Global conveyor belt

Air and water are the two most important fluids on earth. The atmosphere and the hydrosphere are complementary in their role of transporting and transforming chemicals. The atmosphere is the fastest and most efficient global conveyor belt. Yet, certain chemicals prefer the aquatic milieu which is of global dimension, as well. In the hydrosphere typical transport velocities are significantly smaller than in the atmosphere. Therefore, residence times of chemicals in the water are usually much larger than in the atmosphere. 

A natural oceanic phenomenon whereby cold water in the Arctic/North Atlantic sinks to the ocean floor and then circulates at depth around the world, to surface eventually in the Indian and Pacific oceans some hundreds of years later. This circulation is sometimes called the Ocean conveyor belt, the Great ocean conveyer, the Global conveyor belt, or, most commonly, the Meridional overturning circulation (often abbreviated as MOC). 

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