Reasons of Ocean Change

A successful attempt has been made to assess the causes of observed sea level rise. During the peaked of last ice-age (21,000 years ago) the average global sea level was 120m lower than today. As fresh water continue to add into the oceans from melting down of ice sheets and glaciers, sea level rose over a meter per century. Global mean sea level change results from two major processes, mostly related to recent climate change, that alter the volume of water in the global ocean: i) thermal expansion, and ii) the exchange of water between oceans and other reservoirs (glaciers and ice caps, ice sheets, other land water reservoirs - including through anthropogenic change in land hydrology, and the atmosphere).Vertical land movements such as resulting from glacial isostatic adjustment (GIA), tectonics, subsidence and sedimentation influence local sea level measurements but do not alter ocean water volume; nonetheless, they affect global mean sea level through their alteration of the shape and the volume of the ocean basins containing the water. 


Meier.F and J.Whar in 2002 have explained this in much more detail in their article "Sea level is rising: Do we know why?" which can be found on "PNAS May 14, 2002 vol. 99 no. 10, 6524–6526.". So I would recommend this as further reading.