[Reader-list] summer ice

[Nagraj Adve]

Summers free of Arctic ice would be devastating for Arctic
inhabitants, humans and other species, and could prove alarming for
the rest of the world. Seems it might happen sooner than we realize.
Naga


Ice coverage dwindling in extent and thickness


Sea ice in the Arctic is disappearing at a far greater rate than
previously expected, according to data from the first purpose-built
satellite launched to study the thickness of the Earth's polar caps.

Preliminary results from the European Space Agency's CryoSat-2 probe
indicate that 900 cubic kilometres of summer sea ice has disappeared
from the Arctic ocean over the past year.

This rate of loss is 50% higher than most scenarios outlined by polar
scientists and suggests that global warming, triggered by rising
greenhouse gas emissions, is beginning to have a major impact on the
region. In a few years the Arctic ocean could be free of ice in
summer, triggering a rush to exploit its fish stocks, oil, minerals
and sea routes.

Using instruments on earlier satellites, scientists could see that the
area covered by summer sea ice in the Arctic has been dwindling
rapidly. But the new measurements indicate that this ice has been
thinning dramatically at the same time. For example, in regions north
of Canada and Greenland, where ice thickness regularly stayed at
around five to six metres in summer a decade ago, levels have dropped
to one to three metres.

"Preliminary analysis of our data indicates that the rate of loss of
sea ice volume in summer in the Arctic may be far larger than we had
previously suspected," said Dr Seymour Laxon, of the Centre for Polar
Observation and Modelling at University College London (UCL), where
CryoSat-2 data is being analysed. "Very soon we may experience the
iconic moment when, one day in the summer, we look at satellite images
and see no sea ice coverage in the Arctic, just open water."

The consequences of losing the Arctic's ice coverage, even for only
part of the year, could be profound. Without the cap's white
brilliance to reflect sunlight back into space, the region will heat
up even more than at present. As a result, ocean temperatures will
rise and methane deposits on the ocean floor could melt, evaporate and
bubble into the atmosphere. Scientists have recently reported evidence
that methane plumes are now appearing in many areas. Methane is a
particularly powerful greenhouse gas and rising levels of it in the
atmosphere are only likely to accelerate global warming. And with the
disappearance of sea ice around the shores of Greenland, its glaciers
could melt faster and raise sea levels even more rapidly than at
present.

Professor Chris Rapley of UCL said: "With the temperature gradient
between the Arctic and equator dropping, as is happening now, it is
also possible that the jet stream in the upper atmosphere could become
more unstable. That could mean increasing volatility in weather in
lower latitudes, similar to that experienced this year."

CryoSat-2 is the world's first satellite to be built specifically to
study sea-ice thickness and was launched on a Dniepr rocket from
Baikonur cosmodrome, Kazakhstan, on 8 April, 2010. Previous Earth
monitoring satellites had mapped the extent of sea-ice coverage in the
Arctic. However, the thickness of that ice proved more difficult to
measure.

The US probe ICESat made some important measurements of ice thickness
but operated intermittently in only a few regions before it stopped
working completely in 2009. CryoSat was designed specifically to
tackle the issue of ice thickness, both in the Arctic and the
Antarctic. It was fitted with radar that can see through clouds.
(ICESat's lasers could not penetrate clouds.) CryoSat's orbit was also
designed to give better coverage of the Arctic sea.

"Before CryoSat, we could see summer ice coverage was dropping
markedly in the Arctic," said Rapley. "But we only had glimpses of
what was happening to ice thickness. Obviously if it was dropping as
well, the loss of summer ice was even more significant. We needed to
know what was happening – and now CryoSat has given us the answer. It
has shown that the Arctic sea cap is not only shrinking in area but is
also thinning dramatically."

Sea-ice cover in the Arctic varies considerably throughout the year,
reaching a maximum in March. By combining earlier results from ICESat
and data from other studies, including measurements made by submarines
travelling under the polar ice cap, Laxon said preliminary analysis
now gave a clear indication of Arctic sea-ice loss over the past eight
years, both in winter and in summer.

In winter 2004, the volume of sea ice in the central Arctic was
approximately 17,000 cubic kilometres. This winter it was 14,000,
according to CryoSat.

However, the summer figures provide the real shock. In 2004 there was
about 13,000 cubic kilometres of sea ice in the Arctic. In 2012, there
is 7,000 cubic kilometres, almost half the figure eight years ago. If
the current annual loss of around 900 cubic kilometres continues,
summer ice coverage could disappear in about a decade in the Arctic.

However, Laxon urged caution, saying: "First, this is based on
preliminary studies of CryoSat figures, so we should take care before
rushing to conclusions. In addition, the current rate of ice volume
decline could change." Nevertheless, experts say computer models
indicate rates of ice volume decline are only likely to increase over
the next decade.

As to the accuracy of the measurements made by CryoSat, these have
been calibrated by comparing them to measurements made on the ice
surface by scientists including Laxon; by planes flying beneath the
satellite's orbit; and by data supplied by underwater sonar stations
that have analysed ice thickness at selected places in the Arctic. "We
can now say with confidence that CryoSat's maps of ice thickness are
correct to within 10cm," Laxon added.

Laxon also pointed out that the rate of ice loss in winter was much
slower than that in summer. "That suggests that, as winter starts, ice
is growing more rapidly than it did in the past and that this effect
is compensating, partially, for the loss of summer ice." Overall, the
trend for ice coverage in Arctic is definitely downwards, particularly
in summer, however – a point recently backed by Professor Peter
Wadham, who this year used aircraft and submarine surveys of ice
sheets to make estimates of ice volume loss. These also suggest major
reductions in the volume of summer sea ice, around 70% over the past
30 years.

"The Arctic is particularly vulnerable to the impact of global
warming," said Rapley. "Temperatures there are rising far faster than
they are at the equator. Hence the shrinking of sea-ice coverage we
have observed. It is telling us that something highly significant is
happening to Earth. The weather systems of the planet are
interconnected so what happens in the high latitudes affects us all."