One of the big factors in global warming is the slowing of sea currents in their trip from the equator to the northern tundra and glacial flows. The sun warms the ocean at the equator and it is cooled as it flows past the arctic regions, keeping a constant flow that regenerates oxygen levels and moderates ocean temperatures. Once this round trip flow slows sufficiently, or stops, life in the ocean will die off and generate its own methane load that would be much greater than anything to that point. It would be the end for all life.