A very graphical and short simulation film on youtube, titled "Runaway Glaciers in West Antartica", explains what is going on.
The investigated glaciers play a major role in pinning the much larger and much thicker West Antarctic ice shelf down to where it is now. If this ice shelf melts, we'll be in deeper trouble. For this to happen, the already-cited Joughin put the most likely timeframe at between 200 and 500 years, according to the Guardian."
Link to Original Source