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,"
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
"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.