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As part of the ocean conveyor belt, warm water from the tropical Atlantic moves poleward near the surface where it gives up some of its heat to the atmosphere. This process partially moderates the cold temperatures at higher latitudes. As the warm water gives up its heat it becomes more dense and sinks. This circulation loop is closed as the cooled water makes its way slowly back toward the tropics at lower depths in the ocean. If the poles warm, it is possible that melt water from glaciers and the polar ice cap can shut off this circulation and interrupt this circulation system. The melt water is fresher and hence less dense than the ocean water it melts into, and thus the melt water will tend to accumulate near the surface. This layer of fresh water acts as an insulating barrier between the atmosphere and the normal ocean water. The water from the tropics can not release its heat to the atmosphere, and the circulation loop is interrupted. The mechanism has a positive feedback potential in that if the ocean circulation slows, then even less heat will make it to the higher latitudes re-enforcing an effect that will cool the climate at these higher latitudes.
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