Evidence from ocean sediments suggests that at times during the last Ice Age the North Atlantic thermohaline circulation was considerably weaker than it is today, or perhaps it even shut down entirely. One such event took place about 12,900 years ago, during the last deglaciation, and is called the Younger Dryas (after a European cold-dwelling flower that marks it in some terrestrial records). The Younger Dryas began with a dramatic reversal in what was a general warming trend, bringing near-glacial cold to the North Atlantic region. This episode ended with an even more dramatic warming about 1,000 years later. In Greenland and western Europe, the beginning and end of the Younger Dryas involved changes in winter temperature as large as 20 degrees taking place in little more than a decade. But the Younger Dryas was not a purely North Atlantic phenomenon: Manifestations of it also appeared in the tropical and southern Atlantic, in South America and in Asia.
For many years, the leading theory for what caused the Younger Dryas was a release of water from glacial Lake Agassiz, a huge, ice-dammed lake that was once situated near Lake Superior. This sudden outwash of glacial meltwater flooded into the North Atlantic, it was said, lowering the salinity and density of surface waters enough to prevent them from sinking, thus switching off the conveyor. The North Atlantic Drift then ceased flowing north, and, consequently, the northward transport of heat in the ocean diminished. The North Atlantic region was then plunged back into near-glacial conditions. Or so the prevailing reasoning went.
I would expect that any slowdown in thermohaline circulation would have a noticeable but not catastrophic effect on climate.
Instead of creating catastrophe in the North Atlantic region, a slowdown in thermohaline circulation would serve to mitigate the expected anthropogenic warming!
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