We think of the sea level rise projected by climate scientists as a gradual thing, like water filling up a bathtub.

We think of the sea level rise projected by climate scientists as a gradual thing, like water filling up a bathtub. Warmer water expands, glaciers melt, and pieces of the great ice caps break off and drop into the sea like ice cubes into a glass of water, pushing the water-line higher. But there was nothing gradual about the rising of seas off New York and New Jersey a year ago this week. Pushed by Superstorm Sandy, a 14-foot storm surge rose up and hit New York Harbor in just a few hours.

By the time the waters receded, Sandy had caused $68 billion in damage, killed 285 people, and delivered yet another warning about climate change.

Put aside the climate change debate for now. We won't talk about what's causing the sea levels to rise, who's to blame or how to stop it. Let's just look at the water filling the tub.

For 8,000 years — a brief interlude in the life of the planet — sea levels have been stable, Tim Gunn told a conference of opinion journalists last week in Newport, R.I. During those years, we humans settled on the coasts and built our great cities. Now the seas are rising, by about a 10th of an inch per year in Boston, a pace that is accelerating.

Sea level rise brings two kinds of threat. Coastal communities must accommodate two high tides a day, and for 3.7 million Americans, a meter of sea level rise would have high tide lapping at their door. Communities must also prepare for a storm surge that may or may not hit any particular stretch of coastline. The higher the normal sea level, the farther inland the storm surge will hit.

And the sea level is rising. This is more a matter of measurement than theory. We can see in photographs the melting of the glaciers and the polar ice caps. The trend may pause for a few years, as has happened with the Tibetan glaciers recently, but the trend is clear and unstoppable.

How high will the seas rise? Gunn, a retired vice-admiral and former inspector general of the U.S. Navy who directs some 400 scientists and engineers at CNA's Institute for Public Research, said predictions range from half a meter (20 inches) to 6 meters (nearly 20 feet) by the end of this century. That range, he said, doesn't mean there's great disagreement among scientists. Almost everyone agrees global sea levels will rise between a half-meter and a meter. The big question involves the West Antarctic Ice Sheet, a huge and unstable mass of glacial ice on land. If that collapses into the sea, we're all going to have to head for higher ground.

But the more modest consensus estimate of sea level rise is alarming enough, Gunn said, especially for the military. His maps show how a one-meter rise would swamp key U.S. military installations in Tampa, Fla., Hampton Roads, Va., and Diego Garcia, a key base destined to sink beneath the rising Indian Ocean.

Climate change is a "threat multiplier," Gunn said, making regions that are already unstable even more dangerous. Food shortages, which can be related to climate change, for instance, helped light the fire that became the Arab Spring.

Gunn's maps of sea-level change illustrate the danger. A one-meter rise in sea level would submerge the Nile Delta, a vital food-producing region in the Egyptian tinderbox, and the Mekong Delta, where much of Southeast Asia gets its rice. Then there's southeast India and Bangladesh, where some of the poorest people in the world are regularly battered by killer storms. With a meter of sea level rise, much of Bangladesh disappears. That's enough of a threat that India has begun building what's being called "the climate fence" on its eastern border to keep Bangladeshis out when the floods force them to higher ground. We've seen political migration and economic migration, Gunn said. Military experts have begun planning for climate migration, and the violence that is likely to come with it.

On Thursday, the Obama administration announced $162 million in grants to protect Atlantic coastal communities from future storms. Most of it will go toward studies and the restoration of coastal marshes and barrier beaches. Considering the Interior Department has already spent three times that much in the last year cleaning up the damage done by Sandy, it's obviously money well spent.

It's also, forgive the expression, a drop in the ocean. Batten the hatches. The seas are rising, and responding to its threats may be the most expensive task this country has ever undertaken.