The planet's last major cold spell 13,000 years ago was caused by a catastrophic deluge of frigid fresh water from north-west Canada into the Arctic ocean, a new study suggests. Detailed computer simulations show meltwater from the enormous Laurentide Ice Sheet halted the sinking of very dense, saltier, colder water in the North Atlantic.
© Alan CondronDetailed: A new model of flood waters from the melting Laurentide Ice Sheet shows how water first flowed north-west into the Arctic, weakening deep ocean circulation and leading to the Earth's last major cold period
That stopped the large scale ocean circulation - the so-called thermohaline circulation - that transports heat to Europe and North America, causing the continents to dramatically freeze. The findings offer a new explanation for the cause of this last big chill, which scientists had thought was caused by freshwater flowing into the Atlantic through Canada's Gulf of St Lawrence.

It led to a cold spell lasting more than 1,000 years known as the Younger Dryas or 'Big Freeze', during which temperatures in parts of the northern hemisphere fell to about 10 degrees C colder than they are today.

'This episode was the last time the Earth underwent a major cooling, so understanding exactly what caused it is very important for understanding how our modern-day climate might change in the future,' said Alan Condron, of the University of Massachusetts Amherst.

The cooling began after Lake Agassiz, at the southern edge of the Laurentide ice sheet covering much of the Canadian Arctic, broke through an ice dam and dumped thousands of cubic kilometers of cold water into the ocean.
© APArgentina's Patagonia ice-sheet: Much of Europe and North America would have looked like this after the meltwater from the Laurentide Ice Sheet brought about a dramatic global cooling
Using a high resolution, global, ocean-ice circulation model 10 to 20 times more powerful than previous ones, the researchers compared how meltwater from the two different drainage outlets affected ocean circulation.

The original hypothesis, proposed 1989 by Wally Broecker of Columbia University, suggested that Lake Aggasiz drained into the North Atlantic down the St Lawrence River.

But using the new model, Dr Condron, working with Peter Winsor of the University of Alaska, found that this proposed route would have weakened the oceans' thermohaline circulation by less than 15 per cent.

That level of weakening, they say, is unlikely to have accounted for the 1,000-year cold climate event that followed the flood.

Meltwater from the St Lawrence River actually ends up almost 1,900 miles south of the deep water formation regions, too far south to have any significant impact on the sinking of surface waters.

By contrast, Dr Condron and Dr Winsor's model shows that if the meltwater first drains into the Arctic Ocean, narrow coastal boundary currents efficiently deliver it to the deep water formation regions of the sub-polar north Atlantic, weakening the thermohaline circulation by more than 30 per cent.

They conclude that this scenario, showing meltwater discharged first into the Arctic rather than down the St. Lawrence valley, is 'more likely to have triggered the Younger Dryas cooling.'

Dr Condron and Dr Winsor ran their simulations on one of the world's top supercomputers at the National Energy Research Science Computing Centre in Berkeley, California.

Reporting their findings in the journal Proceedings of the National Academy of Sciences, they wrote: 'With this higher resolution modelling, our ability to capture narrow ocean currents dramatically improves our understanding of where the fresh water may be going.'

Dr Condron added: 'The results we obtain are only possible by using a much higher computational power available with faster computers.

'Older models weren't powerful enough to model the different pathways because they contained too few data points to capture smaller-scale, faster-moving coastal currents.'

'Our results are particularly relevant for how we model the melting of the Greenland and Antarctic Ice sheets now and in the future.

'It is apparent from our results that climate scientists are artificially introducing fresh water into their models over large parts of the ocean that freshwater would never have reached.

'In addition, our work points to the Arctic as a primary trigger for climate change. This is especially relevant considering the rapid changes that have been occurring in this region in the last 10 years.'