The scientific definition of permafrost is: “a soil staying at or below 0 °C for more than two years”. The annual average temperature is the most important factor for permafrost existence. In regions covered by permafrost, only the upper 30 to 100 cm of soils (called the active layer) thaws every summer and then completely refreezes during the winter. The thickness of permafrost varies from a few meters to more than 150 meters.
Permafrost typically forms in regions with a mean annual air temperature of less than 0°C. Exceptions are found in moist-wintered forest climates, such as in Northern Scandinavia and North-Eastern Russia west of the Urals, where snow acts as an insulating blanket. The permafrost currently covers 20% of Earth’s land surface (25 million km²) and 25% of the Northern Hemisphere. The extension of the permafrost in Asia is very important (see illustration below).

 

What are the consequences of thawing?
Increasing permafrost temperatures changes many of its physical properties which has some negative effects on man-made infrastructures.

The thaw of the permafrost can also endanger the region's biodiversity. Indeed, permafrost controls plant communities and biomass production by soil temperature, active layer thickness, moisture content, presence of unfrozen water, and surface hydrology. The current changes in the permafrost thermal regime and active layer thickness affect plant diversity and biomass in all the regions covered by this layer of frozen soil.

Long-term permafrost degradation will also continuously improve conditions for the subsurface water drainage (especially in sandy soils) that will lead to increased dryness of soils, inducing significant vegetation stress. Improved drainage conditions will also lead to the reducing of numerous ponds within the degrading permafrost area dramatically affecting aquatic ecosystems.

And, last but not least, the melting of the Arctic permafrost could dramatically worsen global warming by releasing massive amounts of trapped greenhouse gases, especially methane.

Melting permafrost is making this building lean in Dawson, Yukon.
Source: University of Iowa, Department of Geoscience

Drew Point, 2004. Coastal erosion of mud-rich permafrost along Beaufort Sea coastline. Cliff height is 3–4 m. Waves undercut permafrost and cause block slumping (center of photo). Source: USGS

The release of methane as consequence of the melting

From the ocean floor
It's always been a disturbing what-if scenario for climate researchers: gas hydrates stored in the Arctic Ocean floor (hard clumps of ice and methane, conserved by freezing temperatures and high pressure) could grow unstable and release massive amounts of methane into the atmosphere.

Since methane is a greenhouse gas more than 20 times more powerful than carbon dioxide, the result would be a drastic acceleration of global warming. Until now this idea was mostly academic; scientists had warned that such a thing could happen. Now it seems more likely that it will.

Russian polar scientists have shown strong evidence that the first stages of melting are underway. They've studied the largest shelf sea in the world, off the coast of Siberia. In the permafrost bottom of the 200-meter-deep sea, enormous stores of gas hydrates lie dormant in mighty frozen layers of sediment. The carbon content of the ice-and-methane mixture here is estimated at 540 billion tons. This submarine hydrate was considered stable until now but the permafrost has grown porous and the shelf sea has already become "a source of methane passing into the atmosphere".

If this Siberian permafrost-seal thaws completely and all the stored gas escapes, the methane content of the planet's atmosphere would increase twelve fold and the result would be catastrophic global warming.