Greenland is bad, the melting was so bad NASA scientists thought they’d made a mistake:
However the tipping point we should really be worried about is a sudden release – or pulse – of methane into the atmosphere, triggering a runaway greenhouse effect.
In short, rising temperatures help unlock the billions of tonnes of carbon locked up in the under the Siberian permafrost.
The permafrost is a lid, under which is billions of tonnes of methane. Methane is a far more potent GHG than carbon (it traps more heat). More methane, more heat. More heat, more warming. More warming, the greater the risk of other tipping points.
It’s called the clathrate gun hypothesis, because it’s the equivalent of pointing a loaded Magnum 44 at the planet.
How does such an event rate on the scale of probability?
Well it depends.
Do we really want to find out?
Watching the Deniers 
I have blogged that if 97% of the votes of climate scientists isn’t enough to alarm the lying denying type of think tankers, 97% of the ice melting won’t alarm them either.
But their kind of politicians, like Stephen Harper ? Then just 37% of the vote is mandate enough to tear up Canada and turn it into a northern Van Denier’s Land – minus a. Nasty abbott, of course….
More on the new normal
http://www.sciencedaily.com/releases/2012/07/120729142137.htm