The north-west coast of Europe enjoys, as we know, a climate that is anomalously mild. This is a consequence of the benign influence of the Gulf Stream and the North Atlantic Drift. The warm water carried northwards by these surface currents surrenders great quantities of heat to the atmosphere in northern latitudes, and is an important moderator of our present climate.
The powerhouse of the Gulf Stream is in the vicinity of the West Indies. The persistent north-easterly trade winds in the lower latitudes have a tendency to drag the surface waters of the North Atlantic with them, and thereby cause sea level to be slightly higher in the western regions of that ocean than elsewhere. This results in an area of high water pressure centred on the Sargasso Sea comparable to an anti-cyclone in atmosphere.
Impelled by this high pressure, the current flows at first, northeastwards along the eastern seaboard of the US, and then broadens into the North Atlantic Drift to cross the ocean. Then, passing close to the south-west coast of Ireland, the North Atlantic Drift veers north-eastwards to continue along the northern European coastline as the warm Norwegian Current.
So far we have a surface phenomenon - but the Atlantic moves in three dimensions. Its warmth makes the Norwegian current relatively buoyant, but its saltiness results in water potentially more dense, and therefore heavier, than the surrounding northern seas; as the current cools, the balance is tipped in this respect, and it begins to sink. Ultimately the water begins to flow southwards again as a cold current near the ocean floor; upwelling in the south then completes a loop that has come to be called the Atlantic Conveyor.
Studies of the ice deep within the Greenland glaciers have shown that at certain times in the past, the climate seems to have changed dramatically for the worse over a very short time. It has been suggested that this may have come about because the melting of the polar ice cap during a relatively warm period released great quantities of fresh water into the northern seas: this in turn may have diluted the salt in that critical part of the Conveyor, so that sinking no longer occurred, the Conveyor stopped, and the climate of north-western Europe "flipped" into a much colder mode - another ice age.
The obvious fear is that, paradoxically, the present gradual rise in global temperature may trigger such a happening again. Computer models suggest that the balance is very delicate - but whether the Atlantic Conveyor is so fragile in real life is both difficult to demonstrate and controversial.