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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.

 Arctic _sea _ice _extent

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."

 

BPIOMASIce Volume Anomaly Current V2

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".

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Updated on 15th February 2013 to include an additional quote from Dr Kurtz just before the graph showing the PIOMAS trend.

 

 

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