Multiple Equilibria in a Fully Coupled Carbon–Climate Model

Quaternary glacial-interglacial cycles are suggestive of non-linear transitions between multiple stable states. Previous work has found four different stable climatic equilibria in a complex atmosphere-ocean-sea ice model with simplified geometry. These states are associated with global reorganizations of ocean circulation and sea ice extent, and can be achieved from different initial conditions but identical boundary conditions and radiative forcing. However, two major shortcomings hindered the application of this model to realistic glacial cycles: non-conservation of energy due to the lack of frictional heating, and lack of coupling of the carbon cycle to atmospheric radiation.

Here we demonstrate for the first time the existence of multiple climatic equilibria in an improved fully coupled and energy-conserving carbon-climate model. Four radically different climates (known as Warm, Cold, Waterbelt, and Snowball) are achieved from different initial conditions but identical astronomical forcing and total carbon inventory. The carbon cycle introduces a strong positive feedback at very long timescales, as CO2 tends to outgas from the ocean when the climate warms. We study the physical and biogeochemical feedback processes responsible for the multiple stability, and threshold values of astronomical forcing required to trigger transitions between stable states. We will discuss these results and their possible implications for glacial-interglacial cycles.