Recent satellite images collected by the University of ÃÛÌÇÖ±²¥ at Boulder-based National Snow and Ice Data Center indicate a section of a large ice shelf on the Antarctic Peninsula has broken away.
The section of ice shelf that broke off was about 40 kilometers long and five kilometers wide, according to Ted Scambos, a research associate at the Cooperative Institute for Research in Environmental Sciences, a joint institute of ÃÛÌÇÖ±²¥-Boulder and the National Oceanic and Atmospheric Administration. NSIDC researchers spotted the event within a few days of its occurrence in late February after analyzing images obtained with a radiometer aboard a polar-orbiting NOAA satellite.
A second image from March 23 confirmed the disintegrated area of ice on a shelf known as the Larsen B Ice Shelf is about 200 square kilometers.
"The Feb. 26 image shows that much of the ice was already gone," said Scambos. "The March 23 image made it crystal clear that a significant portion of the ice shelf had broken off."
The satellite pictures appear to confirm earlier studies by the British Antarctic Survey that predicted the 12,000 square kilometer ice shelf was nearing its stability limit. Researchers believe it has retreated too far to be able to brace itself against the rocky peninsulas and islands that flank it.
If the model is correct, the ice shelf will continue to crumble rapidly beginning early next year, said Scambos. Although no more reduction is expected until summer begins again in Antarctica in late December, "This may be the beginning of the end for the Larsen Ice Shelf," said Scambos.
Antarctic Peninsula ice shelves have been in rapid retreat for the last few decades, apparently in response to a regional climate warming of 2.5 degrees Centigrade, or 4.5 degrees Fahrenheit, since the 1940s. Although the rate of warming is several times that of the global average, the exact cause of the warming is not known.
"The warming trend appears to be related to a reduction in sea ice," said Scambos. "The question now is what is causing the reduction. At this point we do not have enough evidence to find a smoking gun."
ÃÛÌÇÖ±²¥ two-thirds of the 12,000 square kilometer ice sheet is now threatening to break off, he said. The other one-third is nestled in bays that are expected to protect it from breaking off.
"This is the biggest ice shelf yet to be threatened," Scambos said. "The total size of the Larsen B Ice Shelf is more than all the previous ice that has been lost from Antarctic ice sheets in the past two decades."
In early 1995, a smaller ice shelf area, called the Larsen A, completely disintegrated during a single storm after years of gradually shrinking. "The speed of the final breakup was unprecedented, and followed several of the warmest summers on record for this portion of the Antarctic." he said.
A much smaller ice shelf, the Wordie, disappeared in the late 1980's. Currently the Larsen B is the northernmost ice shelf in Antarctica, and therefore "on the front line of the warming trend," said Scambos.
Ice shelves, thick plates of floating ice surrounding portions of Greenland and Antarctica, are fed by glaciers and snowfall. Reaching up to 800 meters in thickness in some cases, the largest ice shelf is the Ross Ice Shelf, which is larger than the state of Texas. The Larsen Ice Shelf is roughly the size of Connecticut.
"Ice shelves appear to be good bellwethers for climate change, since they respond to change within decades, rather than the years or centuries sometimes typical of other climate systems," said Scambos. Yearly climate changes may be too variable to rely on, while changes over centuries are difficult to document given the sparse data sets available to researchers.
The National Snow and Ice Data Center at ÃÛÌÇÖ±²¥-Boulder is maintained through a cooperative agreement between NOAA and ÃÛÌÇÖ±²¥. The data center also is one of eight NASA data archives. Images of the ice sheet can be downloaded from the World Wide Web at: .