Thawing permafrost could release vast amounts of carbon, accelerate climate change by the end of this century (http://www.physorg.com/news/2011-08-permafrost-vast-amounts-carbon-climate.html)
Irrespective of the cause of global warming we do know the Earth is getting warmer and we can see those effects around us. One of the main problems of global warming are its non-linear outcomes. One such non-linear outcome is when the temperature rise causes a disproportionate release of greenhouse gasses. One such potential release scenario is described below.
Quote(PhysOrg.com) -- Billions of tons of carbon trapped in high-latitude permafrost may be released into the atmosphere by the end of this century as the Earth's climate changes, further accelerating global warming, a new computer modeling study indicates.
The study also found that soil in high-latitude regions could shift from being a sink to a source of carbon dioxide by the end of the 21st century as the soil warms in response to climate change.
Thus the release of Carbon dioxide and methane from the permafrost could well be a tipping point beyond which global warming goes into a positive feedback beyond our ability to influence as natural sources exceed that over which humans have control. If we reach this tipping point our descendents could well have to attempt to live through the greatest mass extinction ever.