| Summary: | The ocean circulation has a great impact on Earth's climate, especially by transporting heat to Europe. Changes in the freshwater supply into the North Atlantic and Nordic Seas have been hypothesized to have a driving role behind changes in the ocean circulation, causing past climate change. This has been a reason for concern, and a collapse of the current Atlantic meridional overturning circulation has been widely discussed. A suggested theory for enabling abrupt climate change is Stommel's classic box-model which connects to the thermohaline circulation. The thermohaline circulation is associated with density differences and upheld by physical processes affecting temperature and salinity. Originating from the fact that temperature and salinity have opposing effects on the driving density difference, Stommel's theory explains possible implications, such as different ocean circulation regimes, which can be either stable or unstable. This thesis investigates how the freshwater supply affects the thermohaline circulation. Stommel's theory about bi-stability in the ocean system is applied to the thermohaline circulation and paleoclimate. Stommel's theory can explain a possible "turn off" of the ocean circulation during the Younger Dryas period, give rise to the quasi periodic Dansgaard-Oeschger events, and explain the seesaw effect between the hemispheres. In conclusion, Stommel's simple box-model provides a conceptual picture for the thermohaline circulation, which could have been a key factor in past climate change but is unlikely to cause abrupt changes in the near future.Â
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