Interactions between carbon dioxide, climate, weathering, and the Antarctic ice sheet in the earliest Oligocene

- We model observed climatic cycles following the Eocene-Oligocene Transition ~ 34 Ma.
- We use 3-D models of global climate, Antarctic ice sheet, and silicate weathering.
- Unlike previous 0-D box modeling of this system, we find no internal cycles.
- Modeled total weathering decreases as atmospheric CO2 increases 0.2 to 1.5x PAL.
- This would have serious implications for the CO2-weathering thermostat mechanism.

A coupled set of models is used to explore the possibility of long-term internal cycles in the CO2-climate-weathering-Antarctic Ice Sheet system. Cycles of this type were found in an earlier study with 0-D box models, and proposed to explain the quasi-periodic oscillations in benthic deep-sea-core records during the Eocene-Oligocene Transition ~ 34 Ma. Here the system is extended using a 3-D Global Climate Model, a 3-D Antarctic ice-sheet model, and two previously published spatially distributed parameterizations of CO2 consumption by silicate weathering. In 6-million year long simulations across an idealized Eocene-Oligocene Transition, no internal cycles are found, and the coupled system just relaxes from the initial state to the final state, with at most one overdamped half-cycle. The absence of cycles is presumably due to features in this 3-D model system that are absent in the 0-D models: powerful Height Mass-Balance Feedback producing strong ice-sheet expansion after initial growth, and hysteresis in ice-sheet response to climate that damps retreat due to moderate warming.With one of the weathering parameterizations, the models indicate a region of negative slope in the relation between CO2 level and global weathering consumption, occurring in the range ~ 0.2 to 1.5x PAL (preindustrial atmospheric level). This contrasts with the monotonically increasing relation usually assumed. If confirmed, it would have serious consequences for the well-known CO2-weathering thermostat mechanism, at least for CO2 levels below ~ 1.5x PAL.