Explain why scientists are concerned about the release of the methane stored in arctic ice

It has been estimated that there is more methane present in frozen arctic ice than the coal left in the coal bed of the Earth. As methane is twenty times more robust than carbon dioxide as a greenhouse gas, its influence could be more massive.  

It is also a concern as while the emission of carbon dioxide resulting due to coal utilization can be alleviated via human activity, the discharge of methane from arctic ice is reliant upon global climate. Thus, if the climate warms enough to liberate huge concentrations of the methane it could create a positive feedback cycle and warm the Earth even more.  

one would be wind another would be water density