes. Are they a sign of things to come? Richard Monastersky searches for clues

STANDING on North America's most visited glacier in summer, you can hear the sound of climate changing. Gurgles and babbles build to a crescendo where water cascades down holes in the ice. The Mendenhall glacier in south-east Alaska is melting and it's raising a ruckus about it.

Situated in the backyard of Juneau, the state's capital city, the Mendenhall attracts more than 300,000 visitors per year, making it one of the most watched glaciers in the world. It's also become a poster child for global warming. Before the public's eyes, the glacier is shrinking. Last summer, its end point retreated up the valley by 100 metres, freeing land that had been locked under ice for centuries. Since the 1930s, the Mendenhall has lost nearly a kilometre of its length.

But glaciers can be slippery subjects when it comes to climate, and appearances can deceive. Individual glaciers can respond to many different factors, making it risky to use any one patch of ice as a global yardstick (see "Running deep"). Four years ago, for instance, the environmental group Greenpeace raised the alarm when the front end of Alaska's huge Bering glacier collapsed. "Rising temperatures shrink world's largest temperate glacier," screamed the press release. Yet researchers immediately pointed out that this apparent catastrophe was part of an expected cycle. Every couple of decades the Bering spends several years surging down its valley. But the longer glacier is unstable, and much of the front part breaks off as icebergs, leaving the end of the glacier further up the valley again.

The Bering brouhaha fits in with a larger pattern of climate catastrophe stories. With the fervour of Chicken Licken, environmentalists have taken to claiming "the ice is melting" at the slightest provocation, no matter what the context. The headlines of late have featured gargantuan icebergs breaking off Antarctica and open water appearing at the North Pole, without mentioning whether these events have happened before or what their significance is.

That's why glaciologist Keith Echelmeyer has taken to the skies. Flying a single-engined, two-seater plane, he is surveying some 90 glaciers from the extremes of northern Alaska down to the Cascade range of Washington State. His project is assessing the health of glaciers both big and small, collecting the data necessary to determine how the ice has fared across the north-west margin of North America and what it has to say about climate. The message coming from the first 8 years of this massive survey isn't good. "I didn't expect to see nearly as large a change," says Echelmeyer, a researcher at the University of Alaska Fairbanks. His measurements show that most of the glaciers are losing weight fast-so quickly in fact that some may not survive the 21st century.

The results could well be a sign of how greenhouse gas pollution will harm the high Arctic in years to come. But some researchers still see evidence of other potential reasons for the shrinkages besides global warming.

It was ignorance, coupled with a love of flying, that prompted Echelmeyer to start making house calls to glaciers. "People would often ask what the glaciers are doing," says Echelmeyer. "`Are they all thinning?' Nobody could answer that." Before his project began in 1993, scientists regularly monitored only two out of the thousands of glaciers in Alaska. With a territory so vast and with many glaciers in remote regions, it was nearly impossible to keep tabs on these icy streams using traditional survey methods.

But the Global Positioning System and compact airborne lasers have made it feasible to track glaciers from the air. In 1992 Echelmeyer and his colleagues built a system that shoots laser light through a window on the bottom of a Piper Supercub and then catches the beam after it bounces off the glacier. By timing the length of the trip, the system can gauge the elevation of the surface relative to the plane, while GPS fixes the plane's altitude and position. The whole procedure can measure the height of the ice and snow to within 10 centimetres.

To map a glacier, Echelmeyer and a crewmate fly several passes along it. They must go low and slow, creeping along at 100 kilometres an hour about 100 metres above the glacier, following its every bump and dip.

For more accurate figures, Echelmeyer sets down his plane on the glacier to measure the snow depth by hand. That's when he's run into the most trouble. "Some of the hairiest moments have been landing or taking off. Sometimes, the snow is so sticky and deep and the winds are blowing with you, and that makes it hard to take off." Sometimes he's taxied down the glacier for 3 kilometres to build up enough speed to get airborne.

If the plane won't take off, Echelmeyer lightens his load by leaving his passenger behind. Then he tries landing and taking off several times to pack down a runway from which he can lift off with his human cargo. Once, he was stuck for 2 days and had to trample down a 650-metre runway by foot, using skis and snowshoes.

Getting thinner

During the past 8 years, Echelmeyer has surveyed glaciers across some 2000 kilometres of western North America. Two of the biggest glaciers each exceeded the surface area of some American states. He expects to continue for another two years, revisiting glaciers that he studied in the early 1990s to see how they have fared since then.

When he has mapped a glacier, Echelmeyer compares his results with charts made from aerial photographs by the US Geological Survey 40 to 50 years ago. This helps determine the glacier's mass balance- whether it is putting on weight or wasting away. Glaciers generally get thicker during winter snowstorms and then thin when they melt in the summer. To stay healthy, they must maintain an even mass balance, putting on at least as much as they lose.

Last summer, flights over the Mendenhall revealed that it hasn't managed to keep its weight up. In the past half-century, it has thinned by about a metre each year. Most glaciers in the survey have had significant bites taken out of them. Some 90 per cent are melting more in summer than they gain in winter, says Echelmeyer. "We find in general the glaciers are thinning from 0.5 to 1.5 metres per year as an average over their entire extent. And that's measured over 40 or 50 years, which is fairly big."

Now he's trying to tease out what has caused these declines. Is the weather warmer? Has snowfall decreased? His studies so far indicate that temperature seems to be the main factor. "The glaciers in Alaska are giving us a clear picture that indeed something is happening to cause them to thin and that has to be climate-related," he says. The marked change has come at the same time as worldwide temperatures have shot up most dramatically.

At this rate, some of the smaller glaciers don't have long to go, says Echelmeyer, pointing in particular to the Lemon Creek glacier, east of Juneau. "The Lemon Creek is now thinning 2 to 3 metres a year and it's a maximum of 200 metres thick. That's a 100 years [to go]. But it will go faster than that because if you thin a glacier, it gets less snowfall because it extends less far up the mountains. If this continues, some of the glaciers will go away in 50 years."

Echelmeyer's study is filling a major gap, says Johannes Oerlemans, a meteorologist at Utrecht University in the Netherlands. "His work is so interesting because there has been so little work on Alaskan glaciers," he says. While the Alps have been studied for many decades, the ice in all the Alpine glaciers combined amounts to no more than one large glacier in Alaska. "Alaska is more remote, but on the other hand it's more important because there is more ice there."

Many glaciers elsewhere are wasting away, but only in Alaska is the rate accelerating, says glaciologist Julian Dowdeswell at the University of Bristol. It may be that Alaskan glaciers are functioning as canaries in the coal mine of global climate, showing changes before they appear in other polar regions.

This doesn't necessarily mean that greenhouse warming is solely to blame, Dowdeswell cautions. Glaciers respond in a complex way to many different climatic forces, he says. "That's why it's actually quite difficult to unravel which bit is having the most influence at any one time."

In fact, the rapid thinning of Alaskan glaciers in the 1990s may have been a temporary setback, according to Dennis Trabant of the US Geological Survey in Fairbanks. Since 1966 the USGS has monitored the mass balance of two Alaskan glaciers-the Wolverine near Anchorage and the Gulkana in the centre of the state-and this shows evidence of a climate cycle, Trabant says. Between 1976 and 1989 one of the glaciers thickened while the other thinned. "Since 1989, both glaciers have lost mass at an unprecedented rate up until the last couple of years. But it looks like that might be ending," he says.

Trabant suggests that the glaciers are responding to a large-scale cycle called the Pacific Decadal Oscillation, which causes the climate of the northern Pacific to swing back and forth every decade or so. Some researchers suspect the PDO is in the process of shifting direction, but they won't be sure for several more years, says Trabant. If Alaska's glaciers are responding to the PDO, then they may start thinning more slowly, he predicts. "I don't think it's a lead-pipe cinch to expect the current trends to be sustained for 100 years in the future," he says. "It's just not that certain."

The Mendenhall glacier is a case in point. Summertime melting thinned the lower reaches of the glacier by 11 metres this year-similar to the average annual loss during the 1990s. But the winter of 1999/2000 deposited an unusually thick layer of snow higher up the glacier. So it ended up with an overall net increase in mass-something that hasn't happened in many years, says Roman Motyka, a glaciologist with the University of Alaska Southeast.

It's too early to tell whether last year's gain is the start of a trend or only a blip. In a report to the US federal government, Motyka said in January that "the long-term trend at the Mendenhall has been decidedly towards glacier thinning and recession".

Over the past winter, snows have once again blanketed the glacier, piling on some much-needed mass and temporarily hiding the rocks strewn across its surface. Come the warm rays of summer, though, the melt-water will once more start trickling across the glacier's surface, first as just a whisper and eventually rising into a sustained chorus. Like scores of other glaciers across the region, the Mendenhall will resume its annual battle to keep from wasting away.

Running deep
When it comes to determining the effects of climate change, the terminus of the famous Mendenhall glacier gives mixed messages, according to Roman Motyka, a glaciologist with the University of Alaska Southeast. In May 2000, he and several colleagues went to the glacier to try to establish why the ice is retreating. They knew already that the region had been getting warmer. Records at nearby Juneau airport show a rise in average annual temperature of 1.6 °C since 1943. But there's another factor confusing the picture:

a lake that laps at the end of the glacier and affects its behaviour. Motyka's group set up a transmitter on the ice and bounced radio signals down to bedrock to measure the glacier's thickness near its terminus. They found that the valley holding the Mendenhall deepens beneath its terminus, where it meets the lake. The presence of that deep water buoys up the end of the glacier and makes the ice less stable. That in turn increases the rate of icebergs calving off the glacier into the lake.

Motyka concludes that the Mendenhall's tip has been responding to two separate influences. "The primary one has probably been the climate," he says. A warming trend that began more than a century ago started the glacial retreat, which has continued through the recent decades of increasing temperatures. "But the retreat probably would have been slower if the glacier had ended on land. Because it fronted in a lake that was in some places quite deep, it accelerated its retreat over and beyond what climate would have done."