Ice is more buoyant and therefore floats. By floating, an amount is above the water and therefore displaces not as much as if it were entirely below the water. This leads to the sea level remaining stable as the amount of ice increases or decreases. The rising and lowering of sea level is influenced by a lot of other things, such as volcanoes expanding the land, thus displacing water, or the huge increase in global shipping meaning that there are millions more ships on the sea that displace large amounts of water (not much in regards to Earth size, but enough to be noticeable) as well as other man induced reasons for less ocean and sea sizes, like the development in Dubai that created island in the sea.
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