There is a continual exchange of heat and moisture between the oceans and the atmosphere. In some parts of the world (such as the subtropics), evaporation from the sea exceeds precipitation into it, whereas the opposite is the case in other regions. Consequently the salinity (or saltiness) of ocean water is not the same everywhere. The density of water changes with its salinity and temperature (warmer or less salty water is less dense, and cooler or saltier water is more dense). This causes large-scale up and down movement of water in the oceans, with denser water sinking in some places and less dense water rising in others. This is known as the 'thermohaline' or density-driven circulation. In simple terms, cool saline water at high latitudes in the Atlantic Ocean sinks and moves towards the equator. Elsewhere, for example in the Indian and Pacific Oceans, deep water rises to the surface, in a process known as upwelling. The overall effect is a slow 'ocean conveyor belt', illustrated below. This re-distributes heat from one region to another.
Simplified global ocean circulation patterns
Source: Bureau of Meteorology, The greenhouse effect and climate change, Bureau of Meteorology, 2003, p. 9.
The oceans can hold much more heat than the atmosphere. Just the top 3.2 metres of ocean holds as much heat as all the world's air. This large heat capacity acts to buffer temperature changes in the atmosphere, while ocean circulation redistributes heat around the globe and influences regional climates. Energy used in evaporation of water from the ocean also represents a major influence on the earth's heat balance.
The ocean temperature, however, is not in equilibrium with the atmosphere because of the long time-scales involved in many oceanic processes, such as the large-scale overturning of the deep ocean which takes thousands of years. Water carried from the surface to the deep ocean is isolated from atmospheric influence. This means that exchange of heat between the atmosphere and oceans, and redistribution of heat within the oceans, may occur over a thousand years or more. Hence the oceans represent a major heat sink, and can influence the rate of climate change by moderating the rate of warming of the atmosphere.
Another important role of the ocean is its ability to absorb CARBON DIOXIDE from the atmosphere. The oceans are currently absorbing about one quarter of the CO2 that is being emitted to the atmosphere through human activities, thereby representing an important greenhouse gas sink. This uptake of CO2, however, is causing ocean acidification, and the rate of CO2 absorption by the oceans will slow with time as the uptake mechanism becomes limited under higher CO2 concentrations.
There is evidence that the salinity of the ocean is changing as a manifestation of climate change producing changes in precipitation, evaporation, river runoff and ice melt. The upper 500 metres of the northern oceans are freshening while the tropical oceans are becoming saltier. The changes in density arising from this are likely to induce changes in ocean circulation patterns, with projections suggesting that the thermohaline circulation will be slowed. This will have implications for future heat transfer between the atmosphere and oceans.
Bureau of Meteorology, The greenhouse effect and climate change, Bureau of Meteorology, 2003.
Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 5 Observations—oceanic climate change and sea level.