With Speed and Violence Fred Pearce - Global Commons Institute

On the ground, one element was a sudden switch in Broecker's ocean
conveyor. It would be going too far to say that the Younger Dryas proves that
the global conveyor is the great climate switch that Broecker claims. But the
event makes a compelling case that events in the far North Atlantic can,
without help from astronomical or any other forces, sometimes have
dramatic and long-lasting effects on global climate.
The unexpected switch of the ocean conveyor was almost certainly
triggered by melting ice. In the final millennia of the ice age, as melting
made fitful but sometimes dramatic progress, a very large amount of liquid
water was produced. Often it did not pour directly into the oceans but
formed giant lakes on the ice or on land around the edges. The largest known
of these is called Lake Agassiz, after the discoverer of the ice ages. It
stretched for more than 600 miles across a wide area of the American
Midwest, from Saskatchewan to Ontario in Canada, and from the Dakotas to
Minnesota in the U.S., generally moving with the advancing front of
warming.
In the early stages of the déglaciation, the lake drained south, down the
Mississippi River into the Gulf of Mexico. But about 12,800 years ago, it
seems, something stopped this and forced the lake to drain east. Perhaps the
route south was blocked by land gradually rising after the weight of the ice
was removed. Perhaps the lake simply passed over a natural watershed as it
moved north with the retreating face of the ice sheet. But at any rate, there
was eventually a huge breakout of freshwater from the heart of North
America into the basin now occupied by the Great Lakes, and on into the
North Atlantic.