[thedoc (OP)]

Jan Skajaa asked the Naked Scientists:
   
Hi, if ocean currents are fed by ice formation around the poles, and the climate getting warmer causing the ice to form at a lower rate thus slowing down the currents, wouldn't it mean that less warm water would be brought to the polar regions causing the temperature to decrease and then having more ice formed? In other words isn't it a self-regulating mechanism? Will the Gulf current slow down, for example, or will it stay steady or even increase by changing climate?

What do you think?

[damocles]

Earth systems are enormously complicated, and we are only just starting to understand them. The problem is that there are both negative feedback (leading to regulation) and positive feedback (leading to amplification and chaos) loops in operation, and there is a very fine balance between them.

For at least the last half million years the climate appears to have been operating in a bistable mode -- relatively longer ice ages leading to relatively shorter interglacials. About 20 years ago there was a theory that the switching mechanism involved a change of ocean circulation when the sea surface temperature reached a certain level that shut down the Gulf Stream. The Gulf Stream is responsible for providing Western Europe with a milder climate than at similar latitudes in Canada or Russia. When the Gulf Stream shut down, glaciation started in Western Europe, and this tipped the balance of a positive feedback leading to extensive glaciation in the whole of the Northern Hemisphere.

I do not know the present status of this hypothesis, or even where it is documented, but if you are really interested I think a little web research (perhaps starting with "Ice Age cause" or "Gulf Stream ice age" as terms in a google search) should get you into it.

[evan_au]

This ocean conveyor belt is driven by temperature differences and by differences in salinity (salt concentration).
Salty water is denser than fresher water, which drives it into lower parts of the ocean.

If melting of land-based polar ice sheets accelerates, this adds more fresh water to the oceans, reducing the salinity of polar seawater, in turn reducing the forces driving these ocean currents. It is thought that breakout of a large freshwater lake near the US/Canadian Great Lakes at the end of the last Ice Age did shut down some of these currents for a while. Scientists are continually monitoring the speed of these ocean currents from satellites and undersea robots.

See: http://en.wikipedia.org/wiki/Shutdown_of_thermohaline_circulation

[CliffordK]

The salt gradient may be caused by the release of brine during the formation of sea ice. 

If, for some reason, less new sea ice was to form, then it is possible the saline gradient may begin to break down.  Sea ice, of course, is very complicated.  Blue water may absorb more sunlight than ice covered water, especially absorbing more heat than thick older ice.  However, thick ice can also act as an insulator, so thin ice may form more rapidly than the formation of layers of ice under a thick sheet.

Anyway, it is concerning that the peak sea ice extent in the Northern Hemisphere seems to be decreasing even though the sea ice in the southern hemisphere may be increasing.