A key component of the conveyor is the formation of cold, dense water in polar oceans, particularly the Arctic. This occurs when ice forms at the surface. As water crystallises the salt is extruded and becomes more concentrated in the water below the ice, increasing its density. The dense, but well oxygenated water then sinks to the bottom of the ocean and is replaced by near surface waters from more temperate latitudes. Due to the topography of the ocean basins and the continents, one of the most important results of this process is the flow of warm water into the north-east Atlantic, Norwegian Sea and adjacent areas of the Arctic, raising the temperature of northern Europe to levels above those in similar latitudes elsewhere in the world. Increasing temperatures are affecting this process in several ways.
1. Reduced ice coverage has restricted the area where the formation of this dense saline water can form.
2. As open water absorbs more solar energy than ice, the reduction in ice cover will tend to enhance the temperature increase and further reduce ice cover.
3. Increased fresh water run-off, due to thawing of the tundra and ice caps, has reduced salinity in the Arctic Ocean. The result is a reduction in density of the near surface waters so that, when ice forms, there is less salt to increase the density to the level at which the water will sink to the ocean floor.
4. The increase in low salinity water has caused increased outflow of surface water from the Arctic, particularly in the Labrador Current.

If the formation of deep saline water is reduced, increasing evidence shows this has been the case in recent decades, the flow of warm water across the Atlantic may not penetrate the higher latitudes to the extent that it has in historical times and a local climatic deterioration in northern Europe could be the consequence. Large influxes of low-density meltwater are thought to have led to a disruption of deep-water formation in the North Atlantic and caused the cold climate period in Europe known as the Younger Dryas.

The flow of well oxygenated water to the ocean floor from the 'sinks' at high latitude help to maintain the deep ocean basins as habitats for a diverse biota. This is still probably the least known component of biodiversity on Earth. In inland waters and areas like the Baltic and Black Seas, the biota of anaerobic areas is seriously impoverished. There is concern that a breakdown of the global conveyor may cause extensive areas of the deep oceans to suffer similar impoverishment.
The circulation of the waters in the global oceans is completed by return of deep waters to the surface in upwelling areas which are often highly productive areas with important fisheries. The implications of any alteration in the nutrient or oxygen content of these upwelling waters are uncertain.

The Global Conveyor Belt