ice
and archimedes
Archimedes’ principle of buoyancy can be applied to the situation
of ice floating in a sea, as well as the situation of Archimedes in his
bath. This principle describes how the mass of a lump of floating ice
(floating partially above and partially below water) is identical to the
mass of the water displaced by the icy lump. If the ice melts and becomes
water, the density of the water decreases but its mass stays the same,
and so the water level is unchanged. Ice floats in fresh water with about
10% above and about 90% below the water.
“Usually 1/8th of an iceberg is above the waterline. That part
consists of snow, which is not very compact. The ice in the cold core
is very compact (and thus relatively heavy) and keeps 7/8ths of the
iceberg under water. [...] An iceberg that has tumbled over several
times has lost is light snow layers and so the iceberg gets relatively
heavier then before (with the snow) and because of the greater compactness,
only 1/10th rises above the surface.” [Quoted from solcomhouse.com]
The North Polar ice is floating in a manner of icebergs, while the Antarctic
ice and Greenland ice
sheets are essentially land-based, as are the world’s glaciers.
So, while the latter will contribute to sea-level rises upon melting,
the Arctic ice will not. Sea-level rises will also occur as the oceans
heat up, and so the volume of the water will expand. According to Robert
Grumbine, for every degree Centigrade of ocean warming, to a depth
of one km, the water level would rise by about 20 cm.
The approximate sea-level rises for a full melting of the Greenland
ice fields are expected to be 6 to 7 metres, while for Antarctic melting
they will be 56 to 80 metres. For more details see water
levels section in Global warming.
greenland
ice cap predicted to melt away [october 2007]
From a useful and reasonable item on Greenland ice:
“Jonathan Gregory, a climatologist at the University of Reading,
UK, says global warming could start runaway melting on Greenland within
50 years, and it will "probably be irreversible this side of a
new ice age". The only good news is that a total meltdown is likely
to take at least 1000 years.
“Greenland has the world's second largest ice cap, a remnant
of the last ice age. It is 3000 metres high and contains 2.85 million
cubic kilometres of ice.”
—
“[...] But Gregory and co-author Philippe Huybrechts, a glaciologist
at the Free University in Brussels, Belgium, calculate that if the island
warms by an annual average of 3 degrees Celsius, melting will exceed
snowfall and the ice sheet will begin to disappear.
“Once under way, the melting will be almost impossible to stop,
argues Gregory. As the ice melts, the cap's surface will sink to lower
altitudes, warming the surface further, reducing snowfall and accelerating
melting.
“ "Even if global climate returned to pre-industrial conditions,
the ice sheet might not regenerate," says Gregory. NASA scientist
Bill Krabill estimates that Greenland may already be losing ice at the
rate of about 50 cubic kilometres a year.”
Seasonal melting of the Greenland Ice Sheet,
1992 and 2002. Courtesy, UNO.
arctic
ice shrink [october 2007]
Wind changes accelerate arctic ice shrinking, they
say, but no comment on why the wind changes are developing. Rising temperatures
will shift climates and local changes, like the melting of the northern
ice, have unpredictable
effects on past established systems.
“From the 1970s through the 1990s, perennial ice declined by
about 500,000 square kilometers (193,000 square miles) each decade.
Since 2000, that amount of decline has nearly tripled.”
—
“ [...] Consequently, the Arctic Ocean was dominated by thinner
seasonal ice that melts faster. This ice is more easily compressed and
responds more quickly to being pushed out of the Arctic by winds. Those
thinner seasonal ice conditions facilitated the ice loss, leading to
this year's record low amount of total Arctic sea ice.”
—
“Nghiem said the rapid decline in winter perennial ice the past
two years was caused by unusual winds. "Unusual atmospheric conditions
set up wind patterns that compressed the sea ice, loaded it into the
Transpolar Drift Stream and then sped its flow out of the Arctic,"
he said. When that sea ice reached lower latitudes, it rapidly melted
in the warmer waters.
“The winds causing this trend in ice reduction were set up by
an unusual pattern of atmospheric pressure that began at the beginning
of this century," Nghiem said.” [Quoted from nasa.gov]
northwest
passage opens [october 2007]
Credit: European
Space Agency, with additions
“The figure was about 1 million sq km less than previous lows
in 2005 and 2006, Pedersen added.
“The Northeast Passage through the Russian Arctic remained partially
blocked, but in the light of the latest developments it may well open
sooner than expected, Pedersen said.”
giant ice
shelf broke free in the arctic [january 2007]
“A giant ice shelf the size of 11,000 football fields has snapped
free from Canada's Arctic, scientists said.
“The mass of ice broke clear 16 months ago from the coast of
Ellesmere Island, about 800 kilometres south of the North Pole, but
no one was present to see it in Canada's remote north.”
—
“The Ayles Ice Shelf, roughly 66 square kilometres in area, was
one of six major ice shelves remaining in Canada's Arctic.” [Quoted
from theage.com.au]
“He said the new island formed by the 66-square-kilometre fragment,
which could be up to 4,500 years old, could present a serious risk to
oil platforms in its drift path in the spring.
“At the longest and widest spans, the remains of the Ayles shelf
are about 15 kilometres long and five kilometres wide. The fragment
is between 30 and 40 metres thick.” [Quoted from cbc.ca]
Note that this huge lump of ice broke away on 13th
August 2005, and was only noticed during a Canadian Ice Service study
of satellite images to monitor ice conditions.
An ice shelf is the floating extension of the ice sheets
that have formed on land from thousands or millions of years of snowfall.
There are still six ice shelves in Canada. They are rare outside Antarctica
and look as if, soon, they will all be gone. As far as I know, the only
other place in the North with ice shelves is in the Russian Arctic Archipelago
of Severnaya Zemlya.
ward
hunt ice shelf breaks free [September
2003]
“The Ward Hunt Ice Shelf has been floating on the sea surface,
attached to the north coast of Canada's Ellesmere Island for around
3,000 years. In spring of last year, large fractures suddenly appeared;
by that August the shelf had broken in two.”
—
“Another consequence of the break-up is the draining of a lake
that was dammed behind the ice shelf in the 30-kilometre-long Disraeli
Fiord. This had a layer of fresh water around 40 metres deep atop more
than 350 metres of salt water. It was the largest lake of its kind in
the northern hemisphere.
“Three cubic kilometres of fresh water has drained from the fiord,
disrupting a rare microbial ecosystem that was only discovered in 1999,"
says Jeffries. "It was lost before scientists had a chance to go
back and really study it." ”
Ward Hunt Island, close to the northern coast of Ellesmere
Island, has been a staging post for many
explorations of the Far North, while five
2002 expeditions used Ward Hunt Island as their starting point.
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related material
pressure
on water resources
A
page about icebergs that includes definitions of iceberg shapes and
sizes. The page also has photos of icebergs.
glossary
- ice shelf:
- floats on the sea/ocean;
- ice sheet:
- situated on land.
- Thus, an ice shelf is effected by water temperature. The water is
quite often frozen right down to the ocean floor. Thus the ice may not
be acting as a shelf, but still be in contact with the water at the
ice face. Shelved ice is under greater stress and, therefore, can be
subject to major break-up.
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