Scientists do not agree on an exact answer to your question, but we do have some idea. Sea level has risen about 15 cm (6 in) in the last 100 years as mountain glaciers melt and their water runs into the oceans.

We expect this type of sea level rise to continue and get faster as the planet warms. That means that in the next 100 years we might should expect 30-100 cm (1-3 feet) of sea level rise. This might not seem like much, but 100 million people live within 3 feet of sea level and that is a lot of people to find new homes for! Certain poor countries like the Maldives (an island nation with a top elevation of 8 ft above sea level) and Bangladesh (Asian country with 70 million within 3 ft of sea level) will suffer the most and have very little money to help them make the adjustment.

There is an unstable portion of Antarctic ice, called the West Antarctic Ice Sheet, that may melt and could cause sea level to rise 6 m (20 ft)! This would flood many of the major cities in the world. However, scientists do not think this will happen in the next 100 years. If it does happen sometime in the future, it would melt gradually and would not cause a sudden (over night) flood. Instead, humans would have to move all their coastal cities uphill over the course of a generation or two.