Superstorm Sandy was no freak, say experts, but rather a hint of a coming era when millions of Americans will struggle to survive killer weather.

They're telling us we shouldn't be surprised that this 900-mile-wide monster marched up the East Coast this week paralyzing cities and claiming scores of lives.

"It's a foretaste of things to come," Princeton University professor Michael Oppenheimer told CNN. "Bigger storms and higher sea levels" will pile on to create a "growing threat" in the coming decades.

And New York, he warned, "is highly vulnerable."

How can cities defend themselves against such powerful enemies? Some of the ideas out there may surprise you.

They range from building higher sea walls and barrier islands to restoring oyster beds and installing massive gates across estuaries.

New York Mayor Michael Bloomberg has been strategizing.

His goal: mitigate future storm surges and flooding along the city's 500 miles of coastline.

That's a huge challenge. The densely populated city is dominated by some of the nation's most expensive real estate and is surrounded by a complex web of estuaries, tides and ocean.

Should New York think of its coastline as a threat? Is it the new Amsterdam? Maybe, say experts. But even a city as inventive as the Big Apple can only do so much.

The Intergovernmental Panel on Climate Change predicted in 2007 that the global average sea level would rise between 7 and 23 inches by the end of this century. More recent projections suggest that the melting of Arctic sea ice could mean a rise in excess of 30 inches. The New York State Sea Level Rise Task Force translated that into a local projection of 2 to 5 inches by the 2020s, and with rapid ice melt, the rise could be as much as 5 to 10 inches over the next 15 years.

New York dodged a bullet by inches last year as the remnants of Hurricane Irene bore down.

As Ben Orlove, director of the master's program in climate and society at Columbia University, wrote on CNN.com: "Irene also arrived at a time of especially high tides, and its storm surge came within inches of flooding the sea wall. Storms and tides are natural, but sea level rise is not. As it continues, New York grows more vulnerable."

Princeton's Oppenheimer, a professor of geosciences, recently modeled the effect of climate change on storm surges for the New York area.

In a paper published by Nature in February, he and three colleagues concluded that the "storm of the century" would become the storm of "every twenty years or less."

New York Gov. Andrew Cuomo agrees.

"After what happened, what has been happening in the last few years, I don't think anyone can sit back anymore and say 'Well, I'm shocked at that weather pattern,' " Cuomo said Tuesday.

The conclusion of Oppenheimer and his colleagues is that storms will become larger and more powerful.

"Climate change will probably increase storm intensity and size simultaneously, resulting in a significant intensification of storm surges," they wrote. Sandy's diameter measured much larger than most storms.

A study of the New York area in 2010 led by Guy Nordenson, an architect and structural engineer whose offices are in Lower Manhattan, concluded: "There is a prevalent risk that the city will be severely paralyzed due to the predicted inundation and wave action associated with storm surge."

If that's not bad enough, future superstorms may threaten drinking water, too.

Ocean saltwater could compromise the quality of drinking water and weaken ecosystems, Nordenson and others concluded in their book "On the Water: Palisade Bay."

But the answer, they argue, is not solely in engineering. "Cities fortify their coasts to protect real estate at the expense of nature ... (T)he hard engineering habit has proven costly, unreliable and ineffective."

Nordenson is helping Bloomberg study ways of turning New York's waterfront into more of a fortified castle to protect against invading superstorms.

The region needs a combination of strategies that includes more "soft infrastructure."