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How will man-made climate change affect the ocean circulation? Is the present system of ocean currents stable, and could it be disrupted if we continue to fill the atmosphere with greenhouse gases? These are questions of great importance not only to the coastal nations of the world. While the ultimate cause of anthropogenic climate change is in the atmosphere, the oceans are nonetheless a vital factor. They do not respond passively to atmospheric changes but are a very active component of the climate system. There is an intense interaction between oceans, atmosphere and ice. Changes in ocean circulation appear to have strongly amplified past climatic swings during the ice ages, and internal oscillations of the ocean circulation may be the ultimate cause of some climate variations.
Our understanding of the stability and variability of the ocean circulation has greatly advanced during the past decade through progress in modelling and new data on past climatic changes. I will not attempt to give a comprehensive review of all the new findings here, but rather I will emphasise four key points.
Ocean currents have a profound influence on climate
Covering some 71 per cent of the Earth and absorbing about twice as much of the sun's radiation as the atmosphere or the land surface, the oceans are a major component of the climate system. With their huge heat capacity, the oceans damp temperature fluctuations, but they play a more active and dynamic role as well. Ocean currents move vast amounts of heat across the planet - roughly the same amount as the atmosphere does. But in contrast to the atmosphere, the oceans are confined by land masses, so that their heat transport is more localised and channelled into specific regions.
The present El Niño event in the Pacific Ocean is an impressive demonstration of how a change in regional ocean currents - in this case, the Humboldt current - can affect climatic conditions around the world. As I write, severe drought conditions are occurring in a number of Western Pacific countries. Catastrophic forest and bush fires have plagued several countries of South-East Asia for months, causing dangerous air pollution levels. Major floods have devastated parts of East Africa. A similar El Niño event in 1982/83 claimed nearly 2,000 lives and global losses of an estimated US$ 13 billion.
Another region that feels the influence of ocean currents particularly strongly is the North Atlantic. It is at the receiving end of a circulation system linking the Antarctic with the Arctic, known as 'thermohaline circulation' or more picturesquely as 'Great Ocean Conveyor Belt' (Fig. 1). The Gulf Stream and its extension towards Scotland play an important part in this system. The term thermohaline circulation describes the driving forces: the temperature (thermo) and salinity (haline) of sea water, which determine the water density differences which ultimately drive the flow. The term 'conveyor belt' describes its function quite well: an upper branch loaded with heat moves north, delivers the heat to the atmosphere, and then returns south at about 2-3 km below the sea surface as North Atlantic Deep Water (NADW). The heat transported to the northern North Atlantic in this way is enormous: it measures around 1 PW, equivalent to the output of a million power stations. If we compare places in Europe with locations at similar latitudes on the North American continent, the effect becomes obvious. Bodö in Norway has average temperatures of -2°C in January and 14°C in July; Nome, on the Pacific Coast of Alaska at the same latitude, has a much colder -15°C in January and only 10°C in July. And satellite images show how the warm current keeps much of the Greenland-Norwegian Sea free of ice even in winter, despite the rest of the Arctic Ocean, even much further south, being frozen.
