Before we start, take a look at the ocean currents. This is going to be playing a very important role in sea level rise. As cold water gets heated, and hot water gets cooled due to these currents. What happens with the water in our oceans is called thermal expansion. Get ready for some facts on Seal level rise!
Thermal Expansion of Water, a rather curious substance.
So everyone probably heard of this analogy: "Take a glass of water and put an ice cube in it, when it melts you'll notice the water in the glass has not risen." Its the No.1 analogy for science deniers to explain that melting sea ice does not rise sea level. Well here's an inconvenient fact, it DOES rise sea level. It does that trough thermal expansion as ocean waters always travel to hotter and colder places.
So what do we know about thermal expansion? We know substances contract when they cool and expand when they get heated. Water on the other hand is an exception. Take 20°C water for example, it will contract as we expect but only to about 4°C then it will start expanding slightly (it crystalizes) . As we further cool the water to freezing point (0°C) it will start to expand by 9%! This is the point where ice is formed. Now if we reverse this traject and start to heat the water. Lets take for example sea ice melting due to seasonal change on the polar regions, first the ice will melt and flow into the ocean where it gets carried away by the ocean currents, by this time the cold water gets mixed with hotter equatorial waters, as this melted sea ice gets heated it starts to expand by approximately 0.002m³ per degree Celsius. Doesn't sound like much but given the amount of ocean water, this is ALOT! Yet not enough to see happening in your glass of water.
What does this mean?
As older sea ice continues to melt every season, the more water gets carried away into the hotter regions of our oceans. This will cause relative sea rise on coastal regions surrounded by hotter water that are low above the ocean surfaces. Of course it is a rather small contribution but one to take account of when predicting sea level rise in different scenarios. However the melt of land ice, which is a volume to volume operation will increase absolute sea level to unprecedented levels in recorded history.
Keep in mind, there is a difference between relative and absolute sea levels and there is a lot of geological activity to be taken account, so again this is an abstract explanation of thermal expansion and sea level rise, it does not cover the whole field of sea level rise, glaciology and geology.