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

摘要

A coupled set of models is used to explore the possibility of long-term internal cycles in the CO_2-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 CO_2 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 CO_2 level and global weathering consumption, occurring in the range ～0.2 to 1.5× PAL (preindustrial atmospheric level). This contrasts with the monotonically increasing relation usually assumed. If confirmed, it would have serious consequences for the well-known CO_2-weathering thermostat mechanism, at least for CO_2 levels below ～1.5× PAL.