Scientists say they have found the trigger of a sharp cooling 13,000 years ago that plunged Europe into a mini ice age.
But one expert says despite the current warming trend, a repeat of this dramatic event isn't likely to happen anytime soon.
The study, which appears today in the journal Nature, shows how a catastrophic flood dumped large amounts of freshwater into the Arctic Ocean shutting down an ocean circulation pattern that brings warmth to Europe.
"We're talking about a lake the size of the UK emptying very quickly," says co-author Dr Mark Bateman of the University of Sheffield in England. "We don't know the exact period of time but we're talking about a catastrophic flood."
The finding has confirmed past theories about the likely cause of a sudden cooling period called the Younger Dryas when temperatures in Europe plunged into ice age conditions that lasted for about 1400 years.
"Our research shows that if you put a large volume of fresh water into the North Atlantic in a very short space of time, this is what happens," says Bateman.
The Gulf Stream acts like a conveyer belt by bringing warm water from the tropics to Europe while cold salty water sinks to the depths in the far north. This "overturning" circulation draws in yet more warm water from the south.
Tracing the flow
Bateman and his team confirmed the path of the floodwaters from Lake Agassiz that covered part of what is now Canada and the northern United States. The lake had formed in front of the ice-sheet that once covered a large part of North America.
Scientists had previously guessed that a giant flood unleashed from the lake probably caused the Younger Dryas cooling but couldn't confirm the route of the floodwaters.
Bateman found that the waters flowed down the Mackenzie River, Canada's longest, rather than the Saint Lawrence Seaway that had previously seemed the most likely route.
Studying sediments from cliff sections along the river delta, he said the evidence spanned a large area at many altitudes. This could only be explained by a mega-flood from Lake Agassiz.
Dating of the sediments helped the team pin down the date of the flooding, showing that it occurred right at the start of the Younger Dryas.
Some climate scientists fear rapid global warming could trigger a sharp increase in the amount of meltwater from Greenland. This surge in freshwater could trigger a tipping point that overwhelms the Gulf Stream, shutting it down and likely plunging Europe into another deep freeze.
Satellite observations and computer models by scientists have shown that the Greenland icesheet is melting at an accelerating rate, dumping large amounts of ice and meltwater into the North Atlantic.
A study published last November in the journal Science claimed recent summers further accelerated Greenland's mass loss to the equivalent of 273 cubic kilometres of water per year in the period 2006-2008.
But a recent study in the journal Geophysical Research Letters has found no significant change in the Gulf Stream.
Dr Josh Willis of the Jet Propulsion Laboratory in California analysed data collected by satellites and robotic ocean floats known as Argo.
He found no change in the strength of the Gulf Stream between 2002 and 2009, while satellite data indicated an increase of 20% between 1993 and 2009.
"The changes we're seeing in overturning strength are probably part of a natural cycle," says Willis. "The slight increase in overturning since 1993 coincides with a decades-long natural pattern of Atlantic heating and cooling."
Willis believes any slowdown in the Gulf Stream isn't likely to result in the same dramatic change that occurred during the Younger Dryas.
"No one is predicting another ice age as a result of changes in the Atlantic overturning," he says. "Even if the overturning was the Godzilla of climate 12,000 years ago, the climate was much colder then. Models of today's warmer conditions suggest that a slowdown would have a much smaller impact now."