50 years ago at school we were told that "permafrost" is a remnant of the last Ice Age. At what rate is the permafrost decaying and how does it decay?

In my area the ground freezes to a depth of less than 1.5 meters. This sets the standards of how deep to bury utilities and by May the ground is generally frost free. In areas of permafrost the top layers generally melt and re freeze in the winter. The depth to which the ground freezes in this area evidently is quite deep. What is the minimum depth of sustainable permafrost, and what average temperature is required to maintain that?

Given that this temperature is likely to be considerably higher than that required to drive the frozen level down to the existing "deepest" levels one can speculate that the temperature is in fact changing. Given that the "edge" (both surface and depth) of permafrost is retreating, can one not create a time scale and projection of global temperature changes over a very long period. On this scale could one see the affect of the recent influx of green house gasses?