Greenland ice sheet losses double

Digital elevation models for Greenland and Antarctica The team has produced elevation models for the ice sheets

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A new assessment from Europe's CryoSat spacecraft shows Greenland to be losing about 375 cu km of ice each year.

Added to the discharges coming from Antarctica, it means Earth's two big ice sheets are now dumping roughly 500 cu km of ice in the oceans annually.

"The contribution of both ice sheets together to sea level rise has doubled since 2009," said Angelika Humbert from Germany's Alfred Wegener Institute.

"To us, that's an incredible number," she told BBC News.

In its report to The Cryosphere journal, the AWI team does not actually calculate a sea-level rise equivalent number, but if this volume is considered to be all ice (a small part will be snow) then the contribution is likely to be on the order of just over a millimetre per year.

This is the latest study to use the precision altimetry data being gathered by the European Space Agency's CryoSat platform.

CryoSat (Esa) Cryosat uses a radar instrument to measure the shape of polar ice surfaces

The satellite was launched in 2010 with a sophisticated radar instrument specifically designed to measure the shape of the polar ice sheets.

The AWI group, led by senior researcher Veit Helm, has taken just over two years' worth of data centred on 2012/2013 to build what are called digital elevation models (DEMs) of Greenland and Antarctica, and to asses their evolution.

These models incorporate a total of 14 million individual height measurements for Greenland and another 200 million for Antarctica.

When compared with similar data-sets assembled by the US space agency's IceSat mission between 2003 and 2009, the scientists are able then to calculate changes in ice volume beyond just the CryoSat snapshot.

Negative shifts are the result of surface melting and ice discharge; positive trends are the consequence of precipitation - snowfall.

Greenland is experiencing the biggest reductions in elevation currently, losing about 375 cu km a year (plus or minus 24 cu km per year), with most of the action occurring at the west and south-east coast of the island.

Significant thinning is seen also in the North East Greenland Ice Stream (NEGIS).

"This has three outlet glaciers and one of these, the Zachariae Isstrom, has retreated quite a bit and some volume loss has already been reported. But we see now that this volume loss is really propagating to upper areas, much further into the interior of the ice sheet than has been recorded before," explained Prof Humbert.

Elevation change The change in height of Greenland's ice sheet between January 2011 and January 2014

In Antarctica, the annual volume loss is about 128 cu km per year (plus or minus 83 cu km per year).

As other studies have found, this is concentrated in the continent's western sector, in the area of the Amundsen Sea Embayment.

Big glaciers here, such as Thwaites and Pine Island, are thinning and retreating at a rapid rate.

Some thickening is seen also, such as in Dronning Maud Land, where colossal snowfalls have been reported. But this accumulation does not offset the losses occurring in West Antarctica.

A British-led group recently reported its own Antarctica DEM, using a different algorithm to process the numbers in the CryoSat data.

The AWI outcomes look very similar, and the German team has transferred the exact same approach to Greenland so it can have confidence in comparing the two ice sheets.

The losses also look consistent with the analysis coming out of the American Grace mission, which uses a different type of satellite to monitor gravity changes in the polar regions - to, in essence, weigh the amount of ice being dumped into the sea.

Prof Andy Shepherd, who was part of the British group that reported its findings in May, commented: "This is yet another exciting result from CryoSat, thanks to the team at AWI, charting yet more new ground by providing the first complete survey of ice volume changes in Greenland.

"However, the increased ice losses that have been detected are a worrying reminder that the polar ice sheets are still experiencing dramatic changes, and will inevitably raise concerns about future global sea-level rise," the Leeds University researcher said.

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos

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