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Study shows Antarctica's impact on sea

ice berg

Ancient Antarctic ice sheet melting contributed to a four metre per century rise in global sea levels (Source: Michael Weber/University of Cologne)


The findings, reported in the journal Nature, shows how closely linked ocean dynamics and ice sheet melting are, which should help scientists currently modelling future sea level increases.

Since the last glacial maximum between 26,000 and 19,000 years ago, when a significant amount of water was locked up on land, sea levels have risen by about 130 metres.

Until now, Antarctica's role in major ice sheet melting events has not been well understood.

"This study suggests that a significant amount, we can't say exactly how much, it could be 50 per cent, was coming from the Antarctic ice sheet." says one of the study's authors Dr Laurie Menviel of the University of New South Wales.

The study led by Dr Michael Weber of the University of Cologne, took marine sediment cores samples from Iceberg Alley in Antarctica's Scotia Sea.

The samples contained layers called iceberg rafted debris, which contains coarse material trapped in the ice as it moves across land and is released during melting.

"These samples have the youngest sediments on the top and progressively older sediments deeper down providing data on movements since the last glaciation," says Menviel.

The core samples revealed a major ice melt episode or "pulse" occurring between 20,000 and 19,000 years ago, followed by seven larger pulse events between 17,000 and 9000 years ago.

"These were times when debris was more abundant than at other times, indicating occasions when more of the Antarctic ice sheet melted," says Menviel.

Major meltdown

The authors found the largest of these pulses occurred between 14,800 and 14,400 years ago, which overlaps with a period of global sea level increases known as melt water pulse 1A, when levels rose by an average of four metres per century.

This information provides the long-sought confirmation of Antarctic contributions to this major jump in sea level rise.

The core data also shows that most of the Antarctic deglaciation occurred earlier than the 11,000 to 9000 years ago previously estimated.

"Early models suggested this melting was caused by thermodynamic effects such as changes in precipitation and air temperature," says Menviel.

"But those are fairly small and would mean deglaciation of the Antarctic would have occurred later than the data shows."

Feedback system

Recent studies have shown that a significant amount of warming occurs directly from the ocean transferring heat to the ice shelves from underneath and causing melt.

"Our models indicate that when you add the fresh water, you initiate a positive feedback through subsurface ocean warming," says Menviel.

Fresh water from the Antarctic ice sheet melts into the Southern Ocean causing stratification of ocean water into separate layers, resulting in cool water on the surface, and warmer sea water pushed deeper down, which further erodes the icesheet.

"So what starts as a small melting can be amplified leading to more rapid melting than just through changes in atmospheric temperature," says Menviel.

Tags: climate-change, oceans-and-reefs