Wednesday, March 7, 2012

Sea ice volume: Multi-year arctic sea ice reducing dramatically

A new study has noted a substantial decadal decline in the Arctic Multiyear ice cover. While ice extent has been shrinking, much of this is seasonal ice that reforms each winter and can grow with variable weather factors such as changes in temperature, wind, weather and ocean currents. A reduction in the thicker multiyear ice indicates a much greater impact and collapse of sea ice is evident.

Sea ice is made up of: seasonal ice that reforms each winter, perennial ice that has survived one summer, and multiyear ice which has lasted more than one year providing thickness to the sea ice. While all multi-year ice is perennial ice, not all perennial ice is multi-year ice.

Sea ice extent measures the area of the ice and can easily be done by satellite imagery. It is a measure of area and provides a useful 2D indicator of sea ice coverage. But for understanding of sea ice trends, it is important to measure the volume of ice extent which involves measuring the relative thickness of ice and assessing the type of ice involved (mult-year/perennial/seasonal). It is more complicated measuring sea ice thickness and the comparison of multiyear to single year ice, but can be done either by individual measurements from a ship, helicopter or by satellite microwave imagery.

NASA scientist Josephino Comiso has been studying sea ice coverage collected from 1980 to 2012, as observed by passive microwave sensors on NASA’s Nimbus-7 satellite and by the Special Sensor Microwave Imager/Sounder (SSMIS) from the Defense Meteorological Satellite Program (DMSP). According to his research perennial ice extent has been shrinking at a rate of –12.2 percent per decade, while its area is declining at a rate of –13.5 percent per decade. These numbers indicate that multiyear ice is declining faster than the perennial ice that surrounds it.

“The Arctic sea ice cover is getting thinner because it’s rapidly losing its thick component,” Comiso said. “At the same time, the surface temperature in the Arctic is going up, which results in a shorter ice-forming season. It would take a persistent cold spell for multi-year sea ice to grow thick enough again to be able to survive the summer melt season and reverse the trend.”

Field reports appear to confirm the satellite measurements with new European data indicating 2011 summer’s loss of ice cover matched the 2007 record.

Here is the abstract for the paper Large Decadal Decline of the Arctic Multiyear Ice Cover published in the American Meteorological Society Journal of Climate:

The perennial ice area was drastically reduced to 38% of its climatological average in 2007 but recovered slightly in 2008, 2009, and 2010 with the areas being 10%, 24%, and 11% higher than in 2007, respectively. However, trends in extent and area remained strongly negative at −12.2% and −13.5% decade−1, respectively. The thick component of the perennial ice, called multiyear ice, as detected by satellite data during the winters of 1979–2011 was studied, and results reveal that the multiyear ice extent and area are declining at an even more rapid rate of −15.1% and −17.2% decade−1, respectively, with a record low value in 2008 followed by higher values in 2009, 2010, and 2011. Such a high rate in the decline of the thick component of the Arctic ice cover means a reduction in the average ice thickness and an even more vulnerable perennial ice cover. The decline of the multiyear ice area from 2007 to 2008 was not as strong as that of the perennial ice area from 2006 to 2007, suggesting a strong role of second-year ice melt in the latter. The sea ice cover is shown to be strongly correlated with surface temperature, which is increasing at about 3 times the global average in the Arctic but appears weakly correlated with the Arctic Oscillation (AO), which controls the atmospheric circulation in the region. An 8–9-yr cycle is apparent in the multiyear ice record, which could explain, in part, the slight recovery in the last 3 yr.

Peter Sinclair's Climate Crock of the week from September 25 2011 beings us up to date with Arctic sea ice extent.

For an earlier report from March 2011 on sea ice extent and multiyear ice loss see Arctic Sea Ice extent winter maximum continues sea ice decline