That's because the ice doesn't just melt. Ice actually flows down valleys like rivers of water. The problem is that we do not completely understand the factors that control how rapidly the ice flows and thus enters the ocean.
One way to approach the problem of not understanding the process is to study how sea level changed in the past. Earthis nearly as warm now as it was during the last interglacial period, about 125,000 years ago. At that time, sea level was 4 to 6 meters (13-20 feet) higher. It seems that this higher sea level was due to the melting Greenland and West Antarctic ice caps.
Perhaps a similar sea level rise is our future. We don't know. We also don't know how rapidly sea level could rise. Will a 4-meter (13-foot) increase take 200, 500, or even 1,000 years? This is a question that a number of scientists are now trying to answer by studying how ice moves.
