Global opvarming fører til raskere avsmelting av Grønlands innlandsis,
mener nå amerikanske glasiologer. Det kan bemerkes, at i Eem-tiden, den
forrige mellomistid for ca. 125000 år siden var Grønlands innlandsis en
del mindre enn nå (hvor mye er omdiskutert) og det vokste skoger langt
nord for polarsirkelen selv på Østgrønland, som har et meget barsk klima
nå. I Eem-tiden antar man at de globale temperaturene var omtrent som de
åpenbart har vært de siste tre månedene, nemlig 2 grader over global
middeltemperatur for 1961-90. Samtidig er det verd å bemerke, at
CO2-innholdet i atmosfæren i Eem var en god del lavere enn det er nå.
Nivået nå er det høyeste på adskillige millioner av år, noe det har
blitt på en rekordkort tidsperiode (mindre enn hundre år). Dette er
aldri før sett i den geologiske historien så vidt forskningen kan vite.
Det ligger med all sannsynlighet en god del ubehagelige overraskelser på
lur, bare i det CO2-nivået vi har allerede nå. Foreløpig kan vi slå
fast, at temperaturene øker med rekordfart og mye raskere enn hittil
antatt. Grønlands innlandsis smelter raskere enn antatt, selv for bare
få år siden. Det samme gjelder for deler av Antarktis, med en mulig
destabilisering av isen i vest-Antarktis som konsekvens.
I Eem-tiden sto havene globalt fire-seks meter høyere enn nå, og den
skandinaviske halvøya var en øy (store deler av Finland var hav, se
Bjørn G. Andersen og Harold W. Borns: Ice Age World, s. 46-49).
Temperaturen da var altså ca. som den nå har vært globalt de siste tre
månedene!
Her er det stoff til ettertanke.
Karsten Johansen
http://news.bbc.co.uk/hi/english/sci/tech/newsid_2030000/2030473.stm
Friday, 7 June, 2002, 00:01 GMT 01:01 UK
Greenland's warming ice flows faster
By Alex Kirby
BBC News Online environment correspondent
New measurements by US scientists show that since 1996 the Greenland ice
sheet has been moving faster during the summer melting season.
Meltwater flows into a large moulin and down to the bedrock
The rate is accelerating because more melted water is trickling down
from the surface of the sheet to the bedrock.
There it lubricates the sheet, which moves faster towards the coast.
The scientists say this suggests the ice may be responding more quickly
than thought to a warming climate.
Their research, reported in Science magazine's online Sciencexpress, was
funded under the US space agency Nasa's ICESat project.
Coastal slide
Using periodic Global Positioning Satellite measurements from 1996 to
1999, the team found that the ice flow accelerated from 31.3 centimetres
(12.3 inches) a day in winter to a peak of 40 cm (15.7 in) in the
summer, when surface melting was greatest.
Jay Zwally is an ICESat project scientist at the Nasa Goddard Space
Flight Center. He said: "This study demonstrates that surface meltwater
travels quickly through the 1,200-metre-thick ice (1,310 yards) to the
bedrock to make the ice slide faster.
"This process was known for decades to enhance the flow of small
mountain glaciers, but was not known to occur in the large ice sheets."
The meltwater drains into crevasses and large tunnels, or moulins, which
can be 10 metres (33 feet) wide. The melting also means a gradual
thinning of the ice as well as its downhill coastward slide.
More significant
In a further twist, the water carries heat from the sheet's surface to
its base. Waleed Abdelati, a polar programme scientist at Nasa HQ, and
Konrad Steffen, of the University of Colorado, undertook a separate
study.
This showed that the melting of the ice sheet surface has increased by
almost 20% over the last 21 years.
In that period summer temperatures have risen by 0.25 Celsius. The
researchers say the link between melting and ice flow suggests that the
increasing rate of melting may be more significant than had been
believed.
They warn that the combination of faster flow, ice thinning and lowering
of the sheet's surface elevation could trigger a feedback process.
This would lead to more melting, something they say has not been
considered in computer models predicting ice sheet response to climate
change.
In December Nasa is due to launch an ICESat mission which will use a
laser altimeter to monitor ice sheet elevations and show changes as
small as one centimetre a year.
The scientists say it is possible that increased movement of the ice
sheet because of more meltwater may have contributed to the
disappearance of the Laurentide ice sheet about 10,000 years ago.
Previous work
They think the process they have identified could also help to explain
the extensive melting of the Greenland ice sheet during the last
interglacial period about 125,000 years ago.
Jay Zwally said: "During this time, when the climate was warmer, the ice
sheet was less extensive. With the predicted greenhouse warming, we may
be returning to similar conditions."
In July 2000, Nasa scientists said the ice around the coast of Greenland
was thinning fast, by more than one metre (three feet) annually.
In March 1999, Nasa reported that the Greenland ice sheet was thinning
by up to one metre a year.
The scientists said then they thought the probable explanation was that
water was filtering down to the bedrock, making the ice likelier to slip
off. They did not then suggest a link with climate change.
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