In 500 years some of man's greatest cities, like London and New York will be underwater. Unless the effects of global warming can be reversed, rising sea levels will continue to rise in the next 500 years and flood most cities on the coast.

Researchers from the Niels Bohr Institute have published a long term outlook for rising sea levels and the news is grim. In a pessimistic scenario, rising greenhouse emissions will increase sea levels by 1.1 meters by the year 2100 and 5.5 meters by the year 2500. Even the best case scenario will still see rising sea levels. By the year 2100, in the optimistic scenario the sea will still have risen by 60 cm and then rise to 1.8 meters by the year 2500.

"In the 20th century sea has risen by an average of 2mm per year, but it is accelerating and over the last decades the rise in sea level has gone approximately 70% faster. Even if we stabilize the concentrations in the atmosphere and stop emitting greenhouse gases into the atmosphere, we can see that the rise in sea level will continue to accelerate for several centuries because of the sea and ice caps long reaction time. So it would be 2-400 years before we returned to the 20th century level of a 2 mm rise per year," says Aslak Grinsted, researcher at the Center for Ice and Climate, the Niels Bohr Institute at the University of Copenhagen.

In either case the impact of rising sea levels will be terrible to humans. Bangladesh will lose 17.5 percent of its lad if the sea level rises about 1 meter. Coastal flooding will displace tens of thousands of people and will threaten the agricultural system. Islands in the Pacific and Indian Oceans will simply disappear off the face of the map. Cities on coastal plains will face severe flooding if the sea level rises 1 meter.

However high the sea level will rise by 2100 or by 2500 the sea will still reclaim most of the land. If emissions continue to heat the planet, Greenland and Antarctic ice sheets will melt to hasten the process. If the sea levels rise more than seven meters many of man's historic cities will be underwater.