Arctic sea-ice thinning as well as receding

28 October 2008

Last winter, the thickness of sea ice in large parts of the Arctic fell by nearly 19 per cent compared with the previous five winters. This followed the dramatic 2007 summer low when Arctic ice extent dropped to its lowest level since records began.

arctic ice

New satellite data shows that last winter the average thickness of sea-ice in the Arctic fell by 26cm.

Up until last winter, the thickness of Arctic sea-ice showed a slow downward trend during the previous five winters. But after summer 2007, when its extent reached a record low, the thickness of the ice also nose-dived. Scientists are now concerned that sea-ice is not just receding - it is also thinning.

Some scientists blamed the record summer 2007 ice extent low on unusually warm weather conditions over the Arctic, but this summer, sea ice extent reached the second lowest level since records began, even though the Arctic had a relatively cool summer.

Dr Katharine Giles, who led the study and is based at the Centre for Polar Observation & Modelling at University College London - part of the National Centre for Earth Observation, says, "This summer's low ice extent doesn't seem to have been driven by warm weather, so the question is, was last winter's thinning behind it?"

This summer's low ice extent doesn't seem to have been driven by warm weather, so was last winter's thinning behind it?

- Dr Katharine Giles, University College London

The team of researchers, including Dr Seymour Laxon and Andy Ridout, used satellites to measure sea-ice thickness over the Arctic from 2002 to 2008. Winter sea-ice in the Arctic is around two and half metres thick on average. Ice thickness is calculated from the time it takes a radar pulse to travel from a satellite to the surface of the ice and back again.

The research - reported in Geophysical Research Letters - showed that last winter the average thickness of sea-ice over the whole Arctic fell by 26cm (10 per cent) compared with the average thickness of the previous five winters, but sea-ice in the western Arctic lost around 49cm of thickness. This region of the Arctic saw the Northwest Passage become ice free and open to shipping for the first time in 30 years during the summer of 2007.

Plugging the gaps

The team is the first to measure ice thickness throughout the Arctic winter, from October to March, over more than half of the Arctic, using the European Space Agency's Envisat satellite.

The European Space Agency's Envisat satellite

The European Space Agency's Envisat satellite

Before this, Christian Haas of the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, had discovered thinner ice in a small region around the North Pole.

Whilst the overall loss of older, thicker ice led researchers to speculate that Arctic sea-ice had probably thinned, this is the first time scientists have been able to say for definite that the ice thinning was widespread and occurred in areas of both young and old ice.

"The extent of sea-ice in the Arctic is down to a number of factors, including warm weather melting it as well as currents and the wind blowing it around, so it's important to know how ice thickness is changing as well as the extent of the ice," adds Giles.

The researchers will continue to monitor the thickness of the ice over this coming winter. Laxon says, "We'll be keeping our eyes on the ice thickness this winter as it'll be interesting to see what happens after a second summer of low ice extent."

Ice satellite to the rescue

The Envisat satellite that provided the UCL scientists with their data doesn't cover the whole of the North Pole. Because of the satellite's orbit, there's a hole north of 81·5 degrees, which is about 600 miles shy of the North Pole.

The Cryosat satellite

The CryoSat satellite, designed to measure ice thickness using radar

But a team, including Laxon, at the Centre for Polar Observation & Modelling has designed a satellite - CryoSat-2 - to plug this hole.

CryoSat-2 is the first radar satellite specifically designed to measure ice thickness. It will do this with greater resolution than is possible with Envisat and so will give scientists a much more detailed picture of what is happening to ice in the Arctic. CryoSat-2 is being prepared for launch at the end of 2009.

'Circumpolar thinning of Arctic sea ice following the 2007 record ice extent minimum' - KA Giles, SW Laxon, and AL Ridout (2008). Geophysical Research Letters, doi:10.1029/2008GL035710.