Evidence from an Antarctic geological research drilling program known as ANDRILL suggests that the vast East Antarctic Ice Sheet, which holds at least four-fifths of the continent’s ice, is more susceptible to melting than previously thought and that an abrupt shrinkage of its ice sheets at some greenhouse gas threshold is possible, perhaps beginning within in this century.

The southern McMurdo Sound core yielded clear evidence of some 74 cycles of ice sheet buildup and retreat during a 6-million-year stretch starting in the Miocene Epoch some 20 million years ago. According to geologist Robert DeConto of the University of Massachusetts, Amherst, the policy implications are grim, as many population centers worldwide are within a few meters of sea level.

Our models may be dramatically underestimating how much worse it’s going to get. We’re seeing ice retreat faster and more dramatically than any model predicts.

The answer to the puzzling disparity between model predictions and the core data could lie in an erroneous assumption about Antarctica itself. Some parts of the land underlying the East ice sheet might be much lower than currently believed. As warming oceans strip away the surrounding ice shelves, significant chunks of the ice sheet could slide into the ocean.

A prior core, extracted from the McMurdo Ice Shelf between October 2006 and January 2007, indicated that the West Antarctic Ice Sheet has frequently advanced and retreated.

An article in Pysorg.com reports that a huge iceberg has broken off of Antarctica:

An iceberg the size of Luxembourg knocked loose from the Antarctic continent earlier this month could disrupt the ocean currents driving weather patterns around the globe, researchers said Thursday.

Another iceberg known as B9B, which had been jammed against the Antarctic continent for more than 20 years, began to drift and smashed into the Metz tongue.

I found this satellite picture of the event at the European Space Agency website:

The 2550 square-kilometer (985 square-mile) block broke off on February 12 or 13 from the Mertz Glacier Tongue, a 160-kilometer spit of floating ice protruding into the Southern Ocean from East Antarctica due south of Melbourne. B9B is a remnant of a 2,000-square-mile iceberg that calved in 1987, making it one of the largest icebergs ever recorded in Antarctica.

The resulting new iceberg, along with B9B, have since drifted into an adjoining area called a ploynya – an area that produce dense water, super cold and rich in salt, that sinks to the bottom of the sea and drives the conveyor-belt like circulation around the globe. The Metz Glacier Polynya is particularly strong and accounts for 20 percent of the “bottom water” in the world.

Benoit Legresy, a French glaciologist who works at the Laboratory for Geophysics and Oceanographic Space Research in Toulouse and who has been monitoring the Metz glacier, explains how the icebergs could possibly disrupt ocean circulation patterns:

[I]f they stay in this area – which is likely – they could block the production of this dense water, essentially putting a lid on the polynya.

The Pine Island glacier in west Antarctica – one of the largest glaciers in Antarctica – is thinning four times faster than it was 10 years ago. Satellite measurements show the surface of the ice is now dropping at a rate of up to 16m a year. At this rate, the glacier could be gone within 100 years.

Scientists fear that the collapse of the Pine Island glacier could lead to a rapid disintegration of the West Antarctic ice sheet (WAIS). Professor Jason Box of Ohio State University said :

It’s like removing a cork from a bottle.

The WAIS is fundamentally far less stable than the Greenland ice sheet because most of it is grounded far below sea level. Its collapse could result in 20-30cm of sea level rise.

Glaciers are rapidly melting in North America, too. A new U.S. Geological Survey report shows the South Cascade Glacier in Washington state has lost nearly half of its volume and a quarter of its mass since 1958. The two others in the study, the Wolverine and Gulkana glaciers in Alaska, have both lost nearly 15% of their mass.

The big news over the weekend was the collapse of the ice bridge which pinned the Wilkins Ice Shelf in place since the beginning of recorded history.

The context of that event is the warming of Antarctia.

Antarctic ice is melting far faster than believed possible. Updated scientific modeling on global warming projects up to one-third of all Antarctic sea ice is likely to melt by the end of the century and that ice melts in the Antarctic Peninsula and Western Ice Shelf will be greater and more rapid than expected, contributing significantly to dangerous sea level rises.

Ice shelves float on the water, so breakages will not directly raise sea levels. But when ice shelves break up, the glaciers and landed ice behind them then slide towards the ocean more rapidly, thus raising sea levels.

The West Antarctic Ice Sheet (WAIS) is fundamentally far less stable than the Greenland ice sheet because most of it is grounded far below sea level. The WAIS rests on bedrock as deep as two kilometers underwater. The collapse of ice shelves provides exit routes for ice from further inland.

Joseph Romm sees this as a potentially catastrophic feedback loop:

The warmer it gets, the more unstable WAIS outlet glaciers will become. Since so much of the ice sheet is grounded underwater, rising sea levels may have the effect of lifting the sheets, allowing more – and increasingly warmer – water underneath, leading to further bottom melting, more ice shelf disintegration, accelerated glacial flow, and further sea level rise, and so on and on, another vicious cycle. The combination of global warming and accelerating sea level rise from Greenland could be the trigger for catastrophic collapse in the WAIS.

You can see the ice bridge and how it pins the Wilkins ice sheet together on this map.

The International Polar Year (IPY) survey has found that warming in the Antarctic is much more widespread than was thought. When the survey began in 2007, Greenland and Antarctica’s land areas were viewed as largely stable despite some worrying signs of fringe melting.

Scientists found that the ocean around the Antarctic has warmed more rapidly than the global average. Shifts in temperature patterns deep underwater are causing the continent’s land ice sheet to melt faster than previously known.

Both the Greenland and the Antarctic ice sheets are losing mass and thus raising sea levels, and the rate of ice loss from Greenland is growing. Arctic sea ice has decreased to its lowest level since satellite records began.

Rising sea levels and changes in ocean temperatures triggered by the melting ice are expected to cause shifts in weather patterns worldwide and more damaging coastal storm surges.

IPY director David Carlson said:

“We’re beginning to get hints of change in ocean circulation, that’ll have a dramatic impact on the global climate system.”

A paper coming out in Nature this week, authored by two of the folks at RealClimate, shows that Antarctica has been warming for the last 50 years, and that it has been warming especially in West Antarctica.

The authors argue that the warming basically has to do with enhanced meridional flow — there is more warm air reaching West Antarctica from farther north (that is, from warmer, lower latitudes). This change in atmospheric circulation goes along with reduced sea ice in the region (while sea ice in Antarctica has been increasing on average, there have been significant declines off the West Antarctic coast for the last 25 years, and probably longer). And this is self reinforcing (less sea ice, warmer water, rising air, lower pressure, enhanced storminess).

A recent paper in Nature Geosciences by Gillet et al. examined trends in temperatures in the both Antarctic and the Arctic and concluded that “temperature changes in both … regions can be attributed to human activity.”

The European Space Agency reports new rifts have developed on the Wilkins Ice Shelf that could lead to the opening of the ice bridge that has been preventing the ice shelf from disintegrating and breaking away from the Antarctic Peninsula.

As Antarctica summer is about to begin, the Wilkins Ice Shelf is experiencing significant changes which could lead to a new break-up. The ESA site has posted animated photos showing the gradual opening of fractures during the last days, in particular on 26 November 2008.

In the past 20 years, seven ice shelves along the Antarctic Peninsula have retreated or disintegrated, including the most spectacular break-up of the Larsen B Ice Shelf in 2002.

Wilkins is the size of the state of Connecticut, or about half the area of Scotland. It is the largest ice shelf on the Antarctic Peninsula yet to be threatened. The ice shelf had been stable for most of the past century before it began retreating in the 1990s.