Sea level rise mechanism :

Three phenomena are responsible for the sea level rise :
- The water oceans expansion due to the water temperature increase,
  this phenomenon is the main cause of sea level rise.
  Moreover, more deep level of oceans will rise in temperature.
- Melting of land ice caps (Greenland, Antarctic Peninsula, etc).
- Melting of mountain glaciers.
For the time being, sea level increase is in average 3.3 mm per year, but many forecasts predict an acceleration of this rise.

To understand the expansion of water, you can perform a simple experiment : completely fill a water bottle to the top, plug it, put in the summer sun (or in a microwave oven).   The temperature will rise.   When you open the bottle, water spurts, because the volume increased.