When were the first defenses built against the sea?
They go back to the earliest civilizations. People have always been drawn to coastlines, and as human settlements developed, empires invested heavily to keep floods from destroying what they'd built. In the 3rd century B.C., Egyptian ruler Ptolemy II had sophisticated dikes built to protect Alexandria's Pharos lighthouse, one of the Seven Wonders of the Ancient World. The breakwaters Herod built to protect Caesarea Harbor are still visible in Israel. For centuries, the Japanese have been fashioning bamboo seawalls as protection against typhoons and tsunamis. Today, the challenges are getting tougher. The sea is warming, providing more energy to storms, and it has risen about 7 inches in the last 100 years. As climate change becomes more pronounced and the Antarctic and Greenland ice sheets melt faster, ocean levels could increase another 3 to 4 feet by the end of this century, according to computer models. The consequences may be catastrophic, as more people move to communities on low-lying coasts; already, 634 million people — about a tenth of the global population — live close to shorelines, said Gordon McGranahan of the International Institute for Environment and Development in London. "People are running toward risk," he says. 

What can be done to protect coastal populations?
Climate activists say that to stop the inexorable rise of the oceans, we should adopt immediate, major restrictions on the burning of fossil fuels. But political resistance to emissions limits is strong in developed and developing countries alike, so there is almost no chance of reaching an agreement on a global reduction of even 20 percent. And it's probably too late anyway: Due to the amount of carbon dioxide we've already pumped into the atmosphere, even the most draconian measures to slash carbon emissions would not reduce rising tides. "Sea-level rise cannot be stopped for at least the next several hundred years," climate scientist Gerald Meehl concluded in an article published this summer in Nature. Curtailing emissions would only slightly slow what now appears to be inevitable.

Can we adapt to higher seas?
It can be done, but it's an epic battle requiring ever-more-heroic means. The city-state of Venice ruled the Eastern Mediterranean from a marshy lagoon for centuries despite frequent tidal flooding from the Adriatic. But in recent decades, ill-considered development, the slow sinking of the islands on which the city is built, and rising sea levels have combined to make acqua alta ("high water") a more regular occurrence. To counter it, construction began in 2003 on a system of 78 giant metal flap gates that will rest flat in the seabed until tides rise to a dangerous level, at which point the gates will be blasted full of compressed air, making them rise up to block the waters. The system, called MOSE (Italian for Moses), is due for completion by 2014 and will cost about $6.7 billion. The Dutch, meanwhile, have spent about $6.4 billion in the last half-century on the Delta Works, a massive integrated project of sea barriers, sluices, and dams to ward off catastrophic flooding. Yet the Dutch government estimates it will have to spend another $11.4 billion on flood-protection measures just to deal with a 10-inch rise in sea levels by 2040; preparing for a 60-inch rise by 2100 would cost an estimated $59 billion.

Are there alternatives to such mega-projects?
Some rural areas prone to flooding have tried "managed retreat" — relocating development farther from threatened shores. It's also possible to strengthen natural flood protections, such as coastal marshes and other wetlands. Katrina would have wrought less devastation on New Orleans in 2005 if the cypress swamps that once absorbed the impact of storms had not been decimated by decades of development. Large-scale projects to restore such wetlands are underway in the Gulf, the Chesapeake Bay, and southern San Francisco Bay. But such measures are less workable in highly developed, dense, and low-lying cities like Singapore and New York, where the cost per square foot of land and buildings is among the highest in the world. 

How can cities be protected?
The hot new concept among urban planners is "resilience" — designing buildings, power lines, and other elements of infrastructure to  function even when the next flood arrives. New York City's environmental commissioner Carter H. Strickland has recommended "a series of small interventions," such as more parkland along the East and Hudson Rivers and stiffer flood-protection requirements for new developments. But many are convinced that such small-bore solutions won't suffice, and several plans are being shopped around to protect all of New York Harbor with elaborate sea gates and surge barriers that resemble the Delta Works in the Netherlands. Cost estimates of constructing sea gates for New York Harbor range from $10 billion to $30 billion — which, until last week, seemed totally unrealistic. But with damage estimates from Sandy rising to $50 billion, grand engineering marvels begin to sound more reasonable. "It's not prudent to sit here and say it's not going to happen again," New York Gov. Andrew Cuomo said last week. "I believe it is going to happen again."

How London defies the sea
London, vulnerable to a tidal Thames much as New York is to the Hudson and East Rivers, acted a generation ago to banish the threat of catastrophic flooding. Construction began in 1974 on the Thames Barrier, about 11 miles downriver from London's city center. When the North Sea surges because of storms, pushing floodwaters upriver, the barrier's 10 enormous steel gates rotate and form a barrier. Since its completion in 1982, the barrier has been closed 119 times, at increasing frequency, but it has never been breached. "Before the tidal defenses were built, there was a contingency plan — sounding air-raid sirens, moving buses onto higher ground, and tube stations closing," said Steve East of the U.K.'s Environment Agency. But when the seas now rise, "London can go about its normal business, without even being aware the barrier is being used."