The gulf stream is the huge warm ocean current that moves water from the gulf of Mexico towards Northern Europe, brining with it heat equivalent to a million nuclear power stations running at full blast all year round, and is the reason why snow is rare in a British winter but at the same latitude on the other side of the Atlantic there are ice bergs floating in the sea.
All this water moving north has to go somewhere, in the winter as ice is forming in the arctic, salt isn't taken into the ice so the sea water becomes more salty. This cold salty water then sinks and returns down the floor of the Atlantic. If this process is stopped then the Gulf Stream would slow down if not stop removing Europe's winter warmer.
Helga Kleiven at the University of Bergen had now found evidence of this very thing having happened 8000 years ago. At this time there was a huge lake covering most of the mid west of Canada called lake Agassiz containing 100 000 km3 of water (as a comparison all the fresh water lakes in the world today contain about 91 0000km3 of water). This Lake was formed as ice sheets and glaciers plugged the rivers which would normally drain this area into Hudson's Bay. But about 8000 years ago the glacial plug melted releasing most of this fresh water into the North Atlantic. Helga's team have found the sediment that this flood brought with it on the floor of the Labrador Sea. They have accurately dated this event and it coincides with an 8°C drop in temperature measured from Greenland ice cores. This very strongly indicates that it did have the effect of stopping the Gulf Stream.
Scientists now have to work out how much water it would take to do the same thing again and whether global warming could melt enough of the Greenland Ice Sheet to have the same effect.