A drop of seawater takes about 1,000 years to circle the globe

Beneath the wind-driven surface currents runs a far slower system: the global conveyor belt, powered by differences in water temperature and salt — thermohaline circulation.

In the cold North Atlantic, sea ice forms and leaves the surrounding water saltier and denser, so it sinks toward the seafloor. That deep water creeps south, loops around Antarctica, and threads into the Indian and Pacific oceans before rising and returning. NOAA estimates a single parcel takes about 1,000 years to complete the loop, moving at just a few centimeters per second — yet carrying more than 100 times the flow of the Amazon River.

The Atlantic arm of this system, the AMOC, is why Western Europe is far milder than its latitude suggests: it ferries tropical heat northward, so London stays gentler than Labrador at the same line of latitude. The slow timescale is humbling, too — the deep water welling up off Antarctica today last touched the surface roughly a millennium ago, around the Middle Ages.

That steadiness can’t be assumed. As Greenland and Arctic ice melt, the influx of fresh, buoyant water makes North Atlantic surface water less inclined to sink, and scientists warn the overturning is slowing. A sharp weakening or shutdown is the classic abrupt-climate-change scenario, with cascading effects on European weather and on the deep ocean’s role in storing heat, nutrients and carbon dioxide out of contact with the sky for centuries at a time.