New satellite data confirms major Arctic ice loss
- 14 Feb 2013, 15:30
- Roz Pidcock
Scientists know from decades of measurements that climate change
is causing Arctic sea ice to shrink. But a new satellite that
estimates ice volume is allowing scientists to examine sea ice in
three-dimensions - providing a more accurate picture of how ice
cover is changing. The volume of sea ice in Autumn - just after its
lowest point in the year - has shrunk by more than a third over the
past decade, the data show.
The European Space Agency's (ESA)
CryoSat-2 satellite has been orbiting the earth since 2010. The
satellite measures the thickness of Arctic sea ice and uses the
data to estimate the volume, and the UK-led team behind the project
has just
published the first two years of data from the mission.
Such satellite volume measurements have been eagerly awaited by
scientists. At a time when the sea ice is
shrinking as the region warms, the data provide an important
new source of information for scientists tracking the effect of
climate change at the top of the planet.
Dr Nathan Kurtz, polar scientist at NASA, told Carbon Brief:
"The [publication of the new results] is
a huge step forward in terms of our knowledge of changing Arctic
sea ice thickness and volume."
Rapid ice loss
Satellite data show Arctic sea ice extent (or area) is
decreasing by about four
per cent per decade. In September 2012, Arctic sea ice reached
its lowest
extent since satellite records began in 1979. The CryoSat-2
data mean that scientists, for the first time, can confirm that the
loss in sea ice area has been accompanied by a loss in volume.
New record low Arctic sea ice extent, reached in September
2012, compared to the average summer minimum extent for the last 30
years in yellow. Source:
NASA
The scientists compared the new CryoSat-2 data with estimates of
ice volume made by the NASA ICESat satellite between 2003 and 2008.
The comparison shows the average volume of sea ice in October and
November fell by 36 per cent between the periods 2003-08 and
2010-12. Average ice volume also decreased in winter, when more of
the ocean is covered by ice, by nine per cent.
The fall in volume is partly a result of there being less older,
thicker multi-year ice. Co-author of the new paper Dr Catherine
Giles explains:
"The data reveals that thick sea ice has
disappeared from a region to the north of Greenland, the Canadian
Archipelago, and to the northeast of Svalbard."
Volume, not just area
During the gap in coverage between the ICESat and CryoSat-2
missions, independent measurements from aircraft, instruments
anchored to the sea floor and a NASA project called IceBridge
have measured ice thickness, but not volume.
Between satellite missions, a team of scientists at the Polar
Ice Centre at the University of Washington has been using a
computer model to estimate how ice volume is changing.
The scientists have used the model, known as the Pan-Arctic
Ice-Ocean Modelling and Assimilation System (
PIOMAS), to predict when the Arctic is likely to become
ice-free in summer. Most recent projections put it within the
next few decades.
Comparing the new CryoSat-2 data with the earlier ICESat data
helps scientists test how well PIOMAS is doing at projecting ice
volume. Kurtz told us:
"PIOMAS had been showing declining ice
volume for quite a while now, but to date this has not been
validated (studies were done comparing it to ICESat, but this was
considered too short a time period to draw conclusions)."
But, Kurtz added, the addition of CryoSat-2 data means
scientists can be more confident that the PIOMAS projections are
realistic:
"The new ICESat and CryoSat-2 time
series of sea ice volume change provides increased confidence in
the PIOMAS data set which shows a loss of sea ice volume over a
much longer time period."
The declining trend in Arctic sea ice between 1979 and 2012
as simulated by the PIOMAS model. Source:
Polar Science Centre
Some important differences exist between the PIOMAS model and
the measurements. The autumn melting observed by CryoSat-2 is 60
per cent greater than PIOMAS expected. Dr Axel Schweiger from the
Polar Ice Centre notes:
"We knew from earlier comparisons with
observations that PIOMAS tended to underestimate the thickness of
thick ice and was therefore conservative in its assessment of
volume loss. The comparison with CryoSAT confirms this view."
Antarctic sea ice
Satellites have also been measuring Antarctic sea ice extent for
a number of decades now. But unlike in the Arctic, Antarctic
sea ice is growing - reaching a record
extent in September this year.
As George Hale from NASA's IceBridge project told Carbon Brief,
scientists have been measuring sea ice thickness in Antarctica as
well as the Arctic, but it's not an easy task.
"[T]here are some technical hurdles to
measuring sea ice thickness in the Southern Ocean that don't really
exist in the Arctic. Part of it is snow accumulation and warming
and refreezing cycles that occur there that make reading our radar
signals difficult."
At the southern end of the planet, most of the ice is on land,
contained in the vast Antarctic ice sheet. Satellite data shows
from 1992 to 2011, the Antarctic ice sheet lost more than
70 billion tonnes of ice.
The future
Polar scientists are hopeful that CryoSat-2 heralds a new level
of scientific understanding of the Arctic sea ice. Schweiger told
us the mission "marks the beginning of a new era of continual
space-based ice thickness measurements".
And as scientists start to get more data from CryoSat-2, they
should be able to improve reconstructions of past changes using
tools like PIOMAS. Axel Schweiger is optimistic that the new data
can give "a better understanding of the past [that] will help us
build better models to predict the future".
-------------------------
Updated on 15th February 2013 to include an additional quote
from Dr Kurtz just before the graph showing the PIOMAS
trend.