De store isfjell som brekker løs fra Antarktis og vekker så mye
oppmerksomhet blant de sensasjonslystne, har muligens lite med global
oppvarmning å gjøre, men er i stedet ledd i en naturlig syklus av vekst
og tilbakegang av isshelfene. Den meget brå oppvarmingen av den
Antarktiske halvø og Larsen-shelfens raske tilbakegang er derimot nok
tegn på en uvanlig rask global oppvarmning.
Karsten Johansen
http://www.lasvegassun.com/sunbin/stories/w-aus/2002/may/23/052306010.ht
ml
May 23, 2002
Antarctic Icebergs Seen As Normal
WASHINGTON- The icebergs breaking away from Antarctica in recent months
- some as big as small states - are part of a process scientists say
marks a return to ice conditions of years past.
Several ice shelves around the continent have been growing in recent
years, a process that has puzzled researchers concerned about possible
global warming.
In the last three months - autumn there - several icebergs, one the size
of Delaware and another nearly as big as Chesapeake Bay, have broken
free.
"The icebergs that have calved in last couple of months probably don't
have much to do with global warming. It is part of a pattern of growth
and retreat that is more or less normal," explained Ted Scambos of the
National Snow and Ice Data Center at the University of Colorado.
University of Chicago researcher Douglas MacAyeal, who placed automated
weather stations and tracking devices on one of the new icebergs, said
the satellite and other technologies are allowing science for the first
time to observe the birth of such large bergs.
The process, he said, is part of a natural cycle in which ice shelves
grow and then calve icebergs over geological time scales.
A couple of the ice shelves along the coasts of the continent,
particularly the Ross and Ronne shelves, have become more extended than
they were in the past and are now returning to the limits that were
normal from about the 1950s to the 1970s, Scambos said.
Scientists are also much better able to track these icebergs using
satellites, Scambos noted. "Anytime an event occurs we're on it in a day
or two."
"It's a grand event, it's astounding event," when these bergs break
loose, he said. "But it probably should not cause alarm ... the shelves
that are calving don't seem to be retreating past their minimum
historical extent."
He added, however, that scientists don't have good long term records of
Antarctic sea ice limits.
H. Jay Zwally of the NASA Goddard Space Flight Center in Greenbelt, Md.,
reports in the June issue of the Journal of Geophysical Research-Oceans
that the amount of floating sea ice surrounding Antarctica has increased
about 1 percent per year over the last 20 years.
The increasing amount of floating ice results from a combination of
processes including changes in salinity and the amount of overturning of
the water. Water with less salt freezes more easily and reducing the
rising and falling ocean water means less heat comes up from below.
This is occurring "possibly despite global warming and possibly as a
result of global warming," he said.
At the same time, at the other end of the planet, Arctic ice has been
decreasing.
And there is also melting on the Larsen Ice Shelf at the Antarctic
peninsula that extends toward the tip of South America.
The collapse of that shelf, much farther north than the Ross and Ronne
shelves, is thought to be related to temperature increases in that
region.
Larsen and the Antarctic peninsula "have experienced an unusual and
profound warming trend," Scambos said. "Ice shelves that had existed
several thousand years are retreating. The surprise is how fast the
turnaround occurred."
Satellite images show that the piece of the Larsen Ice Shelf collapsed
during a five-week period that ended March 7. It splintered into a plume
of drifting icebergs. The Larsen Ice Shelf has been under careful
observation since 1995, when its northernmost sector collapsed in a
similarly dramatic event. The shelf now is about 40 percent of its
original size.
But glaciers elsewhere on the continent are both thickening and thinning
as temperatures show conflicting climate trends.
---
On the Net:
National Ice Center: http://www.natice.noaa.gov
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