What is thermohaline circulation and why is it called the global conveyor belt?

The answer

Thermohaline circulation is a global system of deep ocean currents driven by differences in water density caused by temperature (thermo) and salinity (haline). Cold, salty water sinks in the North Atlantic near Greenland and in the Southern Ocean near Antarctica, flowing along the ocean floor toward the equator and beyond. This deep water eventually resurfaces through upwelling in the Pacific and Indian Oceans, is warmed at the surface, and flows back to the sinking regions, completing a loop that takes approximately 1,000 years. This conveyor belt redistributes an enormous amount of heat, equivalent to about one million nuclear power plants, and its disruption has been linked to abrupt climate shifts in Earth's past, including the Younger Dryas cold period roughly 12,000 years ago.