Page last updated at 18:05 GMT, Saturday, 21 February 2009

Arctic team targets key ice data

By Jonathan Amos
Science reporter, BBC News

Sledge and SPRITE (Martin Hartley)
The yellow SPRITE radar is the expedition's key piece of kit

The retreat of Arctic summer sea-ice in recent years has been dramatic. The past two summers, in particular, have seen record low coverage by mid-September.

And it is not just the area spied by satellites that has undergone this rapid change; the thickness of the ice appears to have experienced an equally significant shrinkage.

Many of the computer models that have tried to forecast these changes have been caught out by events.

Most simulations predict ice-free conditions in summer arriving somewhere between 2040 and 2100.

Route discussion (Martin Hartley)
Prof Maslowski (centre-right) discusses the route with Pen Hadow (centre-left)

On current trends, the real world is on a faster track.

The British Catlin Arctic Survey, which gets under way this month, will attempt to gather important new data about the state of the ice in winter and early spring - when the ice reaches its greatest extent.

The expedition will make measurements of ice thickness along a more-than-1,000km track, from 81 degrees North to the North Pole.

It is intended to give scientists the very latest "ground truth", to better constrain their models and their interpretation of the observations coming from satellites.

"No other information on ice thickness like this is expected to be made available to the scientific community in 2009," explained Arctic ice modeller Professor Wieslaw Maslowski, a science advisor to the survey.

"We would like to use this information in real time to determine our skill in representing the observed ice thickness," he told BBC News.

Route map (BBC)

Professor Maslowski is affiliated to the US Navy's Department of Oceanography at the Postgraduate School in Monterey, California.

He will synthesize the Catlin observations and the latest meteorological data with his high resolution Arctic ice model output.

It should give him an improved understanding of how maximum ice conditions relate to - and influence - the summer minimum.

"It is commonly agreed in the community that the ice thickness distribution at the spring determines the amount of summer melt occurring in the middle of September, where we reach the minimum ice extent based on satellite observations," he said.

Ultimately, Professor Maslowski hopes to finesse his forecast for when the first ice-free summer might arrive.

Currently, he has it down for 2013 - but with an uncertainty range between 2010 and 2016.

Certain assumptions

Measuring the area change in Arctic sea ice is relatively straightforward using spacecraft data; but getting at the thickness is not so easy.

Scientists can - and do - make in-situ measurements; and there is also an array of buoys dotted about the Arctic that autonomously records this information.

In addition, upward-looking sonar from submarines has also provided an invaluable data-set.

And in recent years the overflying satellites have themselves used a neat trick to estimate thickness by measuring the height of that portion of the ice which sticks above the water.

As nine-tenths of the ice lies below the water line, a simple calculation will provide a good grasp of the likely thickness.

But none of these approaches is perfect. It is impractical to do direct measurements everywhere; and the broader satellite method depends on there being cracks in the ice to see the water surface, and in any case relies on a number of assumptions, including the density of the ice.

The Catlin Arctic Survey should fill important data gaps for 2009.


Meet the Catlin Arctic Survey team

The survey is led by the renowned Arctic explorer Pen Hadow. His team-members are Ann Daniels and Martin Hartley. The trio will pull a small radar device, known as SPRITE, across the ice.

The suitcase-sized instrument, attached to the rear of a sledge, can measure the snow and ice thickness every 10cm.

Drilling into the ice (Martin Hartley)
Drilling will confirm the radar data but will also give access to the water

"We've been measuring ice thickness with radar for quite a while but the equipment has been bulky and not very mobile," said SPRITE's designer, Michael Gorman.

"The challenge has been to make something really lightweight - about 4.5kg - and which is really low power - about 18W - because the team will have minimal power on the trip."

At frequent intervals, the team will also gather manually the same data by drilling through the ice. This will put a calibration check on the radar measurements.

Over the course of the expedition, the team hopes to have gathered millions of readings.

Warm water

Professor Maslowski's group is well known for producing results that show much faster ice loss than other modelling teams.

He believes those other models have seriously underestimated some key melting processes - in particular, the way southern warm waters have moved into the Arctic basin.

Professor Maslowski points to the heat brought from the Pacific through the Bering Strait and the Chukchi Sea.

This, he says, is distributed northward by local currents and eddies from the Chukchi Shelf into the Beaufort Sea where it has played a significant role in the major retreat of summer sea-ice in the western Arctic.

"If you think about a piece of ice and you start heating it from above but not from below, this might be an analogue to what [other models] are doing," he explained.

"What we say, based on our understanding, is that heating underneath is just as important as heating from the top. And if you melt the ice from both sides, you can melt much more ice, much faster."

Arctic ice (Martin Hartley)

Of keen interest then will be any data Pen Hadow can gather on ocean conditions. It will be a good test.

When the explorer drills the ice to make a physical measurement of thickness, he will use the hole to lower a 4kg conductivity-temperature device (CTD) into the ocean below.

It will extend down some 250m.

"Every half-metre, it takes a measurement of the temperature and the salinity," said Mr Hadow.

"It is generally thought that there is a critical layer of water below the surface which is warmer than what lies above it; and that this warmer water is making its way up to melt the underside of the ice.

"We will aspire to sample the water column everyday," he told BBC News.

According to the US National Snow and Ice Data Center (NSIDC), Arctic sea-ice covered 4.5 million sq km (1.7 million sq miles) at its lowest point on 12 September last year.

That compared with the 2007 minimum of 4.1 million sq km (1.6 million sq miles).

These figures are the lowest since satellite records began 30 years ago.

Using spacecraft data to make an Arctic-wide assessment of sea ice thickness , a University College London team found the 2008 winter maximum to be about 10% (26cm) below the 2002-2008 average.

Some areas in the western Arctic had thinned by up to 19%.

In recent times, average winter Arctic thickness has been about 2.5m, down from 3.5-4.0m based on climatological data.

In its early stages, the Catlin survey will move over some of the thickest, longest-lived ice. Towards the end, it will experience some of the single-year ice - ice that would not normally be expected to survive summer conditions.

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