Ice melts may cool surface waters, but they also alter salinity levels. Low salinity water has a different density than high (I don't recall which is higher) which can cause vertical movements in the water column.

The ocean circulatory system is dominated by deep counter currents which tend to replace water mass transported by the surface movements, very cold deep water may be transported to the surface, suddenly interrupting this circulation. One scenario I read about predicted global warming could disrupt the Gulf Stream, plunging northern Europe into severe winters and cool summers, like Labrador and Newfoundland which lie at the same latitude. Paradoxically, global warming might have completely different local consequences.

The system is chaotic but it is able to absorb a certain amount of stress, but at some point Le Chatelier's Principle takes over and the system undergoes a change of state. It may never happen, it may be happening slowly right now, or it could happen overnight tomorrow. We simply don't know. Will the earth come to an end? Of course not, these changes have been occuring for billions of years, they haven't destroyed the planet yet, and the earth will eventually arrive at a new equilibrium. It is unstable at short time scales (weather) and long time scales (climate), but on the longest time scales it is extremely stable, life has always been able to evolve and adapt. Even volcqanism, meteor strikes and continental drift due haven't been able to provoke the system into a runaway excursion like Venus or Mars..

But consider a world where the Great Plains become a Gobi Desert, and a northern Europe similar to northern Canada. Or how about a Sahara covered with deciduous forests, or the Siberian permafrost retreating to the shores of the Arctic ocean? And what if it happens in a century, or even less? We don't know if this will happen, but we do know it has happened before, and it will happen again. The question is, can our civilization adapt?