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Short-circuiting the Ocean Conveyor

What factors caused the Northern Hemisphere to develop sea ice and glaciers about 3 million years ago? The closing of the Central American Seaway intensified the Gulf Stream, the upper limb of a conveyor-like global ocean circulation called the Ocean Conveyor. The Gulf Stream transported salt to the North Atlantic, making waters there dense enough to sink and drive the lower limb of the Conveyor. According to one theory, the Gulf Stream also transported moisture to the North Atlantic region and encouraged ice formation. But the Gulf Stream also conveys heat, which would deter glaciation. Neal Driscoll and Gerald Haug proposed a solution to this apparent contradiction (above). (Animation by Jack Cook, Woods Hole Oceanographic Institution)