A nagging question about climate change is how cities will adapt. In the past century, the earth's surface temperature has risen nearly two degrees Fahrenheit, and another four could dramatically alter the planet through extreme storms, flooding, and especially rising sea levels. As the oceans heat up, they expand—up to eight inches in height already—and melting glaciers in Greenland and Antarctica continue to pump up the volume. The flow of ice into the sea has doubled over the past decade and over the next century could cause a 20-foot rise, making densely populated regions like the Nile Delta uninhabitable. In the U.S., even three more feet would flood every city on the Eastern seaboard. If you remember the aerials in An Inconvenient Truth, you know how this might look: Whole coastlines shrink as water spills inland and redraws the map of the world.

The ascent is likely to happen gradually, so there is time to plan. Mass migration is inevitable, but abandoning every affected area isn't practical. Monumental seawalls will spring up, but New Orleans' levees are a tragic demonstration that this strategy isn't fail-safe. “Building barriers is not enough,” says structural engineer Guy Nordenson, leader of the research team that won the AIA's 2007 Latrobe Prize to study the impact of global warming on waterfronts. “We will need more imaginative solutions.” He sees water as a significant but neglected part of the public realm. “We can accommodate climate change through the creation of new urban space.” For example, reshaping the shorelines and constructing strategically placed islands can simultaneously diffuse storm surges and foster new connections between the water community and the land community.

Rethinking the nature of coastal cities can create powerful visions of our watery future. Some proposals for post-Katrina New Orleans suggest letting the lowlands become occupied lagoons, with housing hovering above the tidal basin. Stilt villages have thrived forever in the Gulf of Thailand, so why not the Gulf of Mexico? Graduate students at MIT designed the storm-resistant Lift House for just this purpose, and the Polish design firm Front Architects has designed a modernist twist in its “Single Hauz” concept. An occupied billboard, this simple box perched on a single post works with any terrain. Combine this with the concept of floating parks by the Norwegian architecture firm Jensen & Skodvin, and you have an entire aqueous community. Just add water.

The real test will come with larger metropolitan areas. A report released in December by the Organisation for Economic Co-operation and Development (OECD), an international governmental organization based in Paris, lists New York among the 10 places most threatened by future flooding. For a wealthy city, its protection is minimal, so the images of a deluged Manhattan in the 2004 film The Day After Tomorrow may not be a Hollywood fantasy.

New York's Architecture Research Office (ARO) anticipated this scenario in its winning entry to the History Channel's first City of the Future competition (2006). Fast-forwarding to 2106, ARO imagines a postdiluvian Big Apple as Big Venice—canals for streets and boats in lieu of cars. To maintain comparable density after the flood, ARO inserts new buildings over the public right-of-way. Spanning curb to curb, these unique structures, called “vanes,” would become reeflike foundations for a new communal habitat. “We have nature all around us—it's the water,” says ARO's Adam Yarinsky. “It's not green space, but it's natural.” Rediscovering the city's relationship with the rivers, he feels, can “transform a catastrophe into a revelation.”

The design firm Field Operations pictures a similarly hopeful future in Biopolis, its sketch portrait of Lower Manhattan's projected history from 1660 to 2200. As principal James Corner explains, the first four centuries of the city's development have been driven by economics—for example, about 3,600 acres of landfill have been added to Manhattan to increase available real estate. But he sees the city shifting from economics to ecology, becoming an integrated habitat of people, fauna, and flora—what he calls “a biological engine” and “an incubator for new life.” Instead of containing landscape within clearly defined boundaries—the Central Park model—vegetation would become the backbone of the community's development. “Too often development and sustainability are seen as opposed,” says Corner. “But the two should go hand in hand.”