Arctic sea-ice struggles to build volume

There is likely to be about 10,500 cu km of Arctic sea-ice by the end of the week – a volume that would tie for the lowest on record for a November.

It is another indicator of just how warm conditions in the polar north have been of late. Temperatures of -5C have been logged when -25C would be the norm.

Ice extent – the two-dimensional measure of frozen ocean surface – is also well down, running currently at just over 9.4 million sq km. Ordinarily, it would be at least a million sq km higher.

The latest volume assessment comes from the Earth-orbiting Cryosat mission. This European Space Agency satellite carries a radar altimeter designed specifically for the purpose of studying marine floes. At present, it is the only way to monitor sea-ice volume across the entire Arctic basin.

Average sea-ice thickness stands today at roughly 130cm. This represents the 5th thickest November in the Cryosat record, meaning the low volume is pretty much all down to the low extent. This is most evident at southerly latitudes in the Beaufort, East Siberian and Kara seas, where the warm October/November conditions have been very keenly felt.

‘When the Autumn freeze-up occurs, it is the low latitudes that would normally see the fastest growth in ice, in areas of open water or over thin floes. But because of the warm autumn, these regions simply haven’t been able to build this volume,’ said Rachel Tilling from the Nerc Centre for Polar Observation and Modelling at Leeds University, UK.

Cryosat’s volume determination on Monday was for 10,200 cu km. Scientists expect perhaps 300 cu km to be added to floes in the last days of the month.

The resulting 10,500 cu km would then match what was seen in November 2011 and 2012, taking account of the errors that exist in such measurements.

‘There is no doubt that sea-ice growth this autumn has been sluggish and with Cryosat we’ve witnessed the smallest November growth on record,’ explained Dr Tilling.

‘Usually, it grows by about 160 cu km per day, but this November it’s been 139 cu km per day – just under 10% lower.’

Volume is the key metric when considering the status of Arctic sea-ice. Only thinking about extent, or area, can hide the fact that winds will sometimes spread out the floes and sometimes pile them up. Talking in terms of volume captures more of the overall behaviour in the Arctic system.

Cryosat gauges volume by first measuring the difference in height between the top of the floating part of the sea-ice and the sea surface.

Knowing the size of this ‘freeboard’ enables scientists to estimate the overall thickness of the ice. Multiplying by the area then gives gives the volume.

‘Being able to measure sea-ice thickness means that we can be sure how much is actually there, because changes in melting, snowfall, and drift all affect how thick the ice pack is and this is hidden in maps of extent alone,’ emphasised Leeds University’s Andrew Shepherd, the principal scientific adviser to the Cryosat mission.

‘It’s also of interest for ships attempting to navigate the Arctic’s ice-infested waters. Because Cryosat is the only sensor able to do this, it’s an essential tool for climate scientists and maritime operators alike.’