Arctic ice thickness 'plummets'

Mark Kinver, BBC News 28 Oct 08;

The thickness of Arctic sea ice "plummeted" last winter, thinning by as much as 49 centimetres (1.6ft) in some regions, satellite data has revealed.

A study by UK researchers showed that the ice thickness had been fairly constant for the previous five winters.

The team from University College London added that the results provided the first definitive proof that the overall volume of Arctic ice was decreasing.

The findings have been published in the journal Geophysical Research Letters.

"The ice thickness was fairly constant for the five winters before this, but it plummeted in the winter after the 2007 minimum," lead author Katharine Giles told BBC News.

Sea ice in the Arctic shrank to its smallest size on record in September 2007, when it extended across an area of just 4.13 million sq km (1.59 million sq miles), beating the previous record low of 5.32 million sq km, measured in 2005.

The team from the university's Centre for Polar Observation and Modelling - part of the UK's National Centre for Earth Observation - found that last winter the ice had thinned by an average of 26cm (0.9ft) below the 2002-2008 winter average.

Dr Giles added that the data also showed the western Arctic experienced the greatest impact, where the ice thinned by up to 49cm (1.6ft).

Melting point

The recent record losses of ice cover in the Arctic has led to suggestions that the region could have reached a "tipping point" but some uncertainty over the causes had remained, explained co-author Seymour Laxon.

"The extent can change because the ice can be redistributed, increasing the amount of open water," he told BBC News. "But this does not reduce the overall amount of ice."


"To determine whether the reduction in sea ice extent is the result of ice being piled up against the coast or whether it is the result of melting, you need to measure the thickness."

"I think this is the first time that we can definitively say that the bulk overall volume of ice has decreased," observed Dr Laxon.

"So this means melting; it doesn't mean that the ice has just been pushed up against the coastline."

Dr Giles explained that the measurements gathered by satellite provided a continuous data-set and had a number of advantages over other methods.

"Drilling, submarines or aircraft; all of these techniques can be limited by time and space," she said.

"You can only sample relatively small areas, and you cannot have a continuous time series - it's a very harsh environment, so field experiments in winter are logistically difficult."

"We have been using satellite data, which means we get coverage all across the Arctic Ocean (apart from the very centre) and we get it continuously, so we have great coverage both in terms of time and area."

The measurements were recorded via a radar altimeter onboard the European Space Agency's (Esa) Envisat satellite.

The altimeter fires pulses of electromagnetic waves down on to the ice, which reflects them back up to a receiver on the satellite.

The time taken for the waves to complete this journey is recorded, and it is a fairly straightforward calculation to work out the height of the ice above sea level.

As one tenth of the ice sits above the water, it is then possible to work out the overall volume and thickness of ice in that location.

Dr Laxon said the project's findings are being used to help climate modellers refine their projections of what is going to happen in the future.

"The time when Arctic sea ice is going to disappear is open to a lot of debate," he said.

"About five years ago, the average projection for the sea ice disappearing was about 2080.

"But the ice minimums, and this evidence of melting, suggests that we should favour the models that suggest the sea ice will disappear by 2030-2040, but there is still a lot of uncertainty."

The researchers hope to keep the data series, funded by the EU and the Natural Environmental Research Council (Nerc), running for as long as satellite-based measurements are available.

Big decline in depth of Arctic winter sea ice
Juliette Jowit, The Guardian 28 Oct 08;

The thickness of sea ice in the Arctic dramatically declined last winter for the first time since records began in the early 1990s. The research by British scientists shows a significant loss in the thickness of the northern ice cap after the record loss of ice in the summer of 2007, although the weather was not abnormally warm.

The findings, published in the journal Geophysical Research Letters, raise the possibility that the loss of the Arctic sea ice could accelerate, because as the ice recedes the water temperature rises. This summer the sea ice recorded its second-lowest extent after the record low of 2007, again despite relatively cool air temperatures.

However, Katharine Giles of the Centre for Polar Observation and Modelling at University College London, who led the study, said it was too soon to say whether the downward trend would continue and lead to summer sea ice disappearing even faster than forecast. "It's dangerous to extrapolate out because colder weather would mean the ice could recover again," said Giles. "This data will help climate modellers to validate their models and make them more accurate."

The study, part-funded by the Natural Environment Research Council and the European Union, found the thickness of sea ice in the Arctic was almost unchanged in the five winters from 2002-6, but then declined 10%, or 26cm, last winter. In parts of the western Arctic, where the greatest loss was recorded the previous summer, the loss was nearly double the average.

But Vicky Pope, the Met Office's leading adviser to the government on climate change, warned: "There's clearly a decline over the last 30 years and we can detect a human signal in that, but the change in the last couple of years could be due to natural fluctuations in the weather."

Other causes of sea ice changes could include ocean currents and wind piling up ice, making it important to measure both thickness and extent to calculate total volume, said Giles.

Arctic ice thickness drops by up to 19 per cent
Paul Eccleston, The Telegraph 28 Oct 08;

The thickness of ice in large parts of the Arctic dropped by as much as 19 per cent last winter, according to scientists.

The average thickness fell by almost half a metre in some areas compared to the previous five winters.

After a slow downward trend since 2002 the rate suddenly increased after summer 2007 when the amount of ice dropped to its lowest level since records began.

Following the record low, the amount of sea ice has recovered after cooler summer temperatures so the thinner ice cannot be blamed entirely on warmer Arctic conditions.

Researchers from the Centre for Polar Observation and Modelling at University College London - part of the National Centre for Earth Observation - used satellite technology to measure sea ice thickness over the Arctic from 2002 to 2008.

On average the winter sea ice in the Arctic is two and half metres thick. Its depth 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 team was 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.

Their 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 North-West passage in the summer of 2007 become ice free and open to shipping for the first time in 30 years.

Dr Katharine Giles, who led the study said: "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?

"The western Arctic in the summer of 2007 saw the biggest changes in the amount of ice and this is where we saw the biggest changes in the thickness of the ice so it is hard to imagine that they are not connected.

"We saw an average decrease of 10 per cent which is pretty dramatic."

She said the loss of ice would reduce the Arctic's capacity to deflect sunlight and would lead to a larger areas of water which would absorb heat and produce warmer temperatures - leading to more ice melting.

Dr Seymour Laxon, who also took part in the study, said: "Ice can simply melt and disappear or if the wind changes it can move and be piled up against the shore when you would expect it to become thicker but we saw no evidence of this and rather than getting thicker, the ice was thinner."

Before this latest study, 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 but this is the first time scientists have been able to show that the ice thinning was widespread and occurred in areas of both young and old ice.