Nonlinear stochastic threshold behavior in Arctic Sea Ice

In light of the rapid recent retreat of Arctic sea ice, a number of studies have discussed the possibility of a critical threshold beyond which the ice-albedo feedback causes the ice cover to melt away in an irreversible process. The focus has typically been centered on the annual minimum (September) ice cover, which is often seen as particularly susceptible to destabilization by the ice-albedo feedback. A recent theory by Eisenman and Wettlaufer captures the central physical processes associated with the transition from ice-covered to ice-free Arctic Ocean conditions using a simple nonautonomous ODE that reproduces the systems principal observables. In their approach it was found that the ice-albedo feedback does indeed promote the existence of multiple ice-cover states, the stabilizing thermodynamic effects of sea ice mitigate this when the Arctic Ocean is ice covered during a sufficiently large fraction of the year indicating that threshold behavior is unlikely during the approach from current perennial sea-ice conditions to seasonally ice-free conditions. However, a further warmed climate exhibits a sudden loss of the remaining wintertime-only sea ice cover via a saddle-node bifurcation. Recasting this theory in a stochastic framework reveals that there is an asymmetry in the dwell times between ice free and ice covered states and that both are long lived, questioning the utility of using satellite data to extrapolate information from one year to the next.