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By Roger Harrabin Environment analyst
The Arctic has lost more sea ice this year than at any time since satellite
records began in 1979, Nasa says.
Scientists involved in the calculations say it is part of a fundamental change.
What is more, sea ice normally reaches its low point in September so it is
thought likely that this year's melt will continue to grow.
Nasa says the extent of sea ice was 1.58m sq miles (4.1m sq km) compared with a
previous low of 1.61m sq miles (4.17m sq km) on 18 September 2007.
The sea ice cap grows during the cold Arctic winters and shrinks when
temperatures climb again, but over the last three decades, satellites have
observed a 13% decline per decade in the summertime minimum.
The thickness of the sea ice is also declining, so overall the ice volume has
fallen far - although estimates vary about the actual figure.
Joey Comiso, senior research scientist at Nasa's Goddard Space Flight Center,
said this year's ice retreat was caused by previous warm years reducing the
amount of perennial ice - which is more resistant to melting. It's created a
self-reinforcing trend.
"Unlike 2007, temperatures were not unusually warm in the Arctic this summer.
[But] we are losing the thick component of the ice cover," he said. "And if you
lose [that], the ice in the summer becomes very vulnerable."
'Inevitable death'
Walt Meier, from the National Snow and Ice Data Center that collaborates in the
measurements, said: "In the context of what's happened in the last several
years and throughout the satellite record, it's an indication that the Arctic
sea ice cover is fundamentally changing."
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The summer ice volume is now only 30% of what it was in the 1980s
Prof Peter Wadhams Cambridge University
Professor Peter Wadhams, from Cambridge University, told BBC News: "A number of
scientists who have actually been working with sea ice measurement had
predicted some years ago that the retreat would accelerate and that the summer
Arctic would become ice-free by 2015 or 2016.
"I was one of those scientists - and of course bore my share of ridicule for
daring to make such an alarmist prediction."
But Prof Wadhams said the prediction was now coming true, and the ice had
become so thin that it would inevitably disappear.
"Measurements from submarines have shown that it has lost at least 40% of its
thickness since the 1980s, and if you consider the shrinkage as well it means
that the summer ice volume is now only 30% of what it was in the 1980s," he
added.
"This means an inevitable death for the ice cover, because the summer retreat
is now accelerated by the fact that the huge areas of open water already
generated allow storms to generate big waves which break up the remaining ice
and accelerate its melt.
Infographic
"Implications are serious: the increased open water lowers the average albedo
[reflectivity] of the planet, accelerating global warming; and we are also
finding the open water causing seabed permafrost to melt, releasing large
amounts of methane, a powerful greenhouse gas, to the atmosphere."
Threats and opportunities
Opinions vary on the date of the demise of summer sea ice, but the latest
announcement will give support to those who err on the pessimistic side.
Greenpeace activist boards oil rig, 24 August 2012 Greenpeace is strongly
opposing attempts to exploit the Arctic
A recent paper from Reading University used statistical techniques and
computers to estimate that between 5-30% of the recent ice loss was due to
Atlantic Multi-decadal Oscillation - a natural climate cycle repeating every
65-80 years. It's been in warm phase since the mid 1970s.
But the rest of the warming, the paper estimates, is caused by human activity -
pollution and clearing of forests.
If the ice continues to disappear in summer there will be opportunities as well
as threats.
Some ships are already saving time by sailing a previously impassable route
north of Russia.
Oil, gas and mining firms are jostling to exploit the Arctic - although they're
being strongly opposed by environmentalists. Greenpeace has been protesting at
drilling by the Russian giant Gazprom.
Among the many threats, the warming is bad for Arctic wildlife. Thanks to the
influence of sea ice on the jet stream the changes could affect weather in the
UK.
The changes - if they happen - could unlock frozen deposits of methane which
would further overheat the planet.
Warmer seas could lead to more melting of Greenland's ice cap which would
contribute to raising sea levels and changing the salinity of the sea, which in
turn could alter ocean currents that help govern our climate.