Bevis på global oppvarming: Ice Melting Faster Everywhere
Roald B. Larsen
24 de diciembre de 2009
From the Arctic sea ice to the Antarctic interior and the mountainous peaks
of Peru, Alaska, and Tibet, ice is melting at an alarming rate. The
accelerating loss of ice sheets, sea ice, and glaciers is one of the most
powerful and striking indicators of a warming climate.
The most notable ice loss in recent years has been the shrinking of sea ice
in the Arctic Ocean. From the beginning of the satellite record in 1979
through 1996, ice area decreased at a steady rate of 3 percent per decade in
response to rising temperature. In the following decade, ice area decreased
by 11 percent, reaching a dramatic minimum in 2007. In September of that
year, sea ice occupied only 3.6 million square kilometers, an area 27
percent smaller than the previous record low (in 2005) and 38 percent
smaller than the 1979-2007 average. Summer sea ice coverage has increased
slightly in the last two years, but it is still far below the long-term
average.
Declines in ice thickness and volume are just as dramatic. The combination
of these trends has led to a decrease in the amount of ice that persists in
the Arctic through multiple seasons. Multiyear ice is more stable and less
susceptible to break-up than the thin, short-lived seasonal ice that forms
each winter. Between 1987 and 2007, the amount of ice at least five years
old has plummeted from 57 to just 7 percent. Drastic changes in sea ice
cover have led scientists from the University of Washington and the National
Oceanic and Atmospheric Administration to predict that the summer of 2037
could see the first ice-free Arctic in a million years. Other scientists
have predicted a largely ice-free summertime Arctic as early as 2015.
Declining sea ice is a self-reinforcing trend because of what is known as
the albedo effect. Ice reflects up to 70 percent of the sunlight that
reaches it, while ocean water reflects only 6 percent and absorbs the rest
as heat. This means that as soon as a small amount of sea ice disappears and
exposes the underlying ocean water, the system starts absorbing more energy,
which leads to further ice melt. Dangers associated with this runaway
warming scenario include rapid destruction of diverse ecosystems that
support polar bears, seals, and walruses, among other organisms; a thawing
of the Arctic tundra, which can release copious amounts of the greenhouse
gas methane; and increased warming of nearby Greenland.
Satellite data indicate that the Greenland ice sheet has been experiencing
accelerated melt, particularly over the past several decades. In fact,
Greenland's average annual melt between 2002 and 2005 was triple that of the
1997-2003 period, and the summer melt area on the ice sheet has increased 30
percent since 1979. In recent years, changes in ice dynamics associated with
higher temperatures have caused glaciers to flow faster, leading to
additional ice loss. Melt water lubricates the base of glaciers that carry
ice from the interior to the sea, causing their movement to accelerate (for
example, the speed of Greenland's largest outlet glacier doubled in just
five years). Surface lakes propagate fractures through the ice sheet as they
drain, further lubricating the base and weakening the ice sheet with a
network of cracks. And glaciers have been calving into the ocean with enough
force to be detected on seismometers all over the world. The frequency of
these "glacial earthquakes" has increased in recent years; in 2005, for
example, there were over twice as many quakes as in any year before 2002.
All told, Greenland lost 1,500 gigatons of ice between 2000 and 2008, more
water than is used in U.S. homes and industry over a six-year period.
In the Southern Hemisphere, Antarctica, too, is showing signs of a warming
climate. Annual ice mass loss for the entire continent more than doubled
between the periods 2002-06 and 2006-09. In March 2009, a
400-square-kilometer piece of ice broke off of the Wilkins ice shelf, the
tenth ice shelf collapse on the Antarctic Peninsula in recent times. The
most notable break-up was that of the Larsen B ice shelf in 2002, which
covered some 3,000 square kilometers, roughly the size of Rhode Island. The
West Antarctic ice sheet (WAIS) lost 59 percent more ice in 2006 than it did
in 1996. A fast-flowing drainage glacier of WAIS, the Pine Island glacier,
experienced a quadrupling in its average rate of volume loss between 1995
and 2006. Previously well-established as stable or even gaining mass, the
East Antarctic ice sheet may in fact be shrinking. A late 2009 Nature
Geoscience study points toward a net melting of the ice sheet since 2006.
This new discovery adds to the ever-growing fears of ice sheet collapse and
sea level rise. With increased melting, scientists say sea level could rise
as much as 2 meters by the end of this century.
Mountain glaciers are much smaller in comparison to the polar ice sheets
and, thus, do not pose nearly as great a threat to world sea levels. But due
to their proximity and importance to human settlements, their melting is of
grave and immediate concern. Melting mountain glaciers can create hazards
like rockfalls, avalanches, and outburst floods from glacial lakes; they
also have significant impacts on freshwater supplies. Worldwide, the average
annual rate of mountain glacier melt was over twice as great between 1996
and 2005 as during the previous decade. The World Glacier Monitoring Service
named 2007, the most recent year for which data are available, the
eighteenth consecutive year of retreat for the 30 reference glaciers
measured since 1976.
The glaciers in the Himalayas and on the Tibetan plateau make up the largest
body of ice outside the poles and provide water to Asia's major river
systems, which supply water to over 2 billion people. This water is vital
for drinking and for irrigating the wheat and rice crops in China and India,
the largest in the world. In recent years, Himalayan glaciers have been
retreating at rates ranging from 10 to 60 meters per year. As the glaciers
disappear, the dry-season flows of river systems that depend on them may
decrease by up to 70 percent, making them seasonal rivers. River systems at
risk include the Yangtze, Yellow, Indus, Ganges, and Brahmaputra.
The Andes, home to 90 percent of the world's tropical glaciers, are also
experiencing rapid melt and a shrinking water supply: between the early
1970s and 2006, Peruvian and Bolivian glaciers lost about one third of their
surface area. In Peru, glacier and snow melt provides 80 percent of the
fresh water, used not only for drinking but also for hydroelectricity, which
supplies more than 80 percent of the country's power. In neighboring
Bolivia, the La Paz governor is already anticipating severe water shortages
and considering a program for migration out of the capital city. The
18,000-year-old Chacaltaya glacier, home of the country's only ski resort,
disappeared in 2009.
The glaciers of Tanzania's Mount Kilimanjaro, long cultural and spiritual
icons, decreased in area by 84 percent between 1912 and 2007 and continue to
melt rapidly. In Alaska, 98 percent of glaciers are currently thinning or
retreating. And accelerated melting puts Montana's Glacier National Park on
track to lose its namesakes by 2020.
These current ice loss trends are alarming, but perhaps more disconcerting
is the fact that ice melt is occurring even faster than scientific models
have predicted, emphasizing the need to cut emissions before the world sees
ice sheet collapse, catastrophic inundation of low-lying coastal areas, and
widespread water and food shortages. After all, in the words of Stockholm
University professor Johan Rockstr�m, "We don't know how to refreeze the
Greenland ice sheet."
Alexandra Giese, Earth Policy Institute
Espen Vestre
> response to rising temperature. In the following decade, ice area decreased
> by 11 percent, reaching a dramatic minimum in 2007. In September of that
> year, sea ice occupied only 3.6 million square kilometers, an area 27
> percent smaller than the previous record low (in 2005) and 38 percent
> smaller than the 1979-2007 average. Summer sea ice coverage has increased
> slightly in the last two years, but it is still far below the long-term
> average.
"slightly"? Det �kte 10% i 2008 og 11% i 2009... Fortsetter det slik
rekker faktisk den arktiske sommerhavisen helt til ekvator i 2050 ;-)
> The glaciers of Tanzania's Mount Kilimanjaro, long cultural and spiritual
> icons, decreased in area by 84 percent between 1912 and 2007 and continue to
> melt rapidly. In Alaska, 98 percent of glaciers are currently thinning or
> retreating. And accelerated melting puts Montana's Glacier National Park on
> track to lose its namesakes by 2020.
Kilimanjaro mister is p� grunn av lokal t�rke (pga. lokal avskogning),
ikke p� grunn av global oppvarming.
Skal du n� endelig svare p� svarene du f�r, eller bruker du bare
no.samfunn.naturvern som en spam-postkasse?
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(espen)