The Earth's climate is variable and has exhibited natural changes over the course of millions of years. Climate research has identified these natural causes, however, for the 20^th and 21^st centuries, human-induced changes must also be taken into account by scientists. The greenhouse gas CO₂ plays an important role here. Its atmospheric concentration has nearly doubled since the onset of industrialization (beginning of the 19^th century), through the burning of fossil fuels such as coal and oil. Scientists assume that CO₂ concentrations will continue to increase at a similar rate over the coming decades. One of the important achievements of climate research is that of recognizing that these so-called "anthropogenic" CO₂ emissions are largely responsible for the global warming that we are observing today.

The oceanic CO₂ cycle
Oceans play a key role in the global CO₂ cycle. More than half of the (anthropogenic) CO₂ produced on an annual basis by humans is currently stored in the oceans for a very long time. How much exactly, for how long, and whether this storage will potentially change in the future is not yet known in detail and is the subject of research.

One reason for the high oceanic CO₂ sequestration is an effect of the so-called "thermohaline" circulation. This is a circum-global ocean current that runs through the oceans like a sort of global conveyor belt. In the European Norwegian Sea, the Labrador Sea and close to Antarctica, the surface waters that are very high in salt content cool down in the winter, become heavier and therefore sink to great depths. From there, these waters flow in the direction of the equator, protected from any direct influence from the atmosphere. The cold water masses slowly mix with the warmer layers above and rise slowly while passing through the deep sea. In addition to this mixing effect, the thermohaline circulation can be driven on by the wind (mainly in the southern oceans). The circular flow is accomplished by a compensatory current close to the surface, such as the Gulf Stream, that moves warm water masses back to the northern origins of deep sea water.

The thermohaline circulation is thus composed of a deep current and a current that is close to the surface, and is maintained by cooling at the poles and mixing of water masses in the centre of the oceans. The water close to the surface is cooled down on its way to the poles and sequesters CO₂. The CO₂ that has thus been taken up is transported to the deep sea by the thermohaline circulation and is only released into the atmosphere again when it reaches the surface of the sea. CO₂ can remain in the deep sea for years and even centuries. This effect is also called the "physical CO₂ pump" and is the reason why the ocean has to date taken up more than half of the CO₂ produced by humans. However this effect will decrease with further increases in atmospheric CO₂ concentrations, as the sea water's capacity for additional CO₂ sequestration will be reduced with continued increases in CO₂.
In addition to this physical CO₂ pump, the so-called "biological CO₂ pump" also operates in the global CO₂ cycle. Only the top 30 to 100 meters of surface water provide sufficient sunlight for photosynthesis in unicellular algae (phytoplankton). However, dead algae sink down out of this zone and take the carbon they have absorbed through photosynthesis to the bottom with them. An enormous carbon reservoir is thereby produced in the deep sea.

Contact

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• Prof. Thomas Slawig

  Institution:

  Institut für Informatik

  Phone:

  +49 431 800-4490

  E-mail:

  ts@informatik.uni-kiel.de

• Prof. Andreas Oschlies

  Institution:

  GEOMAR | Helmholtz-Zentrum für Ozeanforschung Kiel

  Phone:

  +49 431 600-1936

  E-mail:

  aoschlies@geomar.de

• Prof. Anand Srivastav

  Institution:

  Institut für Informatik

  Phone:

  +49 431 880-7252

  E-mail:

  asr@numerik.uni-kiel.de