Not All Breakups due to Climate Change

We had just gone to press in March when huge pieces of Antarctica's floating ice shelf had detached from the mainland and disintegrated in the sea. In 2000 and 2001 at least three sections of ice sheets, each one the size of Connecticut, cast off to become floating islands. This year, between January 31 and March 7, a large section of the Larsen B ice shelf on the Antarctic Peninsula (near South America, see map) disintegrated in only 35 days. The satellite photos before and after this breakup are below. The Larsen ice shelf lost 60% of its former summertime extent in the last five years.


A satellite recorded the extraordinarily rapid disintegration of most of the Larsen B ice shelf early this year. Black scratches and dots in the earlier picture are ponds of meltwater, which may have initiated the breakup.

Has a changing climate contributed to the unusual breakup? "Yes" for some events, "No" for others. While many saw this as evidence of a warming planet, the observations show that most of the Antarctic continent is, in fact, cooling. Peter T. Doran and coworkers wrote in Nature (below) that average temperatures have fallen over the last 35 years and that summer temperatures have fallen the most. The few living creatures that call Antarctica their home are sensitive to summer temperatures, because summer is when their body temperature can reach 0ºC, the melting point of water.

The breakups of ice shelves on the Antarctic Peninsula and other ice shelves farther south are caused by different factors. The Peninsula region, including the Larsen shelf, is warming, with temperatures now 3ºC warmer than in the 1940s when the trend began. A theory put forth by Ted Scambos of the National Snow and Ice Data Center [http://nsidc.org], University of Colorado, now has the support of observations during this year's catastrophic breakup.

Scambos thinks that meltwater ponds on the surface of the ice introduce water into deep cracks or crevasses, which forces them apart. The more that water sinks into these fissures, the more pressure it exerts on the walls, until the crack extends all the way through the ice shelf, about 700 feet thick. What is different is that the top surface now remains above freezing for 60 to 70 days each summer, long enough for the ponds to grow large. You can see meltwater ponds in the picture from January, above, as numerous black scratches and dots on the white ice. This region disintegrated into the sea over the 35 days following this picture. Scambos thinks the recent warming is a principal cause of the sudden ice breakups on the Antarctic Peninsula.

Farther south, the four largest ice shelves (Ross, Ronne, Filchner, and Amery) around the margins of the continent almost never reach melting temperatures even in the summer. Scambos credits different factors for causing the breakups in 2000 and 2001: ocean currents, tides, and katabatic (hurricane force) winds off the continent. Even so, these ice shelves had extended farther out than ever before observed and were ready to be broken off. After the recent breakup, the margins of the Ross Ice shelf were whittled back to where they were in the 1960s.



1. Nature, 10 January 2002. www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v415/n6871/abs/nature710_fs.html

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