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Permafrost melting imho is very likely to contribute in a big way to a runaway greenhouse effect. As it melts, vast quantities of methane are released. Methane is a far worse greenhouse gas than carbon dioxide. SBS showed a documentary on melting permafrost in Alaska, and it was frightening. The melt has gotten so bad, that a pastime of some of the locals is to drill a hole in the ice on a frozen lake, and light the gas that comes out. This isn't something they were doing years ago.
It has the second effect (not really mentioned in the doco) that thawed ground doesn’t hold snow for as long, reducing average yearly albedo. Crucially, it is at the end of each season when the sunlight is more present, and at lower latitudes, where the snow disappears from first.
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