http://news.bbc.co.uk/2/hi/science/nature/2558319.stm
Monday, 9 December,
2002, 12:34 GMT
Record ice
loss in Arctic
By Molly Bentley
BBC News Online in San Francisco
QUOTE:
"[Natural variability] can't explain everything."
Dr. Mark Serreze
[ See photograph
of meltwater flowing off the ice... ]
CAPTION #1:
"Meltwater flows into a large moulin on Greenland and down
to the bedrock to "lubricate" the sheet."
[ See photograph
from original article... ]
CAPTION #2:
"Sea ice in September: Lavender line indicates a more
typical ice extent (median for 1988-2000)"
Melting was so severe on the Greenland ice sheet in June that
field
researcher Konrad Steffen wondered just how he would get off the
continent.
"We had come in
with a fixed-wing aircraft landing on skis," said
the University of Colorado climatologist. "If that snow is
melting
then you cannot leave. As it was, we had to charter a helicopter."
Greenland's unusual
summer slush was part of a record-breaking year
of northern polar ice loss, reported by Dr Steffen and other scientists
this weekend at the American Geophysical Union (AGU) conference
in
San Francisco.
According to scientists,
surface melt on Greenland was the highest
in recorded history - and extended to elevations previously untouched
by melt - while the amount of Arctic sea ice also reached a record
low.
While some of the accelerated
melting appears to be linked to natural
atmospheric oscillations, human influence could not be ruled out,
said the scientists.
Glacier 'lubrication'
Greenland glacier and
sea ice melt, combined with disappearing permafrost,
the northern expansion of vegetation, and increased fresh water
run-off
present a "compelling case that something is going on,"
said Larry Hinzman,
of the University of Alaska, Fairbanks.
Measurements of the
Greenland ice sheet taken from passive microwave
satellite sensors show 685,000 square kilometres of melt, an area
more
than double that of 1992.
Such melting encourages
further ice loss. The excess water weight
pushes down on the glacier at the same time that water seeps through
cracks to the underside.
The combination accelerates
the glacier's flow to the sea.
Dr Steffen attributes
the increased melt to an unusual low-pressure
system that "came earlier and stayed longer" on the
island to create
unseasonable warm temperatures.
Ozone depletion
Such a low-pressure
system is characteristic of the Arctic Oscillation,
an atmospheric sea-sawing that produces warm weather and is also
partly
responsible for this year's dramatic Arctic sea ice melt, according
to Mark Serreze, a researcher at the National Snow and Ice Data
Center
in Boulder, Colorado.
He calculates that
warming has produced roughly 20% loss in Arctic
sea ice since 1978. But not all warming is due to natural variability.
"The Arctic Oscillation
can't explain everything," said Dr Serreze.
To what extent these
may be human-induced changes is very difficult
to say, and Arctic Oscillation itself is subject to human influence,
he said.
Stratospheric ozone
depletion can exacerbate the system by cooling
the upper atmosphere, which in turn changes wind and ocean circulation
patterns.
Dr Serreze suspects
that just such an aberrant Arctic Oscillation
was behind unusual circulation patterns last winter that weakened
sea ice.
Positive feedback
Changes in the winter
wind field broke up floating sea ice to create
cracks that suggest the "opening of a Venetian blind,"
said Dr Serreze.
The breaks in the ice allow it to absorb more solar radiation.
This triggered an early
spring melt, which made the ice vulnerable
to the impact of high summer temperatures.
"It was like a
one-two punch," said Dr Serreze.
The loss of Arctic
sea ice is "big news", according to University
of Alaska, Fairbanks, researcher Larry Hinzman.
"Polar sea ice
has an important function in moderating the global
energy balance," he said. He explained that sea ice has albedo
of 0.8.
That is, it reflects 80% of the solar radiation. When the sea
ice melts
you have water, which has an albedo of 0.2.
"The sea ice goes
from absorbing 20% of solar radiation to
absorbing 80%," said Dr Hinzman. This creates positive feedback
for further warming.
In total, the arctic
warming is an unprecedented trend,
according to Dr Hinzman.
"We're experiencing
the most rapid increase in temperature
in recorded history," he said.
###
