The rising sea level is due to a combination of TWO issues. About 2/3 is caused by the melting of the land ice the polar regions. However as the carbon dioxide in the atmosphere increases the ocean temperature also increases. This means that the density of the water decreases which causes thermal expansion of the water. The thermal expansion of the oceans is responsible for about 1/3 of the rising sea level.

(Great Courses – Oceanography)