I know this is a bit old now, but,...
holmes writes:
Okay here's the problem. Cold water sinks while warm water stays on top. It is true that salinity has its say, but there is no definitive concept that fresh water will not mix and so stay around to block incoming gulf currents.
Salinity is increcibly important in the oceanic conveyer. As the water in the artic freezes, the salt is squeezed out and the remaining brine is very cold and salty. That make is significantly heaver than other waters. It sinks and begins a trip around the world that make take a thousand years or more. This conveyer absorbs and gives off huge amounts of energy. Since it takes so long to circulate, it moderates the climate.
Note also that cold salty water does not readily mix with warmer fresher water. That is what enables this oceanic current to remain coherent in its incredibly long journey. And yes, this is known and verified.
I don't remember where, but I read an evaluation about the end of one of the ice ages. That essay said that at one point, an ice dam broke and a huge lake on the North American continent poured into the Atlantic ocean. All that fresh water disrupted the oceanic current. The result was extreme climate changes for the thousands of years that it took for the oceanic current to be restored.
While I did not find the one I remembered, here is one of many web sites:
http://www.soc.soton.ac.uk/.../general_science/conveyor.html
I am not weighing in on one side of the debate or the other, just emphasizing the importance of this aspect.
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