Theory is, if I remember, is that when the ice-sheets melt enough, the salty cold water of the artic will escape into the Atlantic. This water will sink and become so unmovable it shuts down convection currents powering our ocean currents. The currents will no longer circulate warmth and nutrients to the world, and we all die a frozen 'The Day After tomorrow' deaths at the hands of a uber-violent ice age.
That particular theory is really about a more regional disaster caused by
Greenland's sheets melting. They bring freshwater into the ocean, and that technobabble the gulf stream and the eurpean current. It was an overhyped theory to begin with, since it'd really only be Europe that gets somewhat colder. I don't know the effects the polar ice cap melting would have on currents, though there are theories that there will be much more active storm systems in the north if the polar ocean is warm enough to produce energy-laden storms.
There are almost certainly no transacrctic currents (such as, from the atlantic to the pacific directly over the pole), as there is nothing to drive such a current.
I'm seriously starting to think that cap and trade may be effective, if only to create a market for carbon scrubbing, which right now is the only way we're going to avoid substantial temperature increases.
The ice age threat is rediculous. We're looking at a more energetic system, one with more droughts and floods, and a very diffient one, but not anything as simple as sudden universal heat or cold.