Wilkins ice bridge shatters

It had been long expected. It happened on Sunday, 5 April 2009.

Where is it, what happened and what significance does it have?

The first is easy. The Wilkins ice shelf is between three islands off the west coast of the Antarctic Peninsula:

Figure 1: Wilkins ice shelf – location

Here is a cropped map from Wikipedia showing more detail:

Figure 2: Wilkins Sound in context

The area immediately north of the thin bridge between Charcot and Latady Islands is shown as open sea. In fact it is covered in sea ice, remnant from earlier collapses and held in by the bridge. Following are two images from Earth Observatory (I’ve reversed the order):

Figure 3: Wilkins ice bridge 31 March, 2009

Figure 4: Collapsed ice bridge 6 April, 2009

The sun is at a different angle in Figure 4 showing up the gaps and cracks more clearly. Almost the whole ice bridge has broken up, not just the narrow bit. Some sizeable splinters are floating away to the south as newly carved icebergs.

Now have a look at this image via commenter mauri pelto at RealClimate. Focus on the crack above the word “Wilkins”. It was taken on April 2, before the collapse, and shows a large area of the ice shelf near Latady Island as already unstable.

Figure 6: Cracks near Latady Island

There is an animation of the collapse here courtesy of RealClimate.

This youtube is as good as any, though I suspect the footage predates this actual event.

Newsy.com has an interesting news take slipping in the ‘it could mean 6 metres of sea level rise by the end of the century’ meme.

The ice cliffs shown are said to be 60 feet high. On RN Breakfast Dr Ted Scambos said that the ice shelf was as high as a 70 storey building, meaning I think total thickness, which would figure with the 20 metres or so sticking out of the water.

At the time it cracked the bridge had thinned to about 500 metres at the narrowest point.

This is apparently the tenth major ice shelf collapse event in recent times and the closest to the South Pole. The shelf has been there for over 10,000 years.

Since it is floating ice the collapse means nothing in terms of immediate sea level rise. The usual story is that ice shelves buttress ice behind them and after they collapse glaciers are freed up and can flow up to 7 times faster. This happened where the Larsen B ice shelf collapsed. However, Wilkins has nothing much in the way of glaciers behind it. Nevertheless the area will obviously warm more if the ice is dispersed.

Some desperate to see a reason other than AGW have suggested volcanoes. Gavin Schmidt at RealClimate suggests that the maths on the heat involved just don’t add up.

Go here if you want to see what’s happening to sea ice coverage. In the west where the action is it’s reducing. The Antarctic continent is so large that generalisations about what’s happening to the whole area are pretty pointless. If you want to understand more about the sea ice, try Jinlun Zhang’s paper Increasing Antarctic sea Ice under Warming Atmospheric and Oceanic Conditions. The title may be all you need.

But overall I understand that both the sea and the land surface temperatures are warming in trend terms, although the picture in terms of time and place is a bit complex.

This is the long term situation according to new work by Eric Steig and others, described at RealClimate. They found that there has been warming in both west and east Antarctica. This is the sexy version of the image showing the quite distinct warming trend covering the whole of West Antarctica (from NASA Earth Laboratory):

Figure 7: Antarctic warming 1957-2006

That image also well illustrates why the West Antarctic (WAIS) and the East Antarctic ice sheets (EAIS) are considered separate ice sheets. EAIS has higher elevation and is much colder than WAIS.

That was for the period 1957 to 2006. Along the way there has been a cooling story, as can be seen from this image from the RealClimate post representing 1969-2000:

Figure 8a: Antarctic temperature anomalies 1969-2000

During the period of satellite instrumentation, this one (1982-2004) also from NASA also shows cooling:

Figure 8b: Antarctic temperature anomalies 1982-2004

The cooling period is associated with changed circulation patterns linked with large quantities of ozone from the 1970s.

A more recent version using improved techniques shows widespread warming over both land and sea for the period 1981-2007:

Figure 9: Antarctic temperature anomalies 1981-2007

This comment at RealClimate identifies the decadal temperature increases as 0.17C for WAIS, 0.1C for EAIS and 0.12C for global Antarctica. So Antarctica as a whole has warmed 0.6C in 50 years and WAIS 0.86C. This is significant but less than the 3C or more sometimes quoted.

This is where the ice shelves are:

Figure 10: Antarctic ice shelves

I understand that the ice shelves outside the Antarctic Peninsula are not yet seriously in play, but this is where it’s melting according to NASA from Wikipedia:

Figure 11: Antarctic melting

The top image shows the dates that areas of the Antarctic Ice Sheet started melting. The lower image shows the number of days that any area melted in 2005 This is summer surface melt measured by passive microwave satellite. “Started melting” means the first registered melt event by the satellite.

This one from Chris Rapley via The Oil Drum shows that it doesn’t snow much inland from the perimeter:

Figure 12: Antarctic precipitation

The colour scheme is not the best, but the precipitation is around the perimeter while the interior is desert.

This one from the same source shows where and how fast the ice flows:

Figure 13: Antarctic ice movement

Recently it has been discovered that there is a system of rivers and lakes under the ice sheet UNEP Year Book, 2009, Ch3:

Figure 14: Antarctic subglacial lakes

Not much volcanic activity shown there! Scientist don’t yet fully understand the implications of this subglacial water movement.

Finally, again from Rapley again we see the different topography of rock under the Antarctic Peninsular, the West Antarctic ice sheet (WAIS) and the East Antarctic ice sheet.

Figure 15: Topography of subglacial Antarctica

Parts of WAIS are up to 2500 m below sea level.

I think it’s plain that the WAIS is vulnerable and that Wilkins is just part of a developing story with ice shelf collapses like canaries in the coal mine. The problem with WAIS is that it is exposed to and potentially undercut by warming waters, hence it could go more quickly than Greenland, which is roughly saucer shaped.

A recent article by Rignot et al (behind the pay wall) indicates a near zero ice loss for the EAIS, a 59% increase in the loss in the last 10 years to 2006 for the WAIS and a 140% increase for the Antarctic Peninsula. This article cites the Rignot article as indicating a 75% increase in the loss overall in 10 years.

Jo Romm at Climate progress says the ice loss from WAIS increased 75% from 2006 to 2007 alone. I can’t read the articles behind the pay wall, but I’d like to see that confirmed.

Certainly the Pine Island Glacier seems to be in trouble according to a report in Der Spiegel:

Satellite measurements taken between 1992 and 1996, though, show a loss of 1.6 meters in thickness per year on the Pine Island Glacier – a figure that represents 42 times the average melt of the past 4,700 years.

If the 2006-2007 melt stated at Climate Progress is confirmed and continues at a similar rate in say the next 5 years, then the situation is alarming. Certainly my estimate last year of two metres by 2100 which is bad enough would have to be revised. Here’s another thought. A recent study by Stammer reckons that a flux of meltwater will take 50 years or more to have much effect the other side of the equator. A flux from Greenland could be 30 times greater in parts of the North Atlantic than in the Pacific. This image from the UNEP shows how it hugs the coast of the US:

Figure 16: Greenland meltwater dispersion 10 years later, from UNEP Year Book, 2009

If he’s right, bad news for New York, but the world might finally take global warming seriously.

BTW we’ve had Firetree flood maps but here’s a new toy. Wave your cursor over the map and go from there.