A catastrophic 'superflood' following the rupture of a massive glacier-dammed lake in Canada at the end of the Ice Age probably plunged the world into centuries of climatic chaos. That single event was likely responsible for the most dramatic climate change of the last 10,000 years, according to a report by a Canadian team led by Professor Garry Clarke, a geophysicist at the University of British Columbia in Vancouver.
     The 'superflood' was enough to alter ocean circulation in the Northern Hemisphere: analysis of ice cores taken in Greenland reveal that the mean temperature dropped by 5°C, snow accumulation decreased sharply and forest fires became more frequent. Clarke's team found that the water body, known as Lake Agassiz, reached a massive 163,000 cubic km in volume - at least double that of the largest contemporary lake, the Caspian Sea - and that its release was "by far the largest known glacial outburst of the past 100,000 years". It was formed after the vast Laurentide Ice Sheet, which at its maximum formed a 3-kilometre-thick dome over Hudson Bay, began disintegrating rapidly about 8,500 years ago.
     "Modern analogues and the known physics of outburst flooding indicate that tunnelling below the ice is the most probable flood release mechanism," Clarke said. "Once a subglacial path is established, an ice-walled conduit will tend to grow by melting its walls," he added. As water tunnelled its way through the ice dam, its rupture became unavoidable. The team said that on the basis of radiocarbon dating, a full torrent was finally unleashed about 8,450 years ago. It took less than a year to discharge. After the lake water gushed into the Hudson Bay, its freshness altered the strength of ocean circulation, which in turn caused the abrupt climate changes in much of the Northern Hemisphere, the team said.
     The researchers argue that understanding the mechanisms underlying past climate change events is increasingly important as people grow more concerned about the magnitude and rate of future climate change.

Source: ABCE Science Online (15 August 2003)