Deterministic icehouse and greenhouse climates throughout Earth history

Abstract

Some theories posit that icehouse (with polar ice sheets) and greenhouse (ice-free) states throughout Earth history are not deterministic, but bistable—both states may occur for the same level of radiative forcing. If correct, then the climate state that persists for millions of years can depend on which state already existed, giving the system a `memory’ effect. However, on these timescales the negative silicate weathering feedback in the long-term carbon cycle stabilizes global climate—a feedback which is missing from models that simulate a bistable system. Here, we test whether bistability persists on million-year timescales with a model that couples climate, weathering, and the long-term carbon cycle. We show that transitions between bistable states put the long-term carbon cycle out of balance and silicate weathering restores this balance, collapsing bistability. On million-year timescales any memory effect disappears, and the state of global climate is largely deterministic.