If global warming melts the world's glaciers and raises sea levels, the first to know about it will be the citizens of the Maldives, a low-lying chain of island atolls in the Indian Ocean. Belgian architect Vincent Callebaut was thinking of the Maldivians when he designed Lilypad, a floating city for climate refugees.

Each "auto-sufficient amphibious city" would house 50,000 people, and Callebaut says the cities could anchor near coastlines or float free on ocean currents. A deep-set central lagoon would collect and purify rainwater (and also act as ballast), and three mountains would support terraced housing. Artificial wetlands on lower decks would purify wastewater, and the whole city would be studded from top to bottom with solar cells and wind and tidal-power turbines.