STANDING on North America鈥檚 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鈥檚 raising a ruckus about it.
Situated in the backyard of Juneau, the state鈥檚 capital city, the Mendenhall
attracts more than 300,000 visitors per year, making it one of the most watched
glaciers in the world. It鈥檚 also become a poster child for global warming.
Before the public鈥檚 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
鈥淩unning deep鈥�). Four years ago, for instance, the environmental group
Greenpeace raised the alarm when the front end of Alaska鈥檚 huge Bering glacier
collapsed. 鈥淩ising temperatures shrink world鈥檚 largest temperate glacier,鈥�
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.
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The Bering brouhaha fits in with a larger pattern of climate catastrophe
stories. With the fervour of Chicken Licken, environmentalists have taken to
claiming 鈥渢he ice is melting鈥� 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鈥檚 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鈥檛 good. 鈥淚 didn鈥檛 expect to see
nearly as large a change,鈥� says Echelmeyer, a researcher at the University of
Alaska Fairbanks. His measurements show that most of the glaciers are losing
weight fast鈥攕o 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. 鈥淧eople would often ask what the glaciers
are doing,鈥� says Echelmeyer. 鈥渀Are they all thinning?鈥� Nobody could answer
that.鈥� 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鈥檚 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鈥檚 when he鈥檚 run into the most trouble.
鈥淪ome 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.鈥� Sometimes he鈥檚 taxied down the glacier for 3 kilometres to
build up enough speed to get airborne.
If the plane won鈥檛 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鈥檚 mass balance鈥� 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鈥檛 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. 鈥淲e 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鈥檚 measured over 40 or
50 years, which is fairly big.鈥�
Now he鈥檚 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. 鈥淭he 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,鈥� 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鈥檛 have long to go, says
Echelmeyer, pointing in particular to the Lemon Creek glacier, east of Juneau.
鈥淭he Lemon Creek is now thinning 2 to 3 metres a year and it鈥檚 a maximum of 200
metres thick. That鈥檚 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
测别补谤蝉.鈥�
Echelmeyer鈥檚 study is filling a major gap, says Johannes Oerlemans, a
meteorologist at Utrecht University in the Netherlands. 鈥淗is work is so
interesting because there has been so little work on Alaskan glaciers,鈥� 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. 鈥淎laska
is more remote, but on the other hand it鈥檚 more important because there is more
ice there.鈥�
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鈥檛 necessarily mean that greenhouse warming is solely to blame,
Dowdeswell cautions. Glaciers respond in a complex way to many different
climatic forces, he says. 鈥淭hat鈥檚 why it鈥檚 actually quite difficult to unravel
which bit is having the most influence at any one time.鈥�
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鈥攖he Wolverine near Anchorage and the Gulkana in the centre of the
state鈥攁nd this shows evidence of a climate cycle, Trabant says. Between
1976 and 1989 one of the glaciers thickened while the other thinned. 鈥淪ince
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,鈥� 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鈥檛 be sure for several
more years, says Trabant. If Alaska鈥檚 glaciers are responding to the PDO, then
they may start thinning more slowly, he predicts. 鈥淚 don鈥檛 think it鈥檚 a
lead-pipe cinch to expect the current trends to be sustained for 100 years in
the future,鈥� he says. 鈥淚t鈥檚 just not that certain.鈥�
The Mendenhall glacier is a case in point. Summertime melting thinned the
lower reaches of the glacier by 11 metres this year鈥攕imilar 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鈥攕omething that hasn鈥檛 happened in many years, says Roman
Motyka, a glaciologist with the University of Alaska Southeast.
It鈥檚 too early to tell whether last year鈥檚 gain is the start of a trend or
only a blip. In a report to the US federal government, Motyka said in January
that 鈥渢he long-term trend at the Mendenhall has been decidedly towards glacier
thinning and recession鈥�.
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鈥檚 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.
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 掳C
since 1943. But there鈥檚 another factor confusing the picture:
a lake that laps at the end of the glacier and affects its behaviour.
Motyka鈥檚 group set up a transmitter on the ice and bounced radio signals down to
bedrock to measure the glacier鈥檚 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鈥檚 tip has been responding to two
separate influences. 鈥淭he primary one has probably been the climate,鈥� 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. 鈥淏ut
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.鈥�