Bifurcations leading to summer Arctic sea ice loss

There is significant interest in whether there could be a bifurcation, sometimes referred to as a "tipping point," associated with Arctic sea ice loss. A low-order model of Arctic sea ice has recently been proposed and used to argue that a bifurcation associated with summer sea ice loss (the transition from perennial to seasonal ice) is unlikely. Here bifurcations are investigated in a variation of this model that incorporates additional effects, including parameterizations of changes in clouds and heat transport as sea ice is lost. It is shown that bifurcations can separate perennially and seasonally ice-covered states in this model in a robust parameter regime, although smooth loss of summer sea ice is also possible. Hysteresis and jumps associated with bifurcations involving winter sea ice loss are larger than those associated with summer sea ice loss. Finally, in analogy with simulations in global climate models, the low-order model is integrated with time-varying greenhouse gas forcing in both the regime in which summer sea ice is lost via bifurcations and the regime in which it is not. The resulting time series are compared as a preliminary way of investigating ways in which these regimes could be distinguished from each other.