What Is the Thermohaline Circulation? A Current Driven by Weight

Seawater becomes denser when it is colder and when it is saltier. Both conditions are met in a small number of places at high latitude, principally the Nordic and Labrador seas in the North Atlantic and around Antarctica. Water arriving there has already been made salty by evaporation at lower latitudes, and when it cools further and sea ice forms, the freezing process rejects salt into the remaining water, raising its density further still. That dense water sinks, and because the deep ocean is otherwise quiet, it spreads as a slow deep current. The sinking regions are remarkably small relative to the ocean, which means the whole circulation depends on conditions in a handful of places, and that concentration is precisely what makes it vulnerable. The return is diffuse, with deep water rising gradually across large areas and in particular through wind-driven upwelling in the Southern Ocean, which is now understood to play a larger role in driving the whole loop than was once assumed.

The circulation is frequently drawn as a single global loop, sometimes called the ocean conveyor belt, and the diagram is a useful simplification with real distortions:

• •It suggests a single ribbon of water when the reality is a set of interconnected circulations with substantial mixing between them
• •It implies the flow is driven from the sinking end, whereas mixing and Southern Ocean winds supply much of the energy that returns water to the surface
• •It understates the timescale, with a full circuit taking something on the order of a thousand years, so water sinking now surfaces long after everyone reading about it
• •It obscures the distinction between the Atlantic overturning, which is measured and monitored, and the notional global loop, which is a conceptual aid
• •The Atlantic component is what matters most for European climate and is what monitoring arrays actually observe
• •Volume transport is enormous, measured in tens of millions of cubic metres per second, which is many times the combined flow of every river

The Atlantic overturning carries warm surface water northward, and as that water releases heat to the atmosphere it warms the air masses that reach north-western Europe, which is a substantial part of why Britain and Norway are far milder than places at equivalent latitude in Canada and Russia. Attributing all of that to ocean heat transport overstates it, since prevailing westerly winds crossing a relatively warm ocean and the shape of atmospheric circulation contribute as well, and a well-known analysis argued that the ocean's direct contribution is smaller than the popular account suggests. The circulation also moves carbon and nutrients: cold water dissolves carbon dioxide efficiently, so the sinking regions transfer carbon into the deep ocean where it remains for centuries, which is one of the planet's main carbon sinks. Nutrients accumulated in deep water return to the surface where the circulation upwells, which sets where the ocean is biologically productive.

Adding fresh water to the sinking regions reduces salinity and therefore density, which could slow or stop the overturning, and both melting Greenland ice and increased rainfall at high latitudes do exactly that. Evidence from ocean sediments shows the circulation has changed abruptly in the past, with reorganisations associated with rapid regional climate swings during the last glacial period, so the system is known to have more than one stable state. Direct measurement began only in 2004, which is a short record, and analyses of longer indirect indicators have suggested a weakening over the past century, with the size and confidence of that estimate actively disputed among specialists. Assessments generally consider a complete collapse this century unlikely while treating a substantial weakening as plausible, and note that the consequences would be severe and regional rather than a global freeze, including cooling in north-western Europe alongside warming elsewhere, shifts in tropical rainfall belts and changes in sea level along the American east coast. The popular depiction of an abrupt continental deep freeze is not what any model produces.

Cold salty water sinks in a few small high-latitude regions and flows along the deep ocean for centuries before rising, driven by density rather than wind, with salt rejected during sea ice formation adding to the effect. The conveyor belt diagram simplifies a set of connected circulations and understates the role of Southern Ocean winds in returning water to the surface. The Atlantic branch moves heat towards Europe and carries carbon into the deep ocean, and freshening from melting ice could weaken it.