Rising sea levels from thermal expansion and/or ice shelf disintegration may result in mass migrations and the lost of huge tracts of farmland. Some islands may disappear altogether e.g. Tuvalu and The Sundarbans.
Global mean sea level has already risen by around 15cm during the last 100 years, with another 18cm rise predicted due to global warming over the next 30 years. By 2100, if this trend continues, we may see a rise of about 65cm. Though rich countries may be able to cope with these changes by building expensive sea defences, the poor, low lying nations like Bangladesh are likely to be badly hit.