Scientists have revealed that winter Arctic sea ice has been the least extensive on record this year, and it’s volume could also set a new low.
The extent of Arctic sea ice is a headline grabbing measurement, but its volume is in fact the best measure of the health of the marine ice floes.
Scientists waiting on Arctic sea ice volume data
Data beamed back to Earth from Europe’s Cryosat spacecraft shows that the thickness of sea-ice is hovering around a “minimum-maximum.” However there is a chance for further growth to occur this season, and scientists must wait until April before they have a definitive answer.
The extent of the ice is now known and the area of ocean covered by ice is falling from a winter peak, say scientists from the US National Snow and Ice Data Center (NSIDC) and NASA.
March 24 marked the largest average extent at 14.52 million sq km. In 2015 a satellite-era record was set at 14.54 million sq km. While it may appear that 2016 sets a new record, the difference is so small that it falls within the errors built in to the measurement.
Cryosat spacecraft provides valuable data
Arctic ice growth looked as though it would be sluggish for a long time. Air temperatures in the region were 2C-6C above average through December, January and February in nearly every region. This prevented the freeze-up which is usually expected during the dark months of polar winter.
The extent of the ice only measures two dimensionally, while Cryosat can take 3D measurements. The European Space Agency uses a radar altimeter to work out how thick the floes are, and thus calculate the volume of the ice when combined with the area and extent data.
So far Cryosat has noticed slow growth during the Arctic winter, but the ice still retains thick multi-year ice to the north of Canada.
Mass gains despite melting in warmer areas
As a result the maximum volume this winter looks to be close to that of 2013, which boasted a peak of 24,800 cubic km. This set a “minimum maximum¨” which followed a huge summer melt in the Arctic.
Scientists must wait to find out for sure because the greatest volume tends to happen days, if not weeks, after ice extent peaks. Ice in the coldest areas can continue to gain mass even as that in warmer areas erodes.
“Once Arctic sea ice has reached its maximum area for the year it continues to thicken for about a month as seawater beneath the ice freezes to the under-ice surface. We expect the ice to reach its maximum amount towards the end of March or start of April,” explained Rachel Tilling, a Cryosat researcher at the Centre for Polar Observation and Modelling (CPOM) at University College London, UK.
“But the ice has only thickened by about 8cm since February, when usually it thickens by twice this amount.”
Prof Andy Shepherd, the senior scientific advisor to the Cryosat mission, told BBC News: “Early March saw slow growth and if that is maintained then I suspect the winter volume will end up being lower than in 2013 – but it will be very marginal.
“This does all illustrate though why we need a three-dimensional view of the ice. Strong winds can cause the same amount of sea-ice to pile up in a smaller than usual area, which you would miss if you considered just the extent of the floes – how widely the ice is spread across the ocean.
“And if this happens, the ice can actually become more resilient to summer melting depending on where it ends up, which emphasises the importance of knowing how thick it is, too,” the Leeds University expert said.
