The global ocean is not a static pond, but a body in constant motion. Winds blow across its surface, generating waves and currents, while the pull of gravity gently sloshes it back and forth in a lunar rhythm of tides. But beneath these familiar surficial motions lies an enigmatic process which has profound implications for climate: the Great Ocean Conveyor (also called the thermohaline circulation). The Conveyor is one of the great unknowns in humanity's unintentional climate change experiment.

A simplified diagram of the Great Ocean Conveyor.

The Conveyor is slow-moving - at most, 10cm per second - but its vast extent includes a flow equivalent to 100 Amazon rivers. On average, 30 million cubic metres of water enter the Conveyor every second.

Surface water, warmed at the equator, moves to high latitudes where it releases heat to the atmosphere. As a result, the water cools - becoming denser - and sinks to the deep ocean. Deep water slowly travels through the oceanic abyss, eventually mixing up to the surface in distant parts the world, as much as 1000 years later.

Warm surface water cools as it flows into the North Atlantic, releasing heat to the atmosphere and eventually sinking.

Much of the sinking in the modern ocean happens in the North Atlantic limb of the Conveyor. Warm waters are carried north by the Gulf Stream, cooling as they travel. By the time they reach the vicinities of Labrador and Iceland, these waters have grown cold and dense. During winter the coldest, saltiest waters sink thousands of metres below the surface to form North Atlantic Deep Water (NADW). The heat released to the atmosphere during NADW formation is largely responsible for the relatively warm temperatures enjoyed by western Europe.

Heat released by NADW formation allows gardeners in SW England to grow tropical plants like these in the Scilly Isles.

If not for the North Atlantic loop of the Conveyor, European winters would be much colder. Berlin might have the climate of Edmonton, which lies at the same latitude, while Stockholm might be more like Iqaluit.

If the Conveyor were a consistent feature of the earth, all of this would be only of academic interest. However, paleoclimate evidence shows that it can undergo sudden changes, with huge impacts on the entire North Atlantic region and potentially the world.

It is well accepted that the earth - as a whole - is warming as a result of human activity. Ironically, the lessons of distant earth history show that warming may cause a sudden drop in temperature around the North Atlantic.

Computer models with elevated greenhouse gas concentrations tend to show a warming and freshening (i.e. reduced saltiness) in the northern North Atlantic ocean. This simply results from a combination of global warming, changes in rainfall patterns, and melting glaciers. But after a period of warming, the models predict a weakening or complete halt of the NADW formation, similar to the situation during pulses of sudden cooling tens of thousands of years ago.

The North Atlantic, showing Greenland and Iceland, where a dramatic loss of saltiness is currently taking place. From Dickson et al. in Nature, April 2002.

A distressing confirmation of the models has recently emerged. Measurements of the saltiness of the North Atlantic show that the region has been growing gradually fresher over the past 40 years. The rapidity and extent of freshening came as a surprise to oceanographers - the change is equivalent to a 3-4m cap of freshwater appearing over a broad area of the northern North Atlantic. If this continues, the resulting loss of density could prevent sinking, and short-circuit the NADW within decades.

Judging from records of the distant past, this could be disastrous for eastern North America and western Europe. Average temperatures could plunge by 5 degrees C - for comparison, this is about the same difference as between the global average temperature today and during the last Ice Age, when Canada lay beneath 3000m thick glaciers. The resulting colder, snowier winters would require new infrastructure, damage crops and shorten the growing season. The costs associated with such a change would be enormous.

If greenhouse gas emissions continue to rise rapidly, the chances of the NADW shutting down - or of some other abrupt change - will continue to grow. Reducing the use of fossil fuels is the only strategy available to reduce the risk of catastrophic climate surprises (see solutions).

Learn more:

Are we on the brink of a little Ice Age? An in depth article from the Woods Hole Oceanographic Institute.