Talk by Anna von der Heydt

(Institute for Marine and Atmospheric Research, and Center for Complex Systems Science, Department of Physics, Utrecht University)

The Eocene-Oligocene transition { observations, mechanisms, cascading tipping?

Over the last 65 million years, the Earths climate has undergone a large transition from a warm and ice-free greenhouse climate to an icehouse climate with extensive ice sheets on both hemispheres. The gradual cooling may be seen as response to the overall slowly decreasing atmospheric CO2-concentration due to weathering processes in the Earth System, however, continental geometry has changed considerably over this period and the long-term gradual trend was interrupted, by several rapid transitions and periods where temperature and greenhouse gas concentrations seem to be decoupled. The Eocene-Oligocene transition ( 34 Ma) reects a rst major phase of Antarctic ice sheet build-up and global climate cooling. In detail, the transition consists of two distinct steps in the oxygen isotope record, where the rst reects mostly ocean cooling, while during the second a large ice sheet has built up. Here we consider the possibility of two coupled critical transitions in explaining the two-step nature of the Eocene-Oligocene climate change. We introduce a framework of (directionally) coupled tipping elements exhibiting cascading tipping, i.e. a sequence of abrupt transitions as tipping in one subsystem changes the background conditions for another subsystem. A mathematical framework of elementary deterministic cascading tipping points in autonomous dynamical systems is presented containing the double-fold, fold-Hopf, Hopf-fold and double-Hopf as the most generic cases. For the Eocene-Oligocene transition, the leading system could be a bistable ocean meridional overturning circulation, while the second following system reects the bistable land-ice system coupled by the atmospheric CO2 concentration. We investigate the possibility of cascading systems in a simple conceptual climate model. Moreover, the necessary condition of the existence of bistability in the ocean circulation is tested in an ocean general circulation model under Eocene continental boundary conditions.