A paper in the March 14 issue of the journal Science suggests that a catastrophic collapse of an Antarctic ice sheet dumped roughly a million cubic litres per second of freshwater into the southern oceans, changing the climate thousands of kilometres to the north and ushering in a dramatic climate shift known as the B�lling-Aller�d warm interval.

"The paper describes the last gasp of the ice age," says Jerry Mitrovica, the J. Tuzo Wilson Professor of Geophysics at the University of Toronto and co-author of the paper. "These are the spasms that got us from a climate where three kilometres of ice covered Canada to today's conditions.

"We're saying that what pulled us out of this -- what ended the ice age -- was this remarkable sequence of events. It all started in the Antarctic."

Last year, Mitrovica and co-author Professor Peter Clark of the Oregon State University made headlines with their theory that the sudden influx of freshwater that occurred 14,000 years ago came from the Antarctic.

Sea level changes recorded in corals and organic material from places like Barbados and Vietnam indicated that roughly 14,000 years ago, the world's sea level rose by an average of 20 metres over the course of about 200 years -- roughly 100 times faster than today's rate of sea level rise.

The 2002 article countered the long-standing belief that the melting ice came from North America. Instead, said Mitrovica, it mostly came from the Antarctic. The profound climate repercussions of this event, known as meltwater pulse 1A (mwp-1A), are described in the latest paper, which is co-authored with Clark, principal investigator Professor Andrew Weaver and post-doctoral fellow Oleg Saenko of the University of Victoria.

The team created a computer model to simulate the effect of mwp-1A. They found that if a massive influx of freshwater were suddenly deposited in the southern oceans, it intensifies a massive river of warm water called thermohaline circulation.

This conveyor belt-like water current, which is driven by temperature and salinity, rises off Europe, sinks farther north and turns back to the south. "It dramatically influences climate," says Mitrovica, an associate with the Canadian Institute for Advanced Research.

By shunting more warm water towards Europe and the north Atlantic, he explains, the region's climate was significantly heated, leading to the thousand-year-long B�lling-Aller�d warm interval.

The model also successfully predicts the significant cooling of the south -- known as the Antarctic Cold Reversal -- that coincided with the northern warming and that has eluded explanation. In turn, this began to melt the Laurentide and Fennoscandian icesheets that covered North America and northwestern Europe, respectively.

That melting, says Mitrovica, released freshwater into the north Atlantic, shutting down the conveyor belt and cooling the north. "That explains a very famous climate event called the Younger Dryas cold interval," he says, referring to a period around 13,000 years ago when Europe went into a deep freeze.

But despite this chillier interval, says Mitrovica, the melting marked the beginning of the end for the massive glaciers that had covered the Earth's continents. "Once the process of warming the north began, the main deglaciation started and the ice age ended," he says.

The model is able to explain the sequence of dramatic climate changes taking place between roughly 10,000 and 20,000 years ago. "It's like dominoes," says Mitrovica.

"It pieces together all of the major climate events of that period. By taking the meltwater pulse from the Antarctic, everything falls beautifully into place."

The research was funded by the Natural Sciences and Engineering Research Council of Canada, the Killam Foundation, the National Science Foundation and the Canadian Institute for Advanced Research.

Nicolle Wahl is a news services officer with the department of public affairs.

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