would be changing ocean currents due to freshwater disrupting "the conveyor", and the way the Gulf Steam circulates through the N. Atlantic.
A really simplified model: Warm water circulates to the north at the surface, gives up heat and becomes denser, sinks deeper, and returns toward the south. When too much fresh water is entering the system through the runoff of melting ice-sheets and glaciers, it adds a third layer, between the other two, like an insulator. The warm-but-cooling saltwater is not dense enough to sink through the cold fresh water fast enough to continue the current in its present form. The ocean circulation could stall or be diverted in a way that does not bring enough heat north from the equator to keep the northern landmasses warm the way we like it.
Water at the same temperature as land has a higher actual heat content. Without the ocean bringing heat north from the equator to keep the air warm, the landmasses would cool off enough that, in the extreme, it would never be warm enough in the summer to melt all the previous winter's snow.
Aha! a bit of a picture:
http://www.grida.no/climate/vital/32.htm
Edit: And here, if I use term thermohaline:
http://en.wikipedia.org/wiki/Thermohaline_circulation
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