Scientists say they have found the trigger of a sharp cooling 13,000 years ago that plunged Europe into a mini ice age.
Mark Bateman from the University of Sheffield in England said a catastrophic flood unleashed from a giant North American lake dumped large amounts of fresh water into the Arctic Ocean.
This led to the shutting down of the Gulf Stream ocean circulation pattern that brings warmth to Europe.
"We're talking about a lake the size of the (United Kingdom) emptying very quickly," Bateman told Reuters by telephone. "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, similar to today's, quickly returned to ice age conditions. The cooling lasted for about 1,400 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," Bateman said. His team's work is published in the latest issue of the journal Nature.
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.
Climate scientists fear rapid global warming could trigger a sharp increase in the amount of meltwater from Greenland. This surge in fresh water could trigger a tipping point that overwhelms the Gulf Stream, shutting it down and likely plunging Europe into another deep freeze.
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.
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 in the journal Science last November said recent summers further accelerated Greenland's mass loss to the equivalent of 273 cubic kilometers of water per year in the period 2006-2008. The also represented 0.75 millimeters of global sea level rise per year.