The coldest period of the past 10,000 years was caused by the sudden draining of a huge freshwater lake into the North Atlantic, say the scientists, who predict that melting ice sheets and glaciers, caused by global warming, could produce similar effects.
The researchers, writing in the journal Nature, found that the most dramatic fall in global temperatures since the last Ice Age was triggered when the lake, covering millions of square miles in northern Canada, drained violently into the sea when a glacial dam holding it in place suddenly broke.
The mass of freshwater released into the ocean disturbed the Gulf Stream, which keeps Britain and the rest of western Europe from freezing in winter. The scientists warn that if global warming continues to worsen, resulting in the disappear- ance of existing ice sheets and glaciers, another cataclysmic disruption of the Gulf Stream could make London feel like Moscow in winter.
A study of seabed sediments and fossils formed 8,200 years ago indicate that the North Atlantic experienced an influx of freshwater which over a single year was 15 times greater than the existing discharge of the Amazon, the world's largest river. Data gathered from ice cores show that average temperatures in Greenland at that time slumped by 8.3C and in Europe by 3.3C.
Geologists at the University of Colorado at Boulder believe the plummeting temp- eratures were the result of a catastrophic disruption of saline gradients in the North Atlantic Ocean caused by the influx of freshwater. The cold period lasted for 300 years.
The Gulf Stream, which continually brings warm water to western Europe, acts like a conveyor belt: warm surface currents flow north-east, and cold salty currents sink and flow south-west along the ocean floor. The scientists believe the influx of freshwater disturbed the conveyor belt, causing the Gulf Stream to run off course for several centuries, triggering Arctic weather in western Europe.
Their research confirms that sediments from an ice sheet covering much of North America were carried about 800 miles by a large plume of freshwater flowing from Canada, through the Hudson Bay and into the North Atlantic. Radiocarbon dating of seabed sediments, and fossils of freshwater-loving clams, indicates that this plume coincided with a period of climate cooling.