"A system under stress tends to react in such a way as to reduce that stress."

Applying Le Chatelier to thermodynamics, a body reaches thermal equilbrium with its environment by shedding excess heat, through radiation, conduction and convection. Since outer space is a vacuum, the earth can only eliminate heat by radiating it as excess thermal infrared radiation into space, or by reflecting sunlight from the cloud tops and the ice caps. The radiative energy emitted by the earth must eventually exactly equal, on the average, to that being received from the sun (geological or man-made sources of energy are insignificant, sorry Johannes). Otherwise, the earth would not be in thermal equilibrium, i.e., it would be heating up or cooling off. As the amount of summer ice at the N pole decreases, less sunlight gets reflected, more gets absorbed, and the earth system departs from thermal equilibrium: it heats up. It will continue to do so until the increased emission of infrared at the top of the atmosphere makes up for the continuous spectrum sunlight now absorbed by the seas. This will happen at a higher temperature than it does now.

The S pole ice is insensitive to melting because it is thousands of feet thick, no matter how much melts it will still reflect roughly the same amount of sunlight. Even the small growth of sea ice in the Antarctic Ocean will make little difference. Indeed, Antarctic winter sea ice has been rising at a rate of +0.9% per decade over the last 30 years--from 18.5 to 19.0 million km sq.

In contrast, at the N pole, the ice in winter has been dropping at a rate of -2.6% per decade--from 16.2 to 14.9 million km over the last 30 years. This more than makes up for the Antarctic gain. The earth is heating up, and the Arctic is shrinking faster, in both absolute and relative terms, than the Antarctic can make up for it. There are heat sinks like the deep seas, to be sure, but as excess heat goes into the atmosphere and oceans, there will be changes in ecosystems and in the earth's mechanisms for redistributing heat (weather and climate, ocean currents).

Eventually, the temperature will go up a few degrees, the thermal IR flux at the top of the atmosphere will rise, and the earth will stabilize a few degrees warmer than it is now. But it will have readjusted its internal heat budget in order to do so. What effect that has on us remains to be seen. But I'd rather not stick around and find out.