Thursday 15 Jun 2017: The Cenozoic greenhouse-icehouse transition: Tipping points and pathways of heat in the climate system
Anna von der Heydt - Utrecht University
Over the last 65 million years, the Earth’s 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 boundary (~34 Ma) and mid-Miocene climatic transition (~13 Ma) reflect major phases of Antarctic ice sheet build-up and global climate cooling, while Northern Hemisphere ice sheets developed much later (~2.7Ma).
Thresholds in atmospheric CO2-concentration together with feedback mechanisms related to land ice formation are among the favoured mechanisms of these climatic transitions, while the long-proposed ocean circulation changes caused by opening of tectonic gateways seem to play a less direct role. The opening of the Southern Ocean gateways, however, has eventually led to the development of today's strongest ocean current, the Antarctic Circumpolar Current, playing a major role in the transport properties of the global ocean circulation. The overall state of the global ocean circulation, therefore, must precondition the climate system to dramatic events such as major ice sheet formation.
Here we quantify the climate response to a closed Drake Passage in both ocean-only (POP) and coupled climate (CESM) models. We show that the ocean gateway mechanism is robust in the sense that the equatorward expansion of the Southern Ocean sub-polar gyres inevitably leads to widespread warming around Antarctica. Moreover, we provide a framework to characterise the ocean temperature response to a closed Drake Passage in terms of both the mechanical and thermal energy budget of the ocean.