A global system of deep ocean currents driven by differences in water density, which itself depends on temperature and salinity. It moves heat around the planet on a timescale of centuries, and whether it is weakening is one of the more consequential open questions in climate science.

Seawater density rises as it gets colder and as it gets saltier. Where surface water becomes dense enough, it sinks, and water flows in to replace it. That is the engine.

The global circulation in outline, with cold dense water sinking in the North Atlantic and around Antarctica and returning as warmer surface flow. A parcel of water takes roughly a thousand years to complete the circuit.
The global circulation in outline, with cold dense water sinking in the North Atlantic and around Antarctica and returning as warmer surface flow. A parcel of water takes roughly a thousand years to complete the circuit.Credit: Robert Simmon, NASA. Minor modifications by Robert A. Rohde also released to the public domain (Public domain).

Two regions do most of the sinking. In the North Atlantic, warm salty water arriving from the tropics cools as it travels north, and around Greenland and in the Labrador and Nordic seas it becomes dense enough to descend to depth. Around Antarctica, sea ice formation expels salt into the water beneath it, making it dense enough to sink to the sea floor.

The deep water then spreads through the ocean basins and eventually returns to the surface, largely through wind-driven upwelling and mixing. A complete circuit takes roughly a thousand years.

The name is slightly misleading. Wind and tides supply much of the energy that mixes deep water back upward, so the circulation is not driven by density differences alone, and oceanographers increasingly prefer to describe the Atlantic portion by its measured overturning rather than by the older term.

The Gulf Stream, the warm surface flow that carries heat northward across the Atlantic. It is driven mainly by wind, and the overturning circulation acts on it rather than creating it.
The Gulf Stream, the warm surface flow that carries heat northward across the Atlantic. It is driven mainly by wind, and the overturning circulation acts on it rather than creating it.Credit: User Sommerstoffel on de.wikipedia (Public domain).

The Atlantic Meridional Overturning Circulation is monitored continuously by an array of moored instruments across the ocean at 26 degrees north, operating since 2004. It measures transport directly rather than inferring it.

The array records an average northward transport of roughly 17 million cubic metres per second, with large variation between months and years. The heat carried is on the order of a petawatt, which is a substantial fraction of what northwest Europe receives and is why the region is warmer than its latitude would suggest.

The common statement that the Gulf Stream keeps Europe warm needs care. The Gulf Stream is mostly wind-driven and would continue without the overturning. The overturning contributes an additional northward heat transport on top of it, and modelling work suggests the difference in European temperature attributable to the ocean is real but smaller than popular accounts imply.

Adding fresh water to the North Atlantic reduces salinity and therefore density, which should reduce sinking. Greenland ice loss and increased precipitation both do this, so a weakening is expected on physical grounds.

Whether it has happened is harder. The direct record is only two decades long, which is short relative to natural variability, and it shows large swings without a clear trend that can be separated from them.

Longer reconstructions use proxies: sea surface temperature patterns, deep sea sediment grain sizes indicating current speed, and coral records. Several published reconstructions indicate a weakening over the twentieth century, with one widely cited estimate of about fifteen percent since the mid twentieth century, and a suggestion that the circulation is now at its weakest in a millennium. Other analyses find the proxy evidence too indirect to support that conclusion, and the disagreement is active.

The global surface current system. Whether the Atlantic overturning is weakening is contested, since the direct record covers only two decades and proxy reconstructions disagree.
The global surface current system. Whether the Atlantic overturning is weakening is contested, since the direct record covers only two decades and proxy reconstructions disagree.Credit: Dr. Michael Pidwirny (see http://www.physicalgeography.net) (Public domain).

Paleoclimate records show the circulation has changed abruptly before. Dansgaard-Oeschger events during the last glacial period involved Greenland temperature changes of several degrees within decades, and are generally attributed to shifts in North Atlantic overturning.

Models indicate a threshold beyond which the circulation could switch to a substantially weaker state and stay there, because the weakened state carries less salt north and so sustains itself. Where that threshold sits, and how close current conditions are to it, is not known with any confidence.

Assessments by the Intergovernmental Panel on Climate Change consider a complete collapse this century unlikely while stating medium confidence that the circulation will weaken. Several individual studies published since 2023 have argued for a much nearer-term risk, and have been contested on their statistical methods.

The consequences of a collapse would be severe and not confined to the Atlantic: substantial cooling in northwest Europe, shifts in tropical rainfall belts affecting monsoon systems, and regional sea level rise along the North American east coast. That is what makes an unresolved question about an ocean current a matter of general interest.