You may not think much about it, but ocean water off the coast of Portland, Maine, or Sydney, Australia, is not the same water that lapped the shores of these locations a year ago. In fact, in recent millennia that water has been to the furthest reaches of the planet and back many times due to the motion of a great ocean conveyor belt. Known as the thermohaline conveyor, this conveyor belt circulates ocean water around the globe, and in the process redistributes heat and nutrients. Many scientists consider the thermohaline conveyor essential to a healthy ocean ecosystem and to the stability of Earth's climate.
The unequal distribution of solar radiation on Earth's surface is one of the most important factors in the conveyor's circulation. The Sun warms surface water near the equator. This warm water then moves as a current from the tropics toward higher latitudes, where it cools and transfers its heat to the atmosphere. One such current, known as the Gulf Stream, originates in the Gulf of Mexico and carries warm water across the northern Atlantic Ocean. The heat it releases helps to keep northwest Europe warmer than other regions at the same latitude.
Water moving northward in the conveyor cools, and as a result it becomes denser than the warmer water arriving from the tropics behind it. The formation of sea ice at high latitudes also affects the density of water. As ice forms at the ocean surface, salt ions are left behind, increasing their concentration in the liquid water below. Because salt ions have more mass than water molecules, any increase in their concentration increases the solution's density.
Colder and saltier, this dense water begins to sink. Warm water from the tropics moves in to replace it at the surface, and it, too, begins to cool. The cold water descends to the ocean bottom where it forms what oceanographers call the North Atlantic Deep Water, a mass of water that fills most of the deep Atlantic basin. This water moves sluggishly southward until it joins a similar mass called the Antarctic Bottom Water. Here, the conveyor splits in two, with one branch circling Antarctica and then moving on to the ocean south of Australia, and the other branch heading north, up the east coast of Africa. In the Indian Ocean, it warms and rises to the surface, where it warms even more and provides moisture for monsoon rains. This warm water then swings back and joins the other surface currents flowing in from the Pacific. Together, they pass the Cape of Good Hope and head north, past the west coast of Africa and Europe, toward Iceland in the North Atlantic, where the 1,000-year journey begins again.