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.

thickness 層 厚度;濃度;密度

centimetres =centimeter 公分

regions地區,地帶;行政區域  部,部位

revealed展現,顯露出

fairly 公平地,公正地;正當地 頗為,相當地

previous 先的,前的,以前的

provided 提供

definitive 決定性的;最後的 最可靠的

volume 體積;容積 大量,許多

journal 日報;雜誌;期刊 日記;日誌

shrank 收縮,縮短,皺縮 變小,變少,變瘦

impact壓緊;擠滿 對...產生影響

tipping 頂端;尖端

uncertainty 不確定;不確信;易變;不可靠  不確定的事物;難以預料的事物

redistributed重新分配;再分配

determine 決定 是...的決定因素;形成;影響

piled up 積累 堆放起來

melting融化;熔化

definitively 決定性地;最後地

bulk 體積,容積 巨大的東西,大塊,大團

pushed up 向上推

Drilling鑽孔

submarines 潛艇

aircraft航空器;飛機,飛艇[M]

limited 限制;限定

sample取...的樣品;抽樣檢查

harsh 嚴厲的;嚴酷的,惡劣的

logistically 邏輯地;運籌地

terms 期,期限   學期  任期

via 經由;取道 通過,憑藉

radar 雷達

altimeter 測高儀,高度計

electromagnetic 電磁(體)的

pulses 脈衝;脈衝波

reflects 反射;照出,映出

straightforward 一直向前的;徑直的 明確的,肯定的

calculation 計算 估計,推測,預測

refine 提煉,精鍊;精製  使昇華

debate辯論,討論,爭論

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