PhD defence: Carbon Cycle Feedbacks and Climate Tipping Behavior

The climate of the past millions of years has shown a lot of variability, and is strongly coupled to the carbon cycle. This variability might be caused by tipping points, which refer to relatively quick and large changes in an element in the climate system compared to their forcing. The coupling between temperature and the carbon cycle suggests an important role for the carbon cycle by either showing tipping behavior or by providing positive feedbacks to the climate while interacting with a tipping element. A better understanding of these relations is important since they might play a role in future climate change.

In this thesis, we study whether there are tipping points in the marine carbon cycle and how the marine carbon cycle interacts with large changes in the circulation of the Atlantic Ocean, which is a potential tipping element.

By using simple models, we have looked for tipping points in the marine carbon cycle, but we did not find them, making it unlikely that they are present. With the same models, we have looked at the relation between atmospheric pCO₂ and the strength of the circulation in the Atlantic Ocean on long (thousands of years) timescales. We studied the same relation with complex climate models under future climate change up to 2100. Both model types show a weak relation between atmospheric pCO₂ and the strength of the ocean circulation. In the complex models this is because of regionally compensating effects causing a weak global response. Locally a weakening of the ocean circulation can have severe implications for ecosystems and society.