Ocean Currents and Thermohaline Circulation

Surface currents are driven mainly by wind. Global wind belts drag the ocean surface along with them, and the rotation of the Earth curves these moving sheets of water into large circular patterns called gyres. Warm water piles up near the equator and gets pushed poleward along the edges of continents, which is exactly what the Gulf Stream does: it carries warm tropical water up the eastern coast of the United States and then across the North Atlantic toward Western Europe. That heat radiates into the air above, so Britain, sitting at about the same latitude as icy Labrador, ends up with mild winters instead of frozen ones. Labrador gets no such gift; it sits in cold water flowing down from the Arctic instead.

Thermohaline circulation is the deeper, slower partner to these wind-driven currents, and it runs on density instead of wind. In the North Atlantic, the Gulf Stream's warm water eventually loses its heat to the atmosphere as it travels north. Cooling makes it denser, and evaporation along the way leaves it saltier, which makes it denser still. This cold, salty, dense water sinks to the ocean floor and creeps slowly south and around the globe as a deep current, eventually rising back to the surface in the North Pacific through a process called upwelling. This loop, often pictured as a conveyor belt, links surface currents and deep currents into one continuous system that moves heat between the equator and the poles over centuries.

Because sinking depends on salt as much as cold, anything that freshens the North Atlantic threatens the whole loop. Large-scale glacial and ice-sheet melting adds huge volumes of fresh water to exactly the region where sinking normally happens, diluting the salinity, lowering the density, and making that water resist sinking. A slowed or stalled conveyor belt would mean less heat delivered to places like Britain and Western Europe, even as the planet as a whole warms.