By ruputure its the point at which the oil base economy starts breaking down. It would be a combination of war and major depression this is probably somewhere around the 200+ dollars a barrel.

The current predictions for a ice free artic are within 100 years.

http://www.livescience.com/environment/050823_ice_free.html

My assertion is that once a strong feedback loop sets in it will be much sooner, 2030 with the simple metric that the amount of ice lost doubles in ten years.

Positive feedback loops have a nasty effect of soon swamping other mechanisms.

I propose that in both cases peak oil and the Arctic we have already entered positive feedback scenarios.

In the case of peak oil its triggered by the peak of light sweet oil and the "refinery" depletion factor of 25% for replacing with heavy sour grades that greatly magnified the effects of depletion beyond most estimates.

In the case of the arctic the magnifier is simple mechanical break up of ice that's not modeled. Indeed in the collapse of the one antarctic ice sheet mechanical failure prevailed.

In both cases the biggest factor for acceleration into a non-linear or positive feedback condition has been and unmodeled but significant non-linear factor.