Master Thesis Defense by Alfred Bendtzon Hansen

Title: Multistability and tipping points in very high-dimensional systems and implications for abrupt climate change

There is a growing concern that climate subsystems with one dominant positive feedback may cross tipping points, causing abrupt and irreversible qualitative change. One of these is the Atlantic Meridional Overturning Circulation (AMOC), which might collapse due to increased meltwater input. However, the anatomy of tipping points in high-dimensional, spatially extended heterogeneous systems such as the AMOC is not fully understood. Previous work with a global ocean model has shown a high degree of multistability of the AMOC. This suggests a complicated and rugged stability landscape, where the path to tipping is more gradual with many transitions between local minima of the landscape. Furthermore, it is unclear how fast timescale chaotic atmospheric variability might eliminate the pronounced multistability by smoothing out the stability landscape. Here, we emulate fast timescale chaotic atmospheric variability by constructing a noise model which is incorporated into the global ocean model. This allows us to asses the structure of the stability landscape by constructing a quasipotential.
We also examine how the reactive paths evolve in phase space relative to unstable states when noise-induced tipping occurs.

Supervisors: Peter Ditlevsen and Johannes Lohmann   
Censor: Jens Olaf Pepke Petersen