There are simple thermodynamic reasons why the salt is left out of the actual ice crystals. Basically, it doesn't fit in the crystalline structure and thus has lower free energy if it's running around in the liquid. Of course some of it gets left behind in little cracks in the ice, but some of it gets out into the nearby oceans and leaves them more saline.
Some little bubbles of gas (not particularly greenhouse gases more than any others) also can get trapped as snow falls and ice freezes, so that gives a way of seeing what the atmosphere used to be like. I don't believe that the amount of greenhouse gases trapped in the ice caps is enough to have any importance. On the other hand, huge amounts of carbon are trapped in permafrost soil.(http://www.sciencedaily.com/releases/2011/02/110216132100.htm
) As it melts, methane (CH4
, an especially potent greenhouse gas) will be released, accelerating the greenhouse warming. There are also very large amounts of methane trapped in frozen clathrates under the sea. (http://en.wikipedia.org/wiki/Methane_clathrate
) If these are also released by warming (as seems to have happened some 56 million years ago, http://en.wikipedia.org/wiki/Palaeocene-Eocene_Thermal_Maximum
) then a really major acceleration of the warming will occur.
(published on 06/25/2011)