Vol.5 No.21, 10 October 2005

Global Warming 'past the point of no return'

By Steve Connor, Science Editor
Published: 16 September 2005

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

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
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.
Thesurface 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
anotherrecord 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
andthe 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.

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