How could global warming affect sea levels?
Temperatures have increased across the globe due to increased amounts of fossil fuels in the atmosphere. As water warms, it expands. Thus, in many areas sea level will rise because of increased temperatures in the ocean itself.
Melting ice in the arctic is also expected to contribute to sea level rise. Water currently in solid form will melt (and is melting) into liquid form, causing levels to rise.
The graph below shows sea level rise from 1880 to after 2000.
Those who deny sea level rise will often show only a small portion of this graph, such as the dip seen around 1980. By selecting the information they want (no rise in sea level or even a drop) and ignoring the larger trend, those denying climate change are able to fool others.
Bangladesh is predicted to experience extreme sea level rise, as you can see in the image below.
This is a heavily populated region that will be underwater if sea level rises by one meter or one and a half meters.
To learn more about sea level rise, check out this link from National Geographic.
There's also an excellent video called Earth Under Water that is worth checking out.