CARBON CYCLE CONSTRAINTS ON CLIMATE DETERMINISM IN THE GEOLOGIC PAST

How past climate informs the present and future depends, in part, on whether past climate states are deterministic or fully constrained under specified external forcings and boundary conditions. If climate is not deterministic, then greenhouse and icehouse states can co-exist as bistable states for certain sets of parameters, and the realized state depends on the system’s “memory” of its recent past. In other words, icehouse states can persist for greenhouse-like conditions and vice-versa, challenging our ability to link paleoclimate transitions with distinct forcings. Here, we develop a new model framework to test the hypothesis that icehouse-greenhouse transitions in the geologic past are a feature of climate bistability. By coupling a non-deterministic (bistable) energy balance climate model with a model for continental rock weathering and the long-term carbon cycle, we show that climate on million-year timescales is effectively deterministic. Through a series of steady state and long-term perturbation (~1 Myr) experiments, we demonstrate that the memory of global climate is ultimately limited by the response time of silicate weathering, the primary long-term negative feedback on climate. Bistability requires a positive feedback on the timescale of interest, so the negative weathering feedback acts to collapse short-term bistability on the timescales of the silicate weathering response to climate change. Bistability can persist on million-year timescales in our model, but only near the tipping points of the short-term bistability landscape. Consequently, internal variability, such as orbital cyclicity, restricts or fully collapses this short-term bistability on long timescales. Overall, we find that the bistability window—the range of forcings where an icehouse and greenhouse can both persist—is narrow and fragile and largely non-existent on million-year timescales. Long-term climate evolution is therefore largely deterministic, even if the short-term climate system may not be.