Flood risk from Antarctic ice 'overestimated'

Fred Pearce, New Scientist 14 May 09;

The precariously moored West Antarctic ice sheet probably won't collapse into the ocean all in one go as the climate warms. But the bad news, says a researcher, is that the sections most likely to be released into the ocean would raise sea levels globally by 3.3 metres – and rather more on the shores of North America.

The West Antarctic ice sheet, the smaller of the icy continent's two giant slabs of ancient ice, is moored to an archipelago of islands, many of them below sea level. It is held in place by floating ice shelves. Glaciologists had feared that warm water could melt the shelves, releasing the entire sheet into the ocean, raising sea levels by up to 5 metres.

Concern has increased with recent failures of floating Antarctic ice, such as the Wilkins shelf. A recent study (pdf format) found that the West Antarctic ice sheet would likely collapse if sea temperatures rose by more than 5 °Celsius.

But now Jonathan Bamber at Bristol University, UK, has analysed which parts of the West Antarctic ice sheet are vulnerable. He concludes that a third might stay put, mostly because it is moored above sea level. "The area's potential contribution to sea level has been greatly overestimated," he says.
Gravitational shift

Unfortunately, however, the loss of Antarctic ice would shift the Earth's gravitational pull, causing water to pile up in the northern hemisphere. Around North America this could increase anticipated sea-level rise by about a quarter.

In March, Bamber argued that the other giant ice sheet vulnerable to global warming, the Greenland ice sheet, is also more resistant to temperature rise that experts had thought.

But not all glaciologists think it has significantly changed the planet's immediate prognosis.

"The crucial question is how much ice could be lost in the next 100 to 200 years, and Jonathan's work has not really changed that," says David Vaughan of the British Antarctic Survey. Most predictions, he says, put global sea-level rise in the coming century at around 1 metre – but more will follow.

Journal reference: Science (DOI: 10.1126/science.1169335)

West Antarctic ice threat revised; still dire
Alister Doyle, Reuters 14 May 09;

OSLO (Reuters) - A meltdown of West Antarctica's ice sheet would raise sea levels by half as much as previously expected, but the impact would still be catastrophic, especially for U.S. coastal cities, a study showed.

A collapse of the ice sheet, viewed by scientists as more vulnerable than Greenland or East Antarctica because of global warming, would push up world sea levels by 3.3 meters (11 ft) over hundreds of years rather than 5-6 as long estimated.

"The long-term impact of West Antarctica is not be as serious as previously believed," said Jonathan Bamber, a professor at Bristol University in England who led the study in Friday's edition in the journal Science.

"But 17 million people in Bangladesh alone would be displaced by a sea level rise of 1.5 meters," he told Reuters. "The consequences for the planet and stability of society as a whole for even a 1-2 meters rise is very, very serious."

Sea levels off North America would rise more than anywhere else under the new projections, by Bamber and experts at University of Durham in England and Delft University of Technology in the Netherlands.

Antarctica's vast mass exerts a gravitational tug that raises water levels in the Southern Ocean. If that ice were to melt, computer models project that sea levels would rise fastest around North America, while falling in the Southern Ocean.

"Levels on the U.S. seaboards would rise 25 percent more than the global average and threaten cities like New York, Washington DC, and San Francisco," the University of Colorado at Boulder, where Bamber is now a visiting fellow, said in a statement.

ICEBERGS

West Antarctica is believed to be vulnerable because much of its ice rests on bedrock below sea level. Global warming, blamed by the U.N. Climate Panel on human use of fossil fuels, could let water seep in under the ice and make giant chunks buoyant.

Any such collapse would probably last hundreds years, leaving West Antarctica as a series of islands. Bamber said there is evidence that West Antarctica has collapsed in the past, perhaps as recently as 400,000 years ago.

The vast East Antarctic sheet, equivalent to about 50 meters of sea level rise, and Greenland, equivalent to 7 meters, rest on bedrock above sea level.

About 10 ice shelves further north on the Antarctic Peninsula have broken up in recent years, most recently part of the Wilkins Ice Shelf shattered into icebergs last month.

The U.N. Climate Panel projected in a 2007 report that world sea levels would rise by between 18 and 59 cm (7-24 inches) this century because of global warming -- excluding any accelerating thaw of Antarctica or Greenland.

Bamber's study, which updated data from a 1978 report that estimated a 5-6 meter rise, looked solely at the risks of collapse of West Antarctica. The pattern of sea level rise would be different -- especially in the Atlantic -- if Greenland shrank simultaneously.

(Editing by Philippa Fletcher)

Ice sheet melt threat reassessed
Mark Kinver, BBC News 14 May 09;

The collapse of a major polar ice sheet will not raise global sea levels as much as previous projections suggest, a team of scientists has calculated.

Writing in Science, the researchers said that the demise of the West Antarctic Ice Sheet (WAIS) would result in a sea level rise of 3.3m (10 ft).

Previous estimates had forecast a rise in the region of five to six metres.

However, they added, the rise would still pose a serious threat to major coastal cities, such as New York.

"Sea level rise is considered to be the one of the most serious consequence of climate change," lead author Jonathan Bamber told the Science podcast.

"A sea level rise of just 1.5m would displace 17 million people in Bangladesh alone," he added.

"So it is of the utmost importance to understand the potential threats to coastlines and people living in coastal areas."

Threat reassessed

Professor Bamber, from the University of Bristol's Glaciology Centre, said that the WAIS posed "potentially one of the most serious threats".

The world has three ice sheets, Greenland, East Antarctica and West Antarctica, but it is the latter that is considered most vulnerable to climatic shifts.

"It has been hypothesised for more than 30 years now that the WAIS is inherently unstable," he explained.

"This instability means that the ice sheet could potentially rapidly collapse or rapidly put a lot of ice into the oceans."

When the idea first emerged in the late 1970s, it was estimated that global sea level would rise by five metres if the WAIS collapsed.

Current projections suggest that a complete collapse of WAIS would result in an increase of up to six metres.

But Professor Bamber said that no-one had revisited the calculation, despite new data sets becoming available, and scientists developing a better understanding of the dynamics in the vast ice sheets.

The original estimates were based on "very basic ice thickness data", he explained.

"Ice thickness data gives you information about the depth of the bedrock underneath the ice sheet.

"Over the past 30 years, we have acquired much more ice thickness data over the whole of Antarctica, particularly over West Antarctica.

"We also have much better surface topography. Those two data sets are critical in determining two things."

The first was knowing the volume of ice that could contribute to sea level rise, and the second was a better understanding of the proportion of WAIS that was potentially susceptible to this instability.

Instead of assuming that the entire WAIS would collapse, causing sea level to rise by up to six metres, Professor Bamber and colleagues used models based on glaciological theory to simulate how the 2.2 million-cubic-km ice sheet would respond.

"Our reassessment of West Antarctica's contribution to sea level rise if the ice sheet was to collapse is about 3.3 metres," he said.

"That is about half of the value that has been quoted up until now."

The team's study also calculated what regions were likely to experience the biggest increases in sea level.

"Sea level rise is not uniform across the world's oceans, partly as a result of disruptions to the Earth's gravity field," explained Professor Bamber.

"It turns out that the maximum increase in sea level rise is centred at a latitude of about 40 degrees along the Atlantic and Pacific seaboards of North America."

This would include cities such as San Francisco and New York.

These areas could expect increases of one-and-a-quarter times the global average, the team estimated.

In other words, if the global average was one metre, then places like New York could expect to see a rise of 1.25m.

Responding to Professor Bamber's paper in Science, British Antarctic Survey science leader Dr David Vaughan described the findings as "quite sound".

"But for me, the most crucial question is not solely about the total amount of ice in West Antarctica, because that might take several centuries to be lost to the ocean," he told BBC News.

"The crucial question is how much ice could be lost in 100-200 years; that's the sea level rise we have to understand and plan for.

"Even with this new assessment the loss of a fraction of WAIS over those timescales would have serious consequences and costs that we've only really just begun to understand."

Study Halves Prediction of Rising Seas
Andrew C. Revkin, The New York Times 14 May 09

A new analysis halves longstanding projections of how much sea levels could rise if Antarctica’s massive western ice sheets fully disintegrated as a result of global warming.

The flow of ice into the sea would probably raise sea levels about 10 feet rather than 20 feet, according to the analysis, published in the May 15 issue of the journal Science.

The scientists also predicted that seas would rise unevenly, with an additional 1.5-foot increase in levels along the east and west coasts of North America. That is because the shift in a huge mass of ice away from the South Pole would subtly change the strength of gravity locally and the rotation of the Earth, the authors said.

Several Antarctic specialists familiar with the new study had mixed reactions to the projections.

But they and the study’s lead author, Jonathan L. Bamber of the Bristol Glaciology Center, in England agreed that the odds of a disruptive rise in seas over the next century or so from the buildup of greenhouse gases remained serious enough to warrant the world’s attention.

They also uniformly called for renewed investment in ice-probing satellites and field missions that could within a few years substantially clarify the risk.

There is strong consensus that warming waters around Antarctica, and Greenland in the Arctic, would result in centuries of rising seas. But glaciologists and oceanographers still say uncertainty prevails on the vital question of how fast coasts will retreat in a warming world in the next century or two.

The new study combined computer modeling with measurements of the ice and the underlying bedrock, both direct and by satellite.

It did not assess the pace or likelihood of a rise in seas. The goal was to examine as precisely as possible how much ice could flow into the sea if warming seawater penetrated between the West Antarctic ice sheet and the bedrock beneath.

For decades West Antarctic ice has been identified as particularly vulnerable to melting because, although piled more than one mile above sea level in many places, it also rests on bedrock a half mile to a mile beneath sea level in others. That topography means that warm water could progressively melt spots where ice is stuck to the rock, allowing it to flow more freely.

Erik I. Ivins, a scientist at NASA’s Jet Propulsion Laboratory, described the new paper as “good solid science,” but added that the sea-level estimates cannot be verified without renewed investment in satellite missions and other initiatives that are currently lagging.

A particularly valuable satellite program called Grace, which measures subtle variations in gravity related to the mass of ice and rock, “has perhaps a couple of years remaining before its orbit deteriorates,” Dr. Ivins said.

“The sad truth is that we in NASA are watching our earth-observing systems fall by the wayside as they age – without the sufficient resources to see them adequately replaced.”

Robert Bindschadler, a longtime specialist in polar ice at NASA’s Goddard Space Flight Center, said the study only provided a low estimate of Antarctica’s possible long-term contribution to rising seas because it did not deal with other mechanisms that could add water to the ocean.

The prime question, he said, remains what will happen in the next 100 years or so, and other recent work implies that a lot of ice can be shed within that time.

“Even in Bamber’s world,” he said, referring to the study’s lead author, “there is more than enough ice to cause serious harm to the world’s coastlines.”

Sea rise from Antarctic ice melt overestimated: study
Yahoo News 14 May 09;

CHICAGO (AFP) – While a collapse of the West Antarctic ice sheet will have devastating impacts on global sea levels, a study published Thursday found the anticipated impact has been seriously overestimated.

Using new measures of the ice sheet's geometry, British and Dutch researchers predict its collapse would cause sea levels to rise by 3.2 meters (11 feet) rather than previous estimates of five to seven meters.

However, the study published in the journal Science found that even a one meter rise in sea levels would be significant enough to weaken the Earth's gravity field in the southern hemisphere and affect the Earth's rotation.

That rotational shift would cause water to pile up in the northern oceans and could result in dramatic regional differences in sea levels, with the largest rise on the east and west coast of the United States.

"The pattern of sea level rise is independent of how fast or how much of the (Western Antarctic Ice Sheet) WAIS collapses," said lead author Jonathan Bamber of the University of Bristol in England.

"Even if the WAIS contributed only a meter of sea level rise over many years, sea levels along North America's shorelines would still increase 25 percent more than the global average."

Antarctica holds about nine times the volume of ice as Greenland and is considered a sleeping giant when it comes to sea levels.

The western ice sheet is of particular concern because enormous sections sit in inland basins on bedrock that is entirely below sea level.

Vast floating ice shelves currently limit ice loss to the ocean but scientists fear the sheet could collapse if the floating ice shelves break free.

The study authors based their predictions on the assumption that only ice on the downward sloping and inland-facing side of the basins would be lost while ice grounded on bedrock that is above sea level or slopes upward would survive.

Researchers do not know how quickly the shelf would collapse. But if such a large amount of ice melted steadily over 500 years it would raise sea levels by about 6.5 millimeters per year.

That's about twice the current rate due to all sources.

"Though smaller than past predictions, the scale of the fully manifested instability is enormous," cautioned Erik Ivins of the California Institute of Technology in an accompanying article.

"The total mass gained by the oceans ... would be roughly equal to the mass showered to Earth by the impact of about 2000 Halley-sized comets."

Further complicating the situation is the fact that Greenland seems to be losing as much or more ice than Antarctica, even though it doesn't have the same unstable configuration.

"Greenland needs only half the mass loss rate of Antarctica to have an equivalent effect on polar motion due to its less polar position," he wrote.

Even "more ominous" are the current accelerations of ice flow into the Amundsen Sea Embayment in Antarctica, he wrote.

"Should the ice sheet grounding line migrate farther inland, ice resting on bedrock well below sea level could become unstable."