Most ocean currents people have heard of, like the Gulf Stream, are driven by wind pushing surface water around. Underneath that wind-driven layer runs a much slower, much deeper system, driven by nothing but density — water that's cold enough and salty enough to sink under everything around it, dragging a planet-spanning loop of circulation behind it. It's called thermohaline circulation, from the Greek for heat and salt, and a single full loop is estimated to take somewhere on the order of a thousand years to complete.
Where the loop starts: a cold, salty sink in the North Atlantic
Warm, salty surface water travels north on the Gulf Stream toward Greenland and Norway. As it reaches the far North Atlantic, it cools, and cooling alone would make it denser — but this water is also unusually salty, because evaporation along its northward journey concentrated the salt left behind. Cold and salty together make it dense enough to sink, all the way to the ocean floor, forming a mass of water called North Atlantic Deep Water. That sinking is the engine of the whole system: it pulls more warm surface water north to replace what sank, and it sends a slow deep current south, past the equator, around Antarctica, and eventually into the Indian and Pacific basins, where it gradually warms, rises back to the surface, and begins the long journey back.
Why a slow current still moves enormous heat
Speed isn't what makes this circulation matter; volume is. The loop moves roughly twenty million cubic metres of water every second, and every cubic metre of that water carries heat with it. That transport is a major reason Western Europe is markedly milder than other regions at the same latitude — London sits further north than most of Canada, but doesn't share its winters, largely because of heat this circulation delivers across the Atlantic.
What we're still unsure about
Because the engine of this whole system depends on North Atlantic water being cold and salty enough to sink, climate scientists are watching one variable closely: freshwater from melting Greenland ice, which dilutes the salinity that makes the sinking possible in the first place. A continuous monitoring array in place since 2004 has measured real variability in the circulation's strength, but only across a couple of decades — nowhere near long enough to cleanly separate a genuine long-term weakening from the system's ordinary year-to-year swings. Recent studies disagree, sometimes sharply, on how close a serious slowdown might be, which makes this one of the more actively contested questions in current climate research rather than a settled one.
This sits inside Ocean Circulation & the Hydrological Cycle, one of seven topics in Physical Geography, one of five domains in Geography, one of seventeen subjects the app can quiz you on.