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by Jenny Hogan
Exeter
13:38 02 February 2005
NewScientist.com news service
The massive west Antarctic ice sheet,
previously assumed to be stable, is starting to collapse, scientists
warned on Tuesday.
Antarctica contains more than 90% of the world's ice, and the loss
of any significant part of it would cause a substantial sea level
rise. Scientists used to view Antarctica as a "slumbering giant",
said Chris Rapley, from the British Antarctic Survey, but now he
sees it as an "awakened giant".
Rapley presented measurements of the ice sheet at a major climate
conference in Exeter, UK. Glaciers on the Antarctic peninsula, which
protrudes from the continent to the north, were already known to be
retreating. But the data Rapley presented show that glaciers within
the much larger west Antarctic Ice sheet are also starting to
disappear.
If the ice on the peninsula melts entirely it will raise global sea
levels by 0.3 meters, and the west Antarctic ice sheet contains
enough water to contribute meters more. The last report of the
Intergovernmental Panel on Climate Change, published in 2001, said
that collapse of this ice sheet was unlikely during the 21st
century. That may now need to be reassessed, Rapley warned.
Cork from a
bottle
Changes on the peninsula, where 75% of the 400 mountain glaciers are
in retreat, have provided new insights into the ways that ice sheets
may disintegrate.
In March 2002, a huge floating ice shelf known as Larsen B shattered
into icebergs. This turned out to have an effect akin to pulling a
cork from a bottle. With Larsen B no longer impeding movement, the
ice floes that fed the shelf began moving faster towards the sea and
started to thin. The finding took scientists by surprise when
revealed in September 2004 and now modellers are now working to
include such mechanisms in their predictions.
Climate records derived from the analysis of sediments show that ice
shelves off the peninsula have been absent in several earlier eras,
when natural variability warmed the world. But the break-up is
affecting ice closer to the pole than ever recorded, said Rapley.
"It's like the Heineken effect," he said, referring to the beer
adverts that claim Heineken "reaches the parts other beers cannot
reach".
Indications that climate change may be affecting the west Antarctic
ice sheet comes from three glaciers, including Pine Island and
Thwaites. Data reveal they are losing more ice - mainly through the
calving of icebergs - than is being replaced by snowfall. According
to a preliminary analysis, the difference between the mass lost and
mass replaced is about 60%.
Whether the loss of mass by the glaciers is due to natural variation
or is caused by human-influenced warming of the oceans is not known
for sure. Scientists are now making more field measurements to
assess the causes, but warming is a likely culprit, said Rapley:
"The fact that three of them are simultaneously accelerating
suggests that is the case."
The melting of these three glaciers
alone is contributing an estimated 0.24 millimeters per year to sea
level.
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