1 Living with the oceans. – A report on the state of the world's oceans

Great ocean currents


1.13 >  As a rule, icebergs consist of freshwater or contain only small amounts of salt. Because of their slightly lower density compared to seawater, a small fraction extends above the water. The largest part is below the surface. © [M], Bryan & Cherry Alexander/Arcticphoto/laif 1.13 > As a rule, icebergs consist of freshwater or contain only small amounts of salt. Because of their slightly lower density compared to seawater, a small fraction extends above the water. The largest part is below the surface.

The uncertain future of sea ice

Sea ice in the Arctic regions has a significant impact on heat exchange between the atmosphere and ocean, because it acts as an insulating layer to prevent heat from escaping from the water. Considering how large the area of ice is, it is clear that it must have an impact on the global climate. In the Arctic Ocean the sea ice, which is commonly called pack ice, has an average thickness of three metres. In the Southern Ocean it averages around one metre. The total area of sea ice expands and recedes with the seasons. On a yearly average around seven per cent of the oceans (about 23 million square kilometres) is covered with ice, which is equal to about three times the size of Australia. By comparison, the ice masses on land are relatively stable. They permanently cover around ten per cent of the land surface (14.8 million square kilometres). Scientists call the ice-covered areas of the Earth the cryosphere. In addition to land and sea ice, this also includes the shelf ice, the parts of continental ice sheets that extend into the ocean. Changes in the sea ice, including its extent, areal coverage, thickness, and movement, are caused by dynamic processes such as ocean currents and by thermodynamic processes such as freezing and melting. These, in turn, are influenced by solar radiation as well as the heat flux at the sea surface. One of the most conspicuous and important characteristics of climate fluctuations is the change in sea-ice extent in the polar regions. During some winters the Arctic sea ice extends much further to the south than in others. Geophysicists consider the sea ice to be simply a thin, discontinuous layer on the polar oceans that is ­driven by winds and ocean currents, and is variable in thickness and extent. Sea ice forms a boundary between the two large components of the Earth system, the atmosphere and the ocean, and very significantly influences their interaction. Sea ice has a strong reflective property, called albedo, and it reflects a considerable amount of the incoming sunlight. This effect is enhanced when the ice is covered with snow. The sea ice therefore influences the radiation balance of the Earth and thus plays an important role in the climate system.
The impact of sea ice on climate is further amplified by its insulating effect between the atmosphere and ocean. It inhibits the exchange of heat and wind energy between the atmosphere and ocean considerably. The atmosphere is therefore much colder above the sea-ice surface than above the open ocean. This has the effect of increasing the air-temperature difference between the tropics, subtropics, and the polar regions. In warmer regions the air has a greater tendency to rise, which lowers the air pressure significantly. By contrast, in very cold regions the air is heavier, and high pressure zones are created. Accordingly, the compensating air flow between high and low pressure areas is strong and, in concert with the Coriolis force, creates stronger westly winds in the middle latitudes. Of course, sea ice also influences convection processes in the ocean, and thus the formation of deep and bottom water. Sea ice therefore plays an important role in the large-scale ocean circulation, especially with regard to thermohaline circulation. It is not yet known how global warming affects the formation of sea ice and the related processes. Ice melts when it becomes warmer. But it is difficult to predict what effect this has on the currents. In any case, all ­climate models predict an acceleration of warming in the Arctic with a continuing rise in trace-gas concentrations.
In addition, observations indicate a clear decrease in Arctic sea-ice cover in recent decades. This is partly related to a positive feedback mechanism called the ice-albedo feedback. Light surfaces have a very high albedo. When the sea ice retreats as a result of global warming, albedo decreases and more solar energy is available, which leads to additional warming, and melts more ice. This process primarily occurs at the margins of the sea ice. Similar to a spot of grass on the edge of a patchy snow cover, the seawater at the margins of the ice warms more rapidly, and the ice thaws faster there. The further the ice retreats, the larger the area of the open, relatively dark sea surface becomes. The melting is thus amplified. The shrinking of sea ice could therefore amplify climate change in the future. Ironically, this would provide people with something that they have been wanting for a long time: the opening of a northern seaway from Europe across the Arctic to Asia – the Northern Sea Route. In recent years the ice has retreated at such a rate that Arctic waters along the north coast of Russia could be navigable year-round by commercial ships in the future. The route is several thousand kilometres shorter than the trip through the Suez Canal. In the early autumn of 2009 a Bremen shipping company became one of the first private companies in the world to navigate the Northern Sea Route with a merchant vessel. But the negative consequences of climate change will presumably outweigh the advantages of a navigable northern route. There is, for instance, a substantial negative impact on Arctic animals such as the polar bear, whose habitat is melting away.
The large ocean currents and their driving forces have already been intensively investigated, but there are still many unanswered questions in the fine details. For example, thermohaline circulation, with the interplay of its driving factors, has not yet been completely explained. Different mathematical models have produced different conclusions. All models use the same equations, variables, and input parameters. But it is difficult to accurately estimate climate influences at scales of a few kilometers or even smaller and to apply them correctly within the large, global models. Textende