The high heat capacity of seawater and the relatively slow ocean circulation allow the oceans to provide significant 'memory' for the climate system. Bodies of water that descend from the sea surface may reside in the ocean interior for decades and centuries, while preserving their temperature and salinity signature, before they surface again to interact with the overlying atmosphere. In contrast to that, the residence time of water in the atmosphere is about 10 days and the persistence of dynamical states of the atmospheric circulation may last up to a few weeks. Thus, on long time scales ocean dynamics becomes important for climate, which implies that climate variations and climate change can only partially be understood without consideration of ocean dynamics and the

Climate Change: Observed Impacts on Planet Earth

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intricate ocean atmosphere interaction. The El Nino/Southern Oscillation phenomenon in the tropical Pacific is a prominent example of tightly coupled ocean atmosphere dynamics on interannual time scales, other more weakly coupled interactions exist throughout the system.

The oceans' role in climate and climate change is manifold. Ocean circulation transports large amounts of heat and freshwater on hemispheric space scales which have significant impacts on regional climate in the ocean itself but also noticeable consequences via atmospheric teleconnections on land. What is well known for the seasonal cycle with only moderate temperature changes between summer and winter in marine climates compared with much larger swings within the continents, is also true on decadal time scales. Since 1960 the heat uptake of the oceans has been 20 times larger than that of the atmosphere. Thus the oceans have been able to reduce the otherwise much more pronounced temperature rise in the atmospheric climate. Also, over the last 200 a, the oceans have absorbed about half of the CO2 release into the atmosphere by human activities (fossil fuel combustion, de-forestation, cement production), thereby reducing the direct effect of greenhouse gases on atmospheric temperatures.

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