Does the Arctic sea ice have a tipping point?

Two IPCC fourth assessment report climate models have Arctic Ocean simulations that become sea-ice-free year around in 1%/year CO2 increase to quadrupling experiments. These runs are examined for evidence of accelerated climate change associated with the removal of sea ice, particularly due to increasing surface albedo feedback. Both models become seasonally ice-free at an annual mean polar temperature of -9 degrees C without registering much impact on the surface albedo feedback or disturbing the linear relationship between Arctic Ocean climate change and that of the surrounding region. When the polar temperature rises above -5 degrees C, however, there is a sharp increase in the surface albedo feedback of one of the models, driving an abrupt elimination of Arctic ice and an increase in temperature above that expected from warming of the surrounding region. The transition to ice-free conditions is more linear in the other model, with ocean heat flux playing the primary driving role.