A record loss
of sea ice in the Arctic this summer has convinced
scientists that the northern hemisphere may have crossed
a critical threshold beyond which the climate may never
recover. Scientists fear that the Arctic has now entered
an irreversible phase of warming which will accelerate
the loss of the polar sea ice that has helped to keep
the climate stable for thousands of years.
They believe
global warming is melting Arctic ice so rapidly that the
region is beginning to absorb more heat from the sun,
causing the ice to melt still further and so reinforcing
a vicious cycle of melting and heating.
The greatest
fear is that the Arctic has reached a "tipping point"
beyond which nothing can reverse the continual loss of
sea ice and with it the massive land glaciers of
Greenland, which will raise sea levels dramatically.
Satellites
monitoring the Arctic have found that the extent of the
sea ice this August has reached its lowest monthly point
on record, dipping an unprecedented 18.2 per cent below
the long-term average.
Experts
believe that such a loss of Arctic sea ice in summer has
not occurred in hundreds and possibly thousands of
years. It is the fourth year in a row that the sea ice
in August has fallen below the monthly downward trend -
a clear sign that melting has accelerated.
Scientists
are now preparing to report a record loss of Arctic sea
ice for September, when the surface area covered by the
ice traditionally reaches its minimum extent at the end
of the summer melting period.
Sea ice
naturally melts in summer and reforms in winter but for
the first time on record this annual rebound did not
occur last winter when the ice of the Arctic failed to
recover significantly.
Arctic
specialists at the US National Snow and Ice Data Centre
at Colorado University, who have documented the gradual
loss of polar sea ice since 1978, believe that a more
dramatic melt began about four years ago.
In September
2002 the sea ice coverage of the Arctic reached its
lowest level in recorded history. Such lows have
normally been followed the next year by a rebound to
more normal levels, but this did not occur in the
summers of either 2003 or 2004. This summer has been
even worse. The surface area covered by sea ice was at a
record monthly minimum for each of the summer months -
June, July and now August.
Scientists
analysing the latest satellite data for September - the
traditional minimum extent for each summer are preparing
to announce a significant shift in the stability of the
Arctic sea ice, the northern hemisphere's major "heat
sink" that moderates climatic extremes.
"The changes
we've seen in the Arctic over the past few decades are
nothing short of remarkable," said Mark Serreze, one of
the scientists at the Snow and Ice Data Centre who
monitor Arctic sea ice.
Scientists at
the data centre are bracing themselves for the 2005
annual minimum, which is expected to be reached in
mid-September, when another record loss is forecast. A
major announcement is scheduled for 20 September. "It
looks like we're going to exceed it or be real close one
way or the other. It is probably going to be at least as
comparable to September 2002," Dr Serreze said.
"This will be
four Septembers in a row that we've seen a downward
trend. The feeling is we are reaching a tipping point or
threshold beyond which sea ice will not recover."
The extent of
the sea ice in September is the most valuable indicator
of its health. This year's record melt means that more
of the long-term ice formed over many winters - so
called multi-year ice - has disappeared than at any time
in recorded history.
Sea ice
floats on the surface of the Arctic Ocean and its
neighbouring seas and normally covers an area of some 7
million square kilometres (2.4 million square miles)
during September - about the size of Australia. However,
in September 2002, this dwindled to about 2 million
square miles - 16 per cent below average.
Sea ice data
for August closely mirrors that for September and last
month's record low - 18.2 per cent below the monthly
average - strongly suggests that this September will see
the smallest coverage of Arctic sea ice ever recorded.
As more and
more sea ice is lost during the summer, greater expanses
of open ocean are exposed to the sun which increases the
rate at which heat is absorbed in the Arctic region, Dr
Serreze said.
Sea ice
reflects up to 80 per cent of sunlight hitting it but
this "albedo effect" is mostly lost when the sea is
uncovered. "We've exposed all this dark ocean to the
sun's heat so that the overall heat content increases,"
he explained.
Current
computer models suggest that the Arctic will be entirely
ice-free during summer by the year 2070 but some
scientists now believe that even this dire prediction
may be over-optimistic, said Professor Peter Wadhams, an
Arctic ice specialist at Cambridge University.
"When the ice
becomes so thin it breaks up mechanically rather than
thermodynamically. So these predictions may well be on
the over-optimistic side," he said.
As the sea
ice melts, and more of the sun's energy is absorbed by
the exposed ocean, a positive feedback is created
leading to the loss of yet more ice, Professor Wadhams
said.
"If anything
we may be underestimating the dangers. The computer
models may not take into account collaborative positive
feedback," he said.
Sea ice keeps
a cap on frigid water, keeping it cold and protecting it
from heating up. Losing the sea ice of the Arctic is
likely to have major repercussions for the climate, he
said. "There could be dramatic changes to the climate of
the northern region due to the creation of a vast
expanse of open water where there was once effectively
land," Professor Wadhams said. "You're essentially
changing land into ocean and the creation of a huge area
of open ocean where there was once land will have a very
big impact on other climate parameters," he said.
source...
http://news.independent.co.uk/world/science_technology/article312997.ece
