As the ice in the polar oceans melts, the fresh water locked inside the ice is released into the sea water. This dilutes the sea water and lowers the salinity which, in turn, lowers the density. If the density of the water drops, the water will not sink to the bottom. Global ocean currents depend on the sinking of denser, colder water and the rising of warmer, less dense water. If those things aren’t happening, the conveyor belt of global ocean currents will stop. Global ocean currents distribute warm and cold air and water to different areas of the world so if those currents stop, the distribution will stop.