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The oceans store an immense amount of heat energy, much more than the atmosphere, and consequently play a crucial role in the regulation of the global climate. As in the atmosphere, surface ocean currents assist in the transfer of heat from low to high latitudes. Warm water moves poleward whilst cold water returns towards the equator. Energy is also transferred via moisture. Water evaporating from the surface of the oceans stores heat which is subsequently released when the moisture condenses to form clouds and rain.

Heat exchanges also occur vertically within the oceans, between surface water, usually the top 200 metres or so, and the deep water. Seawater in the high latitudes readily sinks, forming deep-water currents. Considerable evaporation of moisture takes place from the warm surface ocean currents as they travel towards the high latitudes. The salt which remains behind in the water after evaporation makes the water heavier or denser. As in the atmosphere the surface and deep-water currents of the world's oceans are inter-linked forming the global ocean circulation. Scientists have proposed that changes in this global ocean circulation influence climate changes over hundreds and thousands of years.

Shifts in the Earth's orbit around the Sun have been shown to be linked to changes in the Earth's climate over hundreds of thousands of years. Scientists now recognise however, that such orbital variations are on their own not enough to account for the shifts in global climate between cold Ice Ages and warm interglacials. Whilst orbital variations may indeed act as a pacemaker to warm-cold climate transitions, additional climate feedback processes have been invoked to explain the large changes in global average temperature of up to 5C. Changes in the composition of the atmosphere, in particular the levels of the greenhouse gas carbon dioxide, have been shown, through reconstruction of palaeoclimatic records, to account for the colder climate of the last Ice Age. A change in ocean circulation however, has been proposed as the trigger mechanism for the transition from a warm interglacial climate to a cold Ice Age 120,000 years ago, and the switch back again 14,000 years ago.

Scientists believe that at the end of the last warm interglacial period 120,000 years ago, high latitude seawater was gradually cooling as a consequence of a reduction in heat received by the Sun due to orbital variations. Colder seawater in the high latitudes will exhibit less deep-water circulation. Sea sediment records have revealed that this change in deep water formation is particularly evident in the Northern Hemisphere. Colder seawater loses less water through evaporation of moisture, and is therefore less salty and lighter. The lighter water finds it more difficult to sink, reducing or even shutting down completely that branch of the global ocean circulation. The warm poleward-moving surface currents too, are slowed or redirected, and consequently less heat is transferred to the polar regions. Less heat, of course, means a colder climate, leading to the growth of ice sheets across the Northern Hemisphere landmasses and the development of a new Ice Age.

Climate models have suggested that the loss of the warm surface Gulf Stream in the North Atlantic, which keeps the climate of western Europe mild, could result in a regional drop in temperature of 6 to 8C. In addition, such changes in ocean circulation could occur over relatively short periods, perhaps within 50 to 100 years. Rather paradoxically, scientists have now begun to speculate that global warming could threaten the present course and intensity of the Gulf Stream. In this climate scenario deep water formation is reduced, not by a reduction in moisture evaporation, but by a melting of ice from the Greenland ice cap. The influx of freshwater would reduce the saltiness of the seawater, making it lighter and more difficult to sink. Worryingly, such a pattern of climate change can be seen the palaeoclimatic record 11,000 years ago, at the beginning of the present interglacial warm period. Large volumes of melting ice from the retreating Northern Hemisphere ice sheets flowed into the North Atlantic, shut down the regional ocean circulation and cause a temporary fall in regional temperature of several degrees Celsius in just a few hundred years.

Global ocean circulation