"Models - They all overstate the warming"
Care to provide a peer reviewed reference to substantiate that claim?
The cause of the melting is very certain.  It's getting warmer.  And the dominant cause of the warming is well documented.  It's human-produced CO2 and CH4.
The rate of warming far exceeds the rate of adaptation for many species.  Some species will flourish, but a large number will vanish.  Overall our biodiversity will plummet over the next 100 years. 

Other periods did warm, and the reason for those periods is well understood.