Antarctic ice loss vaster, faster than thought: study

Yahoo News 22 Nov 09;

PARIS (AFP) – The East Antarctic icesheet, once seen as largely unaffected by global warming, has lost billions of tonnes of ice since 2006 and could boost sea levels in the future, according to a new study.

Published Sunday in Nature Geoscience, the same study shows that the smaller but less stable West Antarctic icesheet is also shedding significant mass.

Scientists worry that rising global temperatures could trigger a rapid disintegration of West Antarctica, which holds enough frozen water to push up the global ocean watermark by about five metres (16 feet).

In 2007 the UN Intergovernmental Panel for Climate Change (IPCC) predicted sea levels would rise 18 to 59 centimetres (7.2 to 23.2 inches) by 2100, but this estimate did not factor in the potential impact of crumbling icesheets in Greenland and Antarctica.

Today many of the same scientist say that even if heat-trapping CO2 emissions are curtailed, the ocean watermark is more likely to go up by nearly a metre, enough to render several small island nations unlivable and damage fertile deltas home to hundreds of millions.

More than 190 nations gather in Copenhagen next month to hammer out a global climate deal to curb greenhouse gases and help poor countries cope with its consequences.

University of Texas professor Jianli Chen and colleagues analysed nearly seven years of data on ocean-icesheet interaction in Antarctica.

Covering the period up January 2009, the data was collected by the twin GRACE satellites, which detect mass flows in the ocean and polar regions by measuring changes in Earth's gravity field.

Consistent with earlier findings based on different methods, they found that West Antarctica dumped, on average, about 132 billion tonnes of ice into the sea each year, give or take 26 billion tonnes.

They also found for the first time that East Antarctica -- on the Eastern Hemisphere side of the continent -- is likewise losing mass, mostly in coastal regions, at a rate of about 57 billion tonnes annually.

The margin or error, they cautioned, is almost as large as the estimate, meaning ice loss could be a little as a few billion tonnes or more than 100.

Up to now, scientists had thought that East Antarctica was in "balance," meaning that it accumulated as much mass and it gave off, perhaps a bit more.

"Acceleration of ice loss in recent years over the entire continent is thus indicated," the authors conclude. "Antarctica may soon be contributing significantly more to global sea level rise."

Another study published last week in the journal Nature reported an upwardly-revised figure for Antarctic temperatures during prior "interglacials", warm periods such as our own that have occurred roughly every 100,000 years.

During the last interglacial which peaked some 128,000 years ago, called the Eemian Period, temperatures in the region were probably six degree Celsius (10.8 degrees Fahrenheit) higher than today, which is about 3 C (5.4 C) above previous estimates, the study said.

The findings suggest that the region may be more sensitive than scientists thought to greenhouse gas concentrations in the atmosphere that were roughly equivalent to present day levels.

During the Eemian, sea levels were five-to-seven metres higher than today.

East Antarctic ice began to melt faster in 2006: study
Nina Chestney, Reuters 22 Nov 09;

LONDON (Reuters) - East Antarctica's ice started to melt faster from 2006, which could cause sea levels to rise sooner than anticipated, according to a study by scientists at the University of Texas.

In the study published in Nature's Geoscience journal, scientists estimated that East Antarctica has been losing ice mass at an average rate of 5 to 109 gigatonnes per year from April 2002 to January 2009, but the rate speeded up from 2006.

The melt rate after 2006 could be even higher, the scientists said.

"The key result is that appear to start seeing a large amount of ice loss in East Antarctica, mostly in the long coastal regions (in Wilkes Land and Victoria Land), since 2006," Jianli Chen at the university's center for space research and one of the study's authors, told Reuters.

"This, if confirmed, could indicate a state change of East Antarctica, which could pose a large impact on global sea levels in the future," Chen said.

Previous estimates for East Antarctica projected anywhere between a 4 gigatonne per year loss and a 22 gigatonne per year gain, according to the report.

The full study is available at www.nature.com/ngeo.

Climate change is turning Antarctica's ice into the one of the biggest risks for coming centuries. Even slight melting could drive up sea levels and could affect world's cities.

Rising temperatures are thought to be the main cause of melting ice, and world leaders are under pressure to agree on a new climate treaty at an upcoming U.N. summit in Copenhagen to curb global warming.

MELTDOWN

The scientists used satellite observations of gravity change over the period April 2002 to January 2009 to calculate the rate of the ice loss in East Antarctica's coastal regions.

The ice sheet's mass has long been difficult to estimate.

"At various times, estimates have disagreed on the sign of the mass balance, as well as its magnitude," the report said.

The whole Antarctic region could be losing ice at a rate of 113-267 gigatonnes a year, with 106-158 gigatonnes coming from West Antarctica, the scientists estimate.

A separate study on Thursday found that melting ice from Greenland and Antarctica will lead to a much sharper rise in sea levels than previously thought.

Climate change will cause a rise of at least 1 meter in sea levels by the end of this century, according to a review of scientific data by environmental group Clean Air-Cool Planet.

The projection is in sharp contrast to a 2007 study by the U.N.'s Intergovernmental Panel on Climate Change, which said world sea levels could increase 18-59 centimeters by 2100.

(Reporting by Nina Chestney; Editing by Angus MacSwan)

East Antarctica 'is losing ice'
Richard Black, BBC News website 22 Nov 09;

The East Antarctic ice sheet has been losing mass for the last three years, according to an analysis of data from a gravity-measuring satellite mission.

The scientists involved say they are "surprised" by the finding, because the giant East Antarctic sheet, unlike the west, has been thought to be stable.

Other scientists say ice loss could not yet be pinned on climate change, and uncertainties in the data are large.

The US-based team reports its findings in the journal Nature Geoscience.

The data comes from Nasa's Gravity Recovery and Climate Experiment (Grace) mission.

Grace has previously shown that the smaller West Antarctic and Greenland ice sheets are losing mass.

These two bodies of ice contain enough water to raise sea levels by about six to seven metres (20ft) each if they melted completely.

Melting the East Antarctic sheet would raise sea levels by much more - about 50-60m.

But scientists have generally discounted the possibility of it happening because the region is so cold.

The Grace measurements suggest there was no net ice loss between 2002 and 2006.

But since then, East Antarctica has been losing 57 billion tonnes (Gt) per year.

"We felt surprised to see this change in East Antarctica," study leader Jianli Chen from the Centre for Space Research at the University of Texas in Austin told BBC News.

The loss still looks small by contrast with West Antarctica, which is losing 132Gt per year, and with Greenland, where a recent analysis combining Grace data with other measurements indicated an annual figure of 273Gt.

Previous Grace analyses - and those from other satellites - had given an inconclusive picture for the giant ice body.

The twin Grace satellites fly in close formation, detecting minute changes in the Earth's gravity through the marginal changes this causes in their relative positions.

Eastern energy

Measuring Antarctic ice loss is a tricky issue because the continent itself is rising and deforming.

Its ice cover was significantly thicker during the last Ice Age; as the ice melted, the weight pressing down on the rock abated, and the rock is "isostatically rebounding".

Readings from satellite missions have to be adjusted to allow for this rebound - and that is one source of uncertainty when trying to assess the significance of the new research, according to Richard Alley, one of the world's leading glaciologists.

"The first thing is that lots of this is dependent on the isostatic [rebound] model, and (recent work has) cast some doubt on the istostatic models that people are using," commented the Penn State University researcher (who was not involved in the paper).

"And then you get into the age-old question of 'is it climate or is it weather?'

"So it energises me as a scientist, but I'm not convinced that as yet it should energise anyone else."

Rising potential

The Grace data gives a picture of where ice is being lost across the continent; and these areas are mainly on the coast.

It is not clear what physical processes could be driving any loss of mass here, although it is not simply melting due to high air temperatures, because temperatures are well below zero.

One clue could lie in research published last year by Leigh Stearns and colleagues, showing that lakes under the ice sheet can periodically overflow, with the liquid water then acting as a lubricant to speed glaciers on their way towards the sea.

Commenting on the new research, Dr Stearns told BBC News: "In these coastal regions the ice loss could be driven by some interaction with the oceans or some weather patterns, or it could be a sub-glacial lake that drained and caused some thinning - so it might not be climate-related.

"It's easy to jump to the conclusion that it's exceptional because it's the first time we've recorded it, but we do need a baseline of how things have been in the past so we do need to be cautious," said the University of Kansas researcher.

"Nevertheless, it awakens us to the fact that the East Antarctic sheet is more dynamic than we thought, and we do need to pay attention to it because its potential for sea level rise is so much greater than in West Antarctica or Greenland."

Dr Chen said that one of his team was currently conducting airborne surveys of one of the regions where mass loss had been detected, hoping to shed some light on the mechanisms involved.