Floods reaching five feet above the current high tide line will become increasingly common along the nation’s coastlines well before the seas climb by five feet. Over the last century, the nearly eight-inch rise of the world’s seas has already doubled the chance of “once in a century” floods for many seaside communities.

We hope that with enough time, most of our great coastal cities and regions will be able to prepare for a five-foot increase. Some will not. Barriers that might work in Manhattan would be futile in South Florida, where water would pass underneath them by pushing through porous bedrock.

According to Dr. Schaeffer’s study, immediate and extreme pollution cuts — measures well beyond any discussion now under way — could limit sea level rise to five feet over 300 years. If we stay on our current path, the oceans could rise five feet by the first half of next century, then continue rising even faster. If instead we make moderate shifts in energy and industry — using the kinds of targets that nations have contemplated in international talks but have failed to pursue — sea level could still climb past 12 feet just after 2300. It is hard to imagine what measures might allow many of our great coastal cities to survive a 12-foot increase…

There are two basic ways to protect ourselves from sea level rise: reduce it by cutting pollution, or prepare for it by defense and retreat. To do the job, we must do both. We have lost our chance for complete prevention; and preparation alone, without slowing emissions, would — sooner or later — turn our coastal cities into so many Atlantises.