There is still some uncertainty about the full volume of glaciers and ice caps on Earth, but if all of them were to melt, the global sea level would rise approximately 230 feet (or 70 meters), flooding every coastal city on the planet. And this has already happened before, geological records of temperature and sea level indicate that during past warm periods over the last several millions of years, the global sea level was higher than it is today.

Source: USGS