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.
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.
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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. |
