Salt and cold drive an ocean conveyor whose oldest water takes 1,000 years
Deep beneath the waves, a slow global current is powered not by wind but by differences in temperature and saltiness. Heavy, cold, salty water sinks near Greenland and Antarctica and creeps through the abyss, resurfacing in the North Pacific after about a thousand years. Both of its main loops appear to be slowing.
Thermohaline combines Greek-derived words for heat and salt, the two factors that set seawater's density. Warmer water is lighter; saltier water is heavier. Unlike fresh water, which is densest at 4 °C, seawater keeps getting denser as it chills until it freezes, which can happen below minus 2 °C. Climate scientist Wallace Broecker coined the phrase global conveyor belt, while many researchers prefer meridional overturning circulation, since wind and tides also shape the flow.
Two sinking zones act as engines. In the North Atlantic, strong winds make evaporation outpace rainfall, leaving salty, cold surface water that plunges in the Norwegian Sea and spills south over undersea ridges linking Greenland, Iceland and Britain, forming North Atlantic Deep Water. Near Antarctica, fierce winds push fresh sea ice offshore, exposing open water that chills sharply; as new ice forms, it expels brine, making the surface extra dense. This Antarctic Bottom Water is heavier still and slides beneath its northern counterpart. Henry Stommel and Arnold Arons established the picture in 1960.
The idea took decades to prove. Wind was known to push surface currents, but in 1908 Johan Sandström showed experimentally that heat-driven flow requires heating deeper than cooling, something only mixing by wind and tides makes possible; salinity was added to the framework in the 1920s. Where deep water rises is still debated. Broecker argued, from high silicon levels, that most of it upwells in the North Pacific, but computer models increasingly place the bulk in the windy Southern Ocean. The RAPID programme has measured the Atlantic flow at 26.5 degrees north only since 2004, a blink for a system that works over centuries.
The stakes are large because the circulation carries heat and dissolved gases around the planet. A flood of meltwater from glacial Lake Agassiz is thought to have disrupted it and triggered Europe's cold Younger Dryas period. Today, melting ice is diluting Antarctic waters, and a collapse of either loop could bring drought to one hemisphere, a much colder Europe and faster sea level rise on North America's east coast. Such a collapse is generally judged more than a century away, though projections are highly uncertain.
Source: Thermohaline circulation
