Question 11:

What is thermohaline circulation?

The world's oceans transport massive amounts of heat. Differences in seawater density, which depend on differences in temperature (thermo) and salinity (haline), drive global ocean currents known as the thermohaline circulation or the meridional overturning circulation. Part of the thermohaline circulation is the Gulf Stream, which warms Western Europe.

The Atlantic thermohaline circulation acts like an oceanic conveyer belt, carrying heat from the tropics to the North Atlantic. As warm water moves into the northern Atlantic, it cools, sinks to the ocean floor, and then returns southward.

The Southern Ocean is also a significant contributor to the thermohaline circulation, linking the shallow and deep ‘limbs' of the ocean conveyor belt, and playing a major role in the ‘heat engine' that influences global climate patterns.

There are concerns that climate change may slow or even halt the thermohaline circulation. This could occur through changing salinity of the oceans due to greater rainfall and influxes of fresh water from melting ice. Surface ocean waters are becoming less salty in some places, and a key current in the North Atlantic appears to have slowed.

The thermohaline circulation has changed abruptly in the distant past; disruption of the thermohaline circulation could lead to rapid changes in the Earth's climate.

A shutdown in thermohaline circulation within decades is most unlikely. However, the Intergovernmental Panel on Climate Change states that it is very likely that the meridional overturning circulation of the Atlantic Ocean will slow during the 21st century. Temperatures in the Atlantic region are projected to increase despite such changes due to the much larger warming associated with projected increases of greenhouse gases. It is very unlikely that there will be a large abrupt change in circulation during the 21st century.

Schematic diagram of the global ocean circulation pathways

Schematic diagram of the global ocean circulation pathways, the ‘conveyor' belt (after W. Broecker)