Rapid disintegration of another ice shelf on the Antarctic Peninsula
The ice shelves that skirt the Antarctic continent are broad plates of floating ice, typically 200-300 m thick at their outer margins. Because these are in direct contact with the underlying ocean and atmosphere, they are much more sensitive to climate change than the grounded ice sheet. Recent events along the Antarctic Peninsula have demonstrated that ice shelves can change very rapidly in a warming world.
In late February 2008, satellite images from NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Terra and Aqua satellites, revealed that the Wilkins Ice Shelf had started an abrupt break-up linked to regional warming (see figure). The Wilkins Ice Shelf is situated on the south-western flank of the Antarctic Peninsula (at 70.25°S, 73.0°W), about 1600 km south of South America. This event was the latest in a series of similar extraordinary ice shelf disintegrations that have stunned glaciologists by their speed and size, and have become iconic indicators of rapid change to the world's ice. For example, the size of the Larsen A Ice Shelf (on the eastern flank of the Peninsula) decreased over many years before collapsing completely in about a week at the end of January 1995; and 3250 sq km of the adjoining Larsen B Ice Shelf disintegrated in just five weeks in 2002. The Wilkins Ice Shelf is more than 500 km further south than Larsen B, but lies on the warmer, western side of the Peninsula.
Series of four NASA MODIS visible satellite images (spatial resolution 250 m) showing the Wilkins Ice Shelf as it began to break up (left), from 28 February to 8 March 2008. The ice shelf location is shown in the inset. The image on the right is a high-resolution, enhanced colour image of a region of the ice-shelf break-up on 8 March 2008 (marked on the MODIS image from that date), showing narrow linear icebergs (about 150 m across) crumbling into house-sized ice rubble as a result of the break-up. This image, from Taiwan's Formosat-2, has a ground dimension of 3.2 x 1.8 km.
MODIS images courtesy of the US National Snow and Ice Data Center (NSIDC). Formosat-2 image courtesy of NSIDC/Dr Cheng-Chien Liu, National Cheng Kung University(NCKU), Taiwan, and Taiwan's National Space Organization; processed at Earth Dynamic System Research Center at NCKU, Taiwan.
These abrupt collapses are linked to intense summer surface melt in the years prior to break-up, associated with a recorded regional air temperature rise of nearly 3ºC over the past 50 years – up to six times greater than the global average. Several models for these break-ups have been proposed, including a hydro-fracturing process whereby an ice shelf, probably thinned and weakened from enhanced melt on its underside, is broken into blocks by the pressure from large volumes of meltwater filling crevasses. Additional factors may come from 'toppling blocks' within the fractured ice shelf and natural weakening by rift formation.
The collapse of the Wilkins Ice Shelf was precipitated by the calving of a long, thin iceberg (41 x 2.5 km in size) from the shelf's southwestern front between 28 and 29 February. This triggered a runaway disintegration of 405 sq km of the shelf behind and led to the calving of thousands of small icebergs over the next few days. The sky-blue pattern of exposed deep glacial ice seen in the figure is characteristic of rapid climate-induced ice shelf disintegrations, and was also seen on the Larsen B Ice Shelf in 2002. As of 23 March 2008, Wilkins Ice Shelf was pinned in place by a narrow beam of ice only six kilometres wide that extended from Charcot Island. When this beam gives way (probably in the summer of 2008-09), a sizeable part of the remaining 13 200 sq km of the Wilkins Ice Sheet will likely rapidly break-up, possibly by the same 'disintegration' process.
This disintegration will not contribute directly to global sea level rise, as the ice released to melt was already afloat. After the Larsen B Ice Shelf collapsed, several of the glaciers feeding into it, no longer constrained by back pressure from the shelf, accelerated significantly, discharging more grounded ice into the ocean and contributing to a slight sea level rise. This is not expected to happen if (when) the Wilkins Ice Shelf collapses completely, as there are no sizeable glaciers feeding it. However, the recent ice shelf break-up events around the Antarctic Peninsula have removed features that appear to have been stable over several centuries to millennia – with major physical and ecological ramifications.
This latest event underlines the complexity of an Antarctic system undergoing rapid change, and highlights the need to more fully understand the processes responsible for such rapid collapse and its impact on the flow of grounded ice behind the ice shelves. With this in mind we are examining the potential role of enhanced wave-ice interaction, due to a reduced sea ice cover, in precipitating the break-up. The Antarctic Peninsula may well be a model for a warmer Antarctica – with future changes occurring at a greater scale and speed, than was previously considered possible.
ROBERT MASSOM1, IAN ALLISON1 and TED SCAMBOS2
1 AAD and ACE CRC
2 US National Snow and Ice Data Center, Boulder, Colorado, USA