Do you think Britain is going to freeze this century, and would America take the refugees?

Ocean Conveyor Belt

As the heads of state gather in Gleneagles, Scotland for the G8 summit, they would do well to ponder the possibility that the place where they are currently enjoying the British summer could be under ice the whole year around in a few decades, thanks to global warming!

Ocean currents

There are various forces at work moving the waters of the oceans. One is the tides, driven by the gravitational pull of the Moon and the Sun.

The second is surface winds; the water at the ocean surface moves at about three per cent of the wind speed. This movement is also affected by the rotation of the Earth. These surface winds move most pollution around the Earth, including the Nike shoes which ended up on the Pacific Coast of America after having been lost at sea in 1999, and the radiation from the Russian submarine Kursk which sank in the Barents Sea north of Norway in summer 2000 - its radiation was detected in the sea north of Iceland. Many surface currents have been given names.

The third is called thermohaline circulation. This is driven by the fact that the density of water (a function of temperature and salinity) varies around the oceans. 'Thermo' means heat and 'haline' means salt. The sun heats the surface water. Evaporation increases salinity. Fresh water arriving from rivers and falling as rain decreases salinity. Cooling water increases its density and it sinks. Heating water decreases its density and it rises. This is convection, the same process that occurs in air. An increase in saltiness also means an increase in density.

Ocean conveyor belt

Ocean Conveyor Belt Courtesy of Argonne National Laboratory

Over time, a pattern of currents has arisen where cold, salty water travels along the bottom of the oceans (which are currently on average 3.7 km deep) and returns nearer the surface. This is called the ocean conveyor belt (see diagram, where red means warmer surface currents, blue means cooler deeper currents, and arrows show the direction of flow). When warmer water arrives near Greenland (top left) it is cooled, and sea ice forms making the waters below more salty, so the waters sink to the ocean floor and begin the journey south. The cold water ends up in either the Indian or the Pacific oceans where the waters become warmer and rise again and begin the journey back near the surface. It can take several hundred years for the water to make the complete circuit. The currents depend on the geography; for example, the water cannot get far enough north in the Pacific to cool and become dense as it does in the Atlantic. Some warm surface water flows from the Pacific around Cape Horn to rejoin the warm current in the Atlantic too, but this is not shown in the diagram. The currents will vary if the water level rises or falls by a large amount, as this can affect barriers to the paths the currents can take.

The Gulf Stream, or North Atlantic Drift, is the name given to the warm current that flows northeastwards from the Gulf of Mexico to north west Europe. It is this current in the Atlantic that keeps Britain and Western Europe much warmer than other places equally far north.

In the past the ocean conveyor belt has ground to a halt several times during glacial periods, as has been shown by Greenland ice cores and ocean sediments. On those occasions it likely happened because ice spread from North American ice sheets into the Atlantic. The trigger during global warming would be different, however.

The ocean conveyor belt can switch off quickly. Experts estimate that there is a 50 percent chance of it happening in the next 100 years. There is already evidence that the salinity is dropping and that it is slowing down as the Greenland ice sheet is melting. As a result of human activity, more fresh water is flowing into the Atlantic Ocean, diluting the saltiness of the seawater and reducing its density.

The current sinks near Greenland because of the change in density of the water there, caused by the forming of sea ice leaving the salt to boost the salinity of the water underneath, which becomes denser and sinks. The extra fresh water from the melting Greenland ice caps, etc., dilutes the water. This means that it cannot sink - end of current. No current, no Gulf Stream. No Gulf Stream, Britain shivers. Unfortunately, nobody knows exactly how dilute the ocean can be before the conveyor belt stops. Nobody knows how to restart it, either. Last time it took over 1,000 years.

Other ocean issues

Ocean level rise: If the oceans become warmer then the water will expand and the sea level will rise. However, the oceans are slow to expand, and would carry on rising for a millennium after the rise started, as the warmth gradually spread through the depths, expanding each layer in turn. Melting ice caps would also add the ocean rise; in fact the melting of any ice that is currently over land. Ice which is already floating on the seas displaces approximately the same volume of water as it takes up when melted. If the West Antarctic ice sheet were to disintegrate, for example, the sea level would rise six metres around the world. That is unlikely to happen this century, however.

Carbon dioxide: The oceans dissolve and store carbon dioxide. If they store some of the excess we are currently producing and it gets into the thermohaline currents, it could resurface to add to global warming many decades or centuries into the future.

Significant changes in atmospheric temperature above an ocean can have a more local affect, such as the El Niño-Southern Oscillation (ENSO) or the North Atlantic Oscillation, which can affect weather patterns.

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