Thin ice formed in late season expected to melt quickly; thick, multi-year ice continues to decline
By Summit Voice
SUMMIT COUNTY — Arctic sea ice extent peaked in late March, reaching its highest level in the past 10 years, according to the National Snow and Ice Data Center’s monthly update.While sea ice extent in March was higher than in recent years, most of the ice that formed late in the season is very thin and expected to melt rapidly.
The average March sea ice extent of 5.87 million square miles is ranked ninth lowest out of the 34 years of satellite data for the month, but it was the highest March average ice extent since 2008 and one of the higher March extents in the past decade.
Ice extent was 205,000 square miles below the 1979 to 2000 average extent, and 301,000 square miles above the record low for the month, set in 2006.
The ice cover remained extensive in the Bering Sea, where it has been above average all winter, in Baffin Bay, as well as in the Sea of Okhotsk. For much of the winter, air temperatures were 11 to 14 degrees below average in those regions.
In the Kara Sea, where ice extent had been below average during January and February, ice extent rebounded to near-average levels in March. Winds that had been pushing the ice cover back shifted, allowing areas of open water in the Kara Sea to freeze over and the ice to spread out.
Ice extent in the Barents Sea remained well below normal. In both the Barents and Kara seas, temperatures remained about 7 to 11 degrees Fahrenheit above average.
Overall, Arctic sea ice grew by about 54,000 square miles in March, which historically has been a month of net ice loss. But according to the NSIDC, the last three Marches have seen net ice growth.
The ice experts said they’re not sure why the maximum ice extent is occurring later given the overall trend of declining sea ice. The long-term trend for March is a decline of about 2.6 percent per decade.
It’s possible that the lower winter ice extent makes it easier for ice to continue growing later in the season. With a lower overall winter ice extent, a late cold snap or northerly wind could spread ice southward over ocean that would normally be ice-covered at that point.
Th researchers don’t expect the late maximum ice extent to strongly influence summer melt. The ice that grew late this winter is quite thin, and will melt rapidly as the sun rises higher in the sky and the air and water get warmer.
Arctic researchers try to predict the summer by looking at how much old, thick ice there is in the Arctic at the end of the winter. Some ice thickness data are available from satellites, but these records are short and discontinuous.
Data from the NASA ICESat satellite covers only 2003 to 2009, and the new European Space Agency CryoSat satellite began collecting data in 2011. So researchers look at ice age data as one indicator of Arctic sea ice thickness. Older ice that has survived multiple melt seasons tends to be thicker than newly formed ice.
Taken together, the data show that the ice cover remains much thinner than it was in the past, with a high proportion of first-year ice, which is thin and vulnerable to summer melt. After the record low minimum of 2007 the Arctic lost a significant amount of older, thicker ice, both from melting and from movement of ice out of the Arctic the following winter. In the last few years, the melt and export of old ice was less extreme than in 2007 and 2008, and multiyear ice started to regrow, with second and third-year ice increasing over the last three years.
However the oldest, thickest ice, more than four years old, continued to decline. Ice older than four years used to make up about a quarter of the winter sea ice cover, but now constitutes only 2 percent. First-year ice (0 to 1 years old) this year makes up 75 percent of the total ice cover, the third highest at this time of year in the satellite record. In 2008 the proportion of first-year ice was 79 percent, and in 2009 it was 76 percent.