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Ocean-Atmosphere-Climate dynamics

Records from Greenland ice cores (Cuffey and Clow 1997) illustrate that abrupt temperature oscillations were the norm over much of the past 100 000 years. Shifts between warm and cold climates occurred rapidly, sometimes within a decade (Alley and others 1993, Alley and others 2003). This suggests that such abrupt changes could occur again.

Over the past 8 000 years these oscillations have been absent, and the Earth has experienced several millennia of relatively stable climate. Modern human civilization developed during this period. It was and is based on permanent agriculture, which depends upon a stable climate with predictable patterns of temperature and rainfall. If abrupt change were to recur, there would be unique challenges to human societies, and to natural ecosystems which have great difficulty adapting to rapid change.

A major factor involved in the abrupt climate changes of the past appears to have been changes in the ocean circulation, which distributes heat from the equator toward the poles. This circulation is controlled in part by differences in seawater density, which is determined by the temperature and salt content of the water. The colder and saltier the water, the more dense it is, and the more readily it sinks. Flows within the oceans related to variations in temperature and salt are called the 'thermohaline circulation' ('thermo' for heat and 'haline' for salt) or the 'Conveyor' (Broecker 1995) (Figure 1).

Figure 1: A schematic diagram of the global ocean Conveyor (thermohaline circulation)

Red indicates warm surface currents, including the Gulf Stream which is important to warming Northern Europe. Blue indicates cold deep saline currents.

Source: WHO 2004

As the waters of the warm Gulf Stream-North Atlantic current system flow northward, the surface waters cool and thus become denser. In some locations, the salty surface waters become dense enough to sink into the deep ocean (Figure 2). This sinking is called ventilation or deep convection and generally occurs in the Greenland, Iceland, Norwegian and Labrador Seas as well as in the subpolar gyre of the North Atlantic (Figure 1).

When the surface waters sink, they pull in additional waters and ultimately form the North Atlantic Deep Water that flows southward. In turn, this draws more warm water at the surface northward (Figure 2).

Figure 2: Vertical cross-section of Atlantic circulation

A diagram depicting the northern flow of surface waters (compensating flow), the deep sinking of dense surface waters in the Greenland, Norwegian and Labrador Seas (ventilation) and the combining of Nordic overflow waters, carried down and mixed with the deep waters of the western North Atlantic waters and Labrador Sea ventilation waters to form the southward flow of North Atlantic Deep Water (NADW). Background colours distinguish the blue Nordic Sea waters from red North Atlantic waters and purple NADW. Green arrows indicate flows.

Source: Modified from Hansen and others 2004

The northward-flowing compensating flow of warm water has a crucial climatic function for northern and western Europe and some parts of northeastern America. It carries heat from lower latitudes, losing much of this to the atmosphere as it moves northward. In doing so it makes northern and western Europe warmer in winter than the west coast of North America at similar latitudes.

The sinking that drives the global thermohaline circulation depends critically on the water being sufficiently cold and salty. Anything that makes the water less cold and less salty can jeopardize the circulation, with potentially serious impacts.

Observations over recent decades suggest that changes in the factors that govern this circulation are occurring, possibly as a result of human activities. This raises concerns about possible abrupt climate changes in the future.

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