Shutdown of Thermohaline Circulation - Thermohaline Circulation and Fresh Water

Thermohaline Circulation and Fresh Water

Heat is transported from the equator polewards mostly by the atmosphere but also by ocean currents, with warm water near the surface and cold water at deeper levels. The best known segment of this circulation is the Gulf Stream, a wind-driven gyre, which transports warm water from the Caribbean northwards. A northwards branch of the Gulf Stream, the North Atlantic Drift, is part of the thermohaline circulation (THC), transporting warmth further north to the North Atlantic, where its effect in warming the atmosphere contributes to warming Europe. Other factors are also important, such as atmospheric waves that bring subtropical air further north, which have been suggested to influence the climate of the British Isles more than the Gulf Stream. The evaporation of ocean water in the North Atlantic increases the salinity of the water as well as cooling it, both actions increasing the density of water at the surface. The formation of sea ice further increases the salinity. This dense water then sinks and the circulation stream continues in a southerly direction. Global warming could lead to an increase in freshwater in the northern oceans, by melting glaciers in Greenland and by increasing precipitation, especially through Siberian rivers. It is by no means clear that sufficient freshwater could be provided to interrupt thermohaline circulation; the Younger Dryas are a period where this might have been the case, although there is some disagreement as to its cause.

Some even fear that global warming may be able to trigger the type of abrupt massive temperature shifts which occurred during the last glacial period: a series of Dansgaard-Oeschger events – rapid climate fluctuations – may be attributed to freshwater forcing at high latitude interrupting the THC. The Younger Dryas event may have been of this sort, too. (See the discussion of chaos theory for related ideas.) However, these events are believed to have been triggered by massive freshwater discharges from the Laurentide ice sheet, rather than from the melting of polar sea-ice and precipitation changes associated with the increased open water in global warming. Meltwater events aside, the climate deterioration into the last ice age appears to have taken about 5,000 years. Also, in coupled Atmosphere-Ocean General Circulation Models the THC tends to weaken somewhat rather than stop, and the warming effects outweigh the cooling, even locally: the IPCC Third Assessment Report notes that "even in models where the THC weakens, there is still a warming over Europe". Model runs in which the THC is forced to shut down do show cooling – locally up to 8 °C (14 °F)— although the largest anomalies occur over the North Atlantic, not over land. However, climate models are not sufficiently sophisticated at present to include climatic factors which give these predictions veracity; e.g., the recent return of deep convection to the subpolar gyre in both the Labrador and Irminger seas and the growing ice mass of Greenland.

Studies of the Florida Current suggest that the Gulf Stream weakens with cooling, being weakest (by ~10%) during the Little Ice Age.